# AWS AppSync resolver mapping template reference for DynamoDB
**Note**
We now primarily support the APPSYNC\$1JS runtime and its documentation. Please consider using the APPSYNC\$1JS runtime and its guides [here](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-reference-js-version.html).
The AWS AppSync DynamoDB function allows you to use [GraphQL](https://graphql.org) to store and retrieve data in existing Amazon DynamoDB tables in your account by mapping an incoming GraphQL request into a DynamoDB call, and then mapping the DynamoDB response back to GraphQL. This section describes the request and response handlers for supported DynamoDB operations:
+ [GetItem](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-getitem.html) - The GetItem request lets you tell the DynamoDB function to make a GetItem request to DynamoDB, and enables you to specify the key of the item in DynamoDB and whether to use a consistent read or not.
+ [ PutItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-putitem.html) - The PutItem request mapping document lets you tell the DynamoDB function to make a PutItem request to DynamoDB, and enables you to specify the key of the item in DynamoDB, the full contents of the item (composed of key and attributeValues), and conditions for the operation to succeed.
+ [ UpdateItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-updateitem.html) - The UpdateItem request enables you to tell the DynamoDB function to make a UpdateItem request to DynamoDB and allows you to specify the key of the item in DynamoDB, an update expression describing how to update the item in DynamoDB, and conditions for the operation to succeed.
+ [ DeleteItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-deleteitem.html) - The DeleteItem request lets you tell the DynamoDB function to make a DeleteItem request to DynamoDB, and enables you to specify the key of the item in DynamoDB and conditions for the operation to succeed.
+ [ Query ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-query.html) - The Query request object lets you tell the DynamoDB resolver to make a Query request to DynamoDB, and enables you to specify the key expression, which index to use, additional filters, how many items to return, whether to use consistent reads, query direction (forward or backward), and pagination tokens.
+ [ Scan ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-scan.html) - The Scan request lets you tell the DynamoDB function to make a Scan request to DynamoDB, and enables you to specify a filter to exclude results, which index to use, how many items to return, whether to use consistent reads, pagination tokens, and parallel scans.
+ [ Sync ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-sync.html) - The Sync request object lets you retrieve all the results from a DynamoDB table and then receive only the data altered since your last query (the delta updates). Sync requests can only be made to versioned DynamoDB data sources. You can specify a filter to exclude results, how many items to return, pagination Tokens, and when your last Sync operation was started.
+ [ BatchGetItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-batch-get-item.html) - The BatchGetItem request object lets you tell the DynamoDB function to make a BatchGetItem request to DynamoDB to retrieve multiple items, potentially across multiple tables. For this request object, you must specify the table names to retrieve the items from and the keys of the items to retrieve from each table.
+ [ BatchDeleteItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-batch-delete-item.html) - The BatchDeleteItem request object lets you tell the DynamoDB function to make a BatchWriteItem request to DynamoDB to delete multiple items, potentially across multiple tables. For this request object, you must specify the table names to delete the items from and the keys of the items to delete from each table.
+ [ BatchPutItem ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-batch-put-item.html) - The BatchPutItem request object lets you tell the DynamoDB function to make a BatchWriteItem request to DynamoDB to put multiple items, potentially across multiple tables. For this request object, you must specify the table names to put the items in and the full items to put in each table.
+ [ TransactGetItems ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-transact-get-items.html) - The TransactGetItems request object lets you to tell the DynamoDB function to make a TransactGetItems request to DynamoDB to retrieve multiple items, potentially across multiple tables. For this request object, you must specify the table name of each request item to retrieve the item from and the key of each request item to retrieve from each table.
+ [ TransactWriteItems ](https://docs.aws.amazon.com/appsync/latest/devguide/js-aws-appsync-resolver-reference-dynamodb-transact-write-items.html) - The TransactWriteItems request object lets you tell the DynamoDB function to make a TransactWriteItems request to DynamoDB to write multiple items, potentially to multiple tables. For this request object, you must specify the destination table name of each request item, the operation of each request item to perform, and the key of each request item to write.
+ [ Type system (request mapping) ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.html) - Learn more about how DynamoDB typing is integrated into AWS AppSync requests.
+ [ Type system (response mapping) ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.html) - Learn more about how DynamoDB types are converted automatically to GraphQL or JSON in a response payload.
+ [Filters](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-filter.html) - Learn more about filters for query and scan operations.
+ [ Condition expressions ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-condition-expressions.html) - Learn more about condition expressions for PutItem, UpdateItem, and DeleteItem operations.
+ [ Transaction condition expressions ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-transaction-condition-expressions.html) - Learn more about condition expressions for TransactWriteItems operations.
+ [ Projections ](https://docs.aws.amazon.com/appsync/latest/devguide/aws-appsync-resolver-mapping-template-reference-dynamodb-projections.html) - Learn more about how to specify attributes in read operations.
# GetItem
The `GetItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `GetItem` request to DynamoDB, and enables you to specify:
+ The key of the item in DynamoDB
+ Whether to use a consistent read or not
The `GetItem` mapping document has the following structure:
```
{
"version" : "2017-02-28",
"operation" : "GetItem",
"key" : {
"foo" : ... typed value,
"bar" : ... typed value
},
"consistentRead" : true,
"projection" : {
...
}
}
```
The fields are defined as follows:
## GetItem fields
### GetItem fields list
**`version`**
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
**`operation`**
The DynamoDB operation to perform. To perform the `GetItem` DynamoDB operation, this must be set to `GetItem`. This value is required.
**`key`**
The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
**`consistentRead`**
Whether or not to perform a strongly consistent read with DynamoDB. This is optional, and defaults to `false`.
**`projection`**
A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see [Projections](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-projections). This field is optional.
The item returned from DynamoDB is automatically converted into GraphQL and JSON primitive types, and is available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
## Example
The following example is a mapping template for a GraphQL query `getThing(foo: String!, bar: String!)`:
```
{
"version" : "2017-02-28",
"operation" : "GetItem",
"key" : {
"foo" : $util.dynamodb.toDynamoDBJson($ctx.args.foo),
"bar" : $util.dynamodb.toDynamoDBJson($ctx.args.bar)
},
"consistentRead" : true
}
```
For more information about the DynamoDB `GetItem` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_GetItem.html).
# PutItem
The `PutItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `PutItem` request to DynamoDB, and enables you to specify the following:
+ The key of the item in DynamoDB
+ The full contents of the item (composed of `key` and `attributeValues`)
+ Conditions for the operation to succeed
The `PutItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "PutItem",
"customPartitionKey" : "foo",
"populateIndexFields" : boolean value,
"key": {
"foo" : ... typed value,
"bar" : ... typed value
},
"attributeValues" : {
"baz" : ... typed value
},
"condition" : {
...
},
"_version" : 1
}
```
The fields are defined as follows:
## PutItem fields
### PutItem fields list
**`version`**
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
**`operation`**
The DynamoDB operation to perform. To perform the `PutItem` DynamoDB operation, this must be set to `PutItem`. This value is required.
**`key`**
The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
**`attributeValues`**
The rest of the attributes of the item to be put into DynamoDB. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This field is optional.
**`condition`**
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `PutItem` request overwrites any existing entry for that item. For more information about conditions, see [Condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-condition-expressions.md). This value is optional.
**`_version`**
A numeric value that represents the latest known version of an item. This value is optional. This field is used for *Conflict Detection* and is only supported on versioned data sources.
**`customPartitionKey`**
When enabled, this string value modifies the format of the `ds_sk` and `ds_pk` records used by the delta sync table when versioning has been enabled (for more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*). When enabled, the processing of the `populateIndexFields` entry is also enabled. This field is optional.
**`populateIndexFields`**
A boolean value that, when enabled **along with the `customPartitionKey`**, creates new entries for each record in the delta sync table, specifically in the `gsi_ds_pk` and `gsi_ds_sk` columns. For more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*. This field is optional.
The item written to DynamoDB is automatically converted into GraphQL and JSON primitive types and is available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
## Example 1
The following example is a mapping template for a GraphQL mutation `updateThing(foo: String!, bar: String!, name: String!, version: Int!)`.
If no item with the specified key exists, it’s created. If an item already exists with the specified key, it’s overwritten.
```
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key": {
"foo" : $util.dynamodb.toDynamoDBJson($ctx.args.foo),
"bar" : $util.dynamodb.toDynamoDBJson($ctx.args.bar)
},
"attributeValues" : {
"name" : $util.dynamodb.toDynamoDBJson($ctx.args.name),
"version" : $util.dynamodb.toDynamoDBJson($ctx.args.version)
}
}
```
## Example 2
The following example is a mapping template for a GraphQL mutation `updateThing(foo: String!, bar: String!, name: String!, expectedVersion: Int!)`.
This example checks to be sure the item currently in DynamoDB has the `version` field set to `expectedVersion`.
```
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key": {
"foo" : $util.dynamodb.toDynamoDBJson($ctx.args.foo),
"bar" : $util.dynamodb.toDynamoDBJson($ctx.args.bar)
},
"attributeValues" : {
"name" : $util.dynamodb.toDynamoDBJson($ctx.args.name),
#set( $newVersion = $context.arguments.expectedVersion + 1 )
"version" : $util.dynamodb.toDynamoDBJson($newVersion)
},
"condition" : {
"expression" : "version = :expectedVersion",
"expressionValues" : {
":expectedVersion" : $util.dynamodb.toDynamoDBJson($expectedVersion)
}
}
}
```
For more information about the DynamoDB `PutItem` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_PutItem.html).
# UpdateItem
The `UpdateItem` request mapping document enables you to tell the AWS AppSync DynamoDB resolver to make a `UpdateItem` request to DynamoDB and allows you to specify the following:
+ The key of the item in DynamoDB
+ An update expression describing how to update the item in DynamoDB
+ Conditions for the operation to succeed
The `UpdateItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "UpdateItem",
"customPartitionKey" : "foo",
"populateIndexFields" : boolean value,
"key": {
"foo" : ... typed value,
"bar" : ... typed value
},
"update" : {
"expression" : "someExpression",
"expressionNames" : {
"#foo" : "foo"
},
"expressionValues" : {
":bar" : ... typed value
}
},
"condition" : {
...
},
"_version" : 1
}
```
The fields are defined as follows:
## UpdateItem fields
### UpdateItem fields list
**`version`**
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
**`operation`**
The DynamoDB operation to perform. To perform the `UpdateItem` DynamoDB operation, this must be set to `UpdateItem`. This value is required.
**`key`**
The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about specifying a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
**`update`**
The `update` section lets you specify an update expression that describes how to update the item in DynamoDB. For more information about how to write update expressions, see the [DynamoDB UpdateExpressions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html). This section is required.
The `update` section has three components:
** `expression` **
The update expression. This value is required.
** `expressionNames` **
The substitutions for expression attribute *name* placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the `expression`, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the `expression`.
** `expressionValues` **
The substitutions for expression attribute *value* placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the `expression`, and the value must be a typed value. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the `expression`.
**`condition`**
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `UpdateItem` request updates the existing entry regardless of its current state. For more information about conditions, see [Condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-condition-expressions.md). This value is optional.
**`_version`**
A numeric value that represents the latest known version of an item. This value is optional. This field is used for *Conflict Detection* and is only supported on versioned data sources.
**`customPartitionKey`**
When enabled, this string value modifies the format of the `ds_sk` and `ds_pk` records used by the delta sync table when versioning has been enabled (for more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*). When enabled, the processing of the `populateIndexFields` entry is also enabled. This field is optional.
**`populateIndexFields`**
A boolean value that, when enabled **along with the `customPartitionKey`**, creates new entries for each record in the delta sync table, specifically in the `gsi_ds_pk` and `gsi_ds_sk` columns. For more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*. This field is optional.
The item updated in DynamoDB is automatically converted into GraphQL and JSON primitive types and is available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
## Example 1
The following example is a mapping template for the GraphQL mutation `upvote(id: ID!)`.
In this example, an item in DynamoDB has its `upvotes` and `version` fields incremented by 1.
```
{
"version" : "2017-02-28",
"operation" : "UpdateItem",
"key" : {
"id" : $util.dynamodb.toDynamoDBJson($ctx.args.id)
},
"update" : {
"expression" : "ADD #votefield :plusOne, version :plusOne",
"expressionNames" : {
"#votefield" : "upvotes"
},
"expressionValues" : {
":plusOne" : { "N" : 1 }
}
}
}
```
## Example 2
The following example is a mapping template for a GraphQL mutation `updateItem(id: ID!, title: String, author: String, expectedVersion: Int!)`.
This is a complex example that inspects the arguments and dynamically generates the update expression that only includes the arguments that have been provided by the client. For example, if `title` and `author` are omitted, they are not updated. If an argument is specified but its value is `null`, then that field is deleted from the object in DynamoDB. Finally, the operation has a condition, which verifies whether the item currently in DynamoDB has the `version` field set to `expectedVersion`:
```
{
"version" : "2017-02-28",
"operation" : "UpdateItem",
"key" : {
"id" : $util.dynamodb.toDynamoDBJson($ctx.args.id)
},
## Set up some space to keep track of things we're updating **
#set( $expNames = {} )
#set( $expValues = {} )
#set( $expSet = {} )
#set( $expAdd = {} )
#set( $expRemove = [] )
## Increment "version" by 1 **
$!{expAdd.put("version", ":newVersion")}
$!{expValues.put(":newVersion", { "N" : 1 })}
## Iterate through each argument, skipping "id" and "expectedVersion" **
#foreach( $entry in $context.arguments.entrySet() )
#if( $entry.key != "id" && $entry.key != "expectedVersion" )
#if( (!$entry.value) && ("$!{entry.value}" == "") )
## If the argument is set to "null", then remove that attribute from the item in DynamoDB **
#set( $discard = ${expRemove.add("#${entry.key}")} )
$!{expNames.put("#${entry.key}", "$entry.key")}
#else
## Otherwise set (or update) the attribute on the item in DynamoDB **
$!{expSet.put("#${entry.key}", ":${entry.key}")}
$!{expNames.put("#${entry.key}", "$entry.key")}
#if( $entry.key == "ups" || $entry.key == "downs" )
$!{expValues.put(":${entry.key}", { "N" : $entry.value })}
#else
$!{expValues.put(":${entry.key}", { "S" : "${entry.value}" })}
#end
#end
#end
#end
## Start building the update expression, starting with attributes we're going to SET **
#set( $expression = "" )
#if( !${expSet.isEmpty()} )
#set( $expression = "SET" )
#foreach( $entry in $expSet.entrySet() )
#set( $expression = "${expression} ${entry.key} = ${entry.value}" )
#if ( $foreach.hasNext )
#set( $expression = "${expression}," )
#end
#end
#end
## Continue building the update expression, adding attributes we're going to ADD **
#if( !${expAdd.isEmpty()} )
#set( $expression = "${expression} ADD" )
#foreach( $entry in $expAdd.entrySet() )
#set( $expression = "${expression} ${entry.key} ${entry.value}" )
#if ( $foreach.hasNext )
#set( $expression = "${expression}," )
#end
#end
#end
## Continue building the update expression, adding attributes we're going to REMOVE **
#if( !${expRemove.isEmpty()} )
#set( $expression = "${expression} REMOVE" )
#foreach( $entry in $expRemove )
#set( $expression = "${expression} ${entry}" )
#if ( $foreach.hasNext )
#set( $expression = "${expression}," )
#end
#end
#end
## Finally, write the update expression into the document, along with any expressionNames and expressionValues **
"update" : {
"expression" : "${expression}"
#if( !${expNames.isEmpty()} )
,"expressionNames" : $utils.toJson($expNames)
#end
#if( !${expValues.isEmpty()} )
,"expressionValues" : $utils.toJson($expValues)
#end
},
"condition" : {
"expression" : "version = :expectedVersion",
"expressionValues" : {
":expectedVersion" : $util.dynamodb.toDynamoDBJson($ctx.args.expectedVersion)
}
}
}
```
For more information about the DynamoDB `UpdateItem` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateItem.html).
# DeleteItem
The `DeleteItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `DeleteItem` request to DynamoDB, and enables you to specify the following:
+ The key of the item in DynamoDB
+ Conditions for the operation to succeed
The `DeleteItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "DeleteItem",
"customPartitionKey" : "foo",
"populateIndexFields" : boolean value,
"key": {
"foo" : ... typed value,
"bar" : ... typed value
},
"condition" : {
...
},
"_version" : 1
}
```
The fields are defined as follows:
## DeleteItem fields
### DeleteItem fields list
** `version` **
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `DeleteItem` DynamoDB operation, this must be set to `DeleteItem`. This value is required.
** `key` **
The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about specifying a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
** `condition` **
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `DeleteItem` request deletes an item regardless of its current state. For more information about conditions, see [Condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-condition-expressions.md). This value is optional.
** `_version` **
A numeric value that represents the latest known version of an item. This value is optional. This field is used for *Conflict Detection* and is only supported on versioned data sources.
**`customPartitionKey`**
When enabled, this string value modifies the format of the `ds_sk` and `ds_pk` records used by the delta sync table when versioning has been enabled (for more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*). When enabled, the processing of the `populateIndexFields` entry is also enabled. This field is optional.
**`populateIndexFields`**
A boolean value that, when enabled **along with the `customPartitionKey`**, creates new entries for each record in the delta sync table, specifically in the `gsi_ds_pk` and `gsi_ds_sk` columns. For more information, see [Conflict detection and sync](https://docs.aws.amazon.com/appsync/latest/devguide/conflict-detection-and-sync.html) in the *AWS AppSync Developer Guide*. This field is optional.
The item deleted from DynamoDB is automatically converted into GraphQL and JSON primitive types and is available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
## Example 1
The following example is a mapping template for a GraphQL mutation `deleteItem(id: ID!)`. If an item exists with this ID, it’s deleted.
```
{
"version" : "2017-02-28",
"operation" : "DeleteItem",
"key" : {
"id" : $util.dynamodb.toDynamoDBJson($ctx.args.id)
}
}
```
## Example 2
The following example is a mapping template for a GraphQL mutation `deleteItem(id: ID!, expectedVersion: Int!)`. If an item exists with this ID, it’s deleted, but only if its `version` field set to `expectedVersion`:
```
{
"version" : "2017-02-28",
"operation" : "DeleteItem",
"key" : {
"id" : $util.dynamodb.toDynamoDBJson($ctx.args.id)
},
"condition" : {
"expression" : "attribute_not_exists(id) OR version = :expectedVersion",
"expressionValues" : {
":expectedVersion" : $util.dynamodb.toDynamoDBJson($expectedVersion)
}
}
}
```
For more information about the DynamoDB `DeleteItem` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_DeleteItem.html).
# Query
The `Query` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `Query` request to DynamoDB, and enables you to specify the following:
+ Key expression
+ Which index to use
+ Any additional filter
+ How many items to return
+ Whether to use consistent reads
+ query direction (forward or backward)
+ Pagination token
The `Query` mapping document has the following structure:
```
{
"version" : "2017-02-28",
"operation" : "Query",
"query" : {
"expression" : "some expression",
"expressionNames" : {
"#foo" : "foo"
},
"expressionValues" : {
":bar" : ... typed value
}
},
"index" : "fooIndex",
"nextToken" : "a pagination token",
"limit" : 10,
"scanIndexForward" : true,
"consistentRead" : false,
"select" : "ALL_ATTRIBUTES" | "ALL_PROJECTED_ATTRIBUTES" | "SPECIFIC_ATTRIBUTES",
"filter" : {
...
},
"projection" : {
...
}
}
```
The fields are defined as follows:
## Query fields
### Query fields list
** `version` **
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `Query` DynamoDB operation, this must be set to `Query`. This value is required.
** `query` **
The `query` section lets you specify a key condition expression that describes which items to retrieve from DynamoDB. For more information about how to write key condition expressions, see the [DynamoDB KeyConditions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/LegacyConditionalParameters.KeyConditions.html) . This section must be specified.
** `expression` **
The query expression. This field must be specified.
** `expressionNames` **
The substitutions for expression attribute *name* placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the `expression`, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the `expression`.
** `expressionValues` **
The substitutions for expression attribute *value* placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the `expression`, and the value must be a typed value. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the `expression`.
** `filter` **
An additional filter that can be used to filter the results from DynamoDB before they are returned. For more information about filters, see [Filters](aws-appsync-resolver-mapping-template-reference-dynamodb-filter.md). This field is optional.
** `index` **
The name of the index to query. The DynamoDB query operation allows you to scan on Local Secondary Indexes and Global Secondary Indexes in addition to the primary key index for a hash key. If specified, this tells DynamoDB to query the specified index. If omitted, the primary key index is queried.
** `nextToken` **
The pagination token to continue a previous query. This would have been obtained from a previous query. This field is optional.
** `limit` **
The maximum number of items to evaluate (not necessarily the number of matching items). This field is optional.
** `scanIndexForward` **
A boolean indicating whether to query forwards or backwards. This field is optional, and defaults to `true`.
** `consistentRead` **
A boolean indicating whether to use consistent reads when querying DynamoDB. This field is optional, and defaults to `false`.
** `select` **
By default, the AWS AppSync DynamoDB resolver only returns attributes that are projected into the index. If more attributes are required, you can set this field. This field is optional. The supported values are:
** `ALL_ATTRIBUTES` **
Returns all of the item attributes from the specified table or index. If you query a local secondary index, DynamoDB fetches the entire item from the parent table for each matching item in the index. If the index is configured to project all item attributes, all of the data can be obtained from the local secondary index and no fetching is required.
** `ALL_PROJECTED_ATTRIBUTES` **
Allowed only when querying an index. Retrieves all attributes that have been projected into the index. If the index is configured to project all attributes, this return value is equivalent to specifying `ALL_ATTRIBUTES`.
**`SPECIFIC_ATTRIBUTES`**
Returns only the attributes listed in the `projection`'s `expression`. This return value is equivalent to specifying the `projection`'s `expression` without specifying any value for `Select`.
**`projection`**
A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see [Projections](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-projections). This field is optional.
The results from DynamoDB are automatically converted into GraphQL and JSON primitive types and are available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
The results have the following structure:
```
{
items = [ ... ],
nextToken = "a pagination token",
scannedCount = 10
}
```
The fields are defined as follows:
** `items` **
A list containing the items returned by the DynamoDB query.
** `nextToken` **
If there might be more results, `nextToken` contains a pagination token that you can use in another request. Note that AWS AppSync encrypts and obfuscates the pagination token returned from DynamoDB. This prevents your table data from being inadvertently leaked to the caller. Also note that these pagination tokens cannot be used across different resolvers.
** `scannedCount` **
The number of items that matched the query condition expression, before a filter expression (if present) was applied.
## Example
The following example is a mapping template for a GraphQL query `getPosts(owner: ID!)`.
In this example, a global secondary index on a table is queried to return all posts owned by the specified ID.
```
{
"version" : "2017-02-28",
"operation" : "Query",
"query" : {
"expression" : "ownerId = :ownerId",
"expressionValues" : {
":ownerId" : $util.dynamodb.toDynamoDBJson($context.arguments.owner)
}
},
"index" : "owner-index"
}
```
For more information about the DynamoDB `Query` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html).
# Scan
The `Scan` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `Scan` request to DynamoDB, and enables you to specify the following:
+ A filter to exclude results
+ Which index to use
+ How many items to return
+ Whether to use consistent reads
+ Pagination token
+ Parallel scans
The `Scan` mapping document has the following structure:
```
{
"version" : "2017-02-28",
"operation" : "Scan",
"index" : "fooIndex",
"limit" : 10,
"consistentRead" : false,
"nextToken" : "aPaginationToken",
"totalSegments" : 10,
"segment" : 1,
"filter" : {
...
},
"projection" : {
...
}
}
```
The fields are defined as follows:
## Scan fields
### Scan fields list
** `version` **
The template definition version. `2017-02-28` and `2018-05-29` are currently supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `Scan` DynamoDB operation, this must be set to `Scan`. This value is required.
** `filter` **
A filter that can be used to filter the results from DynamoDB before they are returned. For more information about filters, see [Filters](aws-appsync-resolver-mapping-template-reference-dynamodb-filter.md). This field is optional.
** `index` **
The name of the index to query. The DynamoDB query operation allows you to scan on Local Secondary Indexes and Global Secondary Indexes in addition to the primary key index for a hash key. If specified, this tells DynamoDB to query the specified index. If omitted, the primary key index is queried.
** `limit` **
The maximum number of items to evaluate at a single time. This field is optional.
** `consistentRead` **
A Boolean that indicates whether to use consistent reads when querying DynamoDB. This field is optional, and defaults to `false`.
** `nextToken` **
The pagination token to continue a previous query. This would have been obtained from a previous query. This field is optional.
** `select` **
By default, the AWS AppSync DynamoDB resolver only returns whatever attributes are projected into the index. If more attributes are required, then this field can be set. This field is optional. The supported values are:
** `ALL_ATTRIBUTES` **
Returns all of the item attributes from the specified table or index. If you query a local secondary index, DynamoDB fetches the entire item from the parent table for each matching item in the index. If the index is configured to project all item attributes, all of the data can be obtained from the local secondary index and no fetching is required.
** `ALL_PROJECTED_ATTRIBUTES` **
Allowed only when querying an index. Retrieves all attributes that have been projected into the index. If the index is configured to project all attributes, this return value is equivalent to specifying `ALL_ATTRIBUTES`.
**`SPECIFIC_ATTRIBUTES`**
Returns only the attributes listed in the `projection`'s `expression`. This return value is equivalent to specifying the `projection`'s `expression` without specifying any value for `Select`.
** `totalSegments` **
The number of segments to partition the table by when performing a parallel scan. This field is optional, but must be specified if `segment` is specified.
** `segment` **
The table segment in this operation when performing a parallel scan. This field is optional, but must be specified if `totalSegments` is specified.
**`projection`**
A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see [Projections](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-projections). This field is optional.
The results returned by the DynamoDB scan are automatically converted into GraphQL and JSON primitive types and is available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
The results have the following structure:
```
{
items = [ ... ],
nextToken = "a pagination token",
scannedCount = 10
}
```
The fields are defined as follows:
** `items` **
A list containing the items returned by the DynamoDB scan.
** `nextToken` **
If there might be more results, `nextToken` contains a pagination token that you can use in another request. AWS AppSync encrypts and obfuscates the pagination token returned from DynamoDB. This prevents your table data from being inadvertently leaked to the caller. Also, these pagination tokens can’t be used across different resolvers.
** `scannedCount` **
The number of items that were retrieved by DynamoDB before a filter expression (if present) was applied.
## Example 1
The following example is a mapping template for the GraphQL query: `allPosts`.
In this example, all entries in the table are returned.
```
{
"version" : "2017-02-28",
"operation" : "Scan"
}
```
## Example 2
The following example is a mapping template for the GraphQL query: `postsMatching(title: String!)`.
In this example, all entries in the table are returned where the title starts with the `title` argument.
```
{
"version" : "2017-02-28",
"operation" : "Scan",
"filter" : {
"expression" : "begins_with(title, :title)",
"expressionValues" : {
":title" : $util.dynamodb.toDynamoDBJson($context.arguments.title)
},
}
}
```
For more information about the DynamoDB `Scan` API, see the [DynamoDB API documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Scan.html).
# Sync
The `Sync` request mapping document lets you retrieve all the results from a DynamoDB table and then receive only the data altered since your last query (the delta updates). `Sync` requests can only be made to versioned DynamoDB data sources. You can specify the following:
+ A filter to exclude results
+ How many items to return
+ Pagination Token
+ When your last `Sync` operation was started
The `Sync` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "Sync",
"basePartitionKey": "Base Tables PartitionKey",
"deltaIndexName": "delta-index-name",
"limit" : 10,
"nextToken" : "aPaginationToken",
"lastSync" : 1550000000000,
"filter" : {
...
}
}
```
The fields are defined as follows:
## Sync fields
### Sync fields list
** `version` **
The template definition version. Only `2018-05-29` is currently supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `Sync` operation, this must be set to `Sync`. This value is required.
** `filter` **
A filter that can be used to filter the results from DynamoDB before they are returned. For more information about filters, see [Filters](aws-appsync-resolver-mapping-template-reference-dynamodb-filter.md). This field is optional.
** `limit` **
The maximum number of items to evaluate at a single time. This field is optional. If omitted, the default limit will be set to `100` items. The maximum value for this field is `1000` items.
** `nextToken` **
The pagination token to continue a previous query. This would have been obtained from a previous query. This field is optional.
** `lastSync` **
The moment, in epoch milliseconds, when the last successful `Sync` operation started. If specified, only items that have changed after `lastSync` are returned. This field is optional, and should only be populated after retrieving all pages from an initial `Sync` operation. If omitted, results from the *Base* table will be returned, otherwise, results from the *Delta* table will be returned.
**`basePartitionKey`**
The partition key of the *Base* table used when performing a `Sync` operation. This field allows a `Sync` operation to be performed when the table utilizes a custom partition key. This is an optional field.
**`deltaIndexName`**
The index used for the `Sync` operation. This index is required to enable a `Sync` operation on the whole delta store table when the table uses a custom partition key. The `Sync` operation will be performed on the GSI (created on `gsi_ds_pk` and `gsi_ds_sk`). This field is optional.
The results returned by the DynamoDB sync are automatically converted into GraphQL and JSON primitive types and are available in the mapping context (`$context.result`).
For more information about DynamoDB type conversion, see [Type system (response mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-responses.md).
For more information about response mapping templates, see [Resolver mapping template overview](resolver-mapping-template-reference-overview.md#aws-appsync-resolver-mapping-template-reference-overview).
The results have the following structure:
```
{
items = [ ... ],
nextToken = "a pagination token",
scannedCount = 10,
startedAt = 1550000000000
}
```
The fields are defined as follows:
** `items` **
A list containing the items returned by the sync.
** `nextToken` **
If there might be more results, `nextToken` contains a pagination token that you can use in another request. AWS AppSync encrypts and obfuscates the pagination token returned from DynamoDB. This prevents your table data from being inadvertently leaked to the caller. Also, these pagination tokens can’t be used across different resolvers.
** `scannedCount` **
The number of items that were retrieved by DynamoDB before a filter expression (if present) was applied.
** `startedAt` **
The moment, in epoch milliseconds, when the sync operation started that you can store locally and use in another request as your `lastSync` argument. If a pagination token was included in the request, this value will be the same as the one returned by the request for the first page of results.
## Example
The following example is a mapping template for the GraphQL query: `syncPosts(nextToken: String, lastSync: AWSTimestamp)`.
In this example, if `lastSync` is omitted, all entries in the base table are returned. If `lastSync` is supplied, only the entries in the delta sync table that have changed since `lastSync` are returned.
```
{
"version" : "2018-05-29",
"operation" : "Sync",
"limit": 100,
"nextToken": $util.toJson($util.defaultIfNull($ctx.args.nextToken, null)),
"lastSync": $util.toJson($util.defaultIfNull($ctx.args.lastSync, null))
}
```
# BatchGetItem
The `BatchGetItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `BatchGetItem` request to DynamoDB to retrieve multiple items, potentially across multiple tables. For this request template, you must specify the following:
+ The table names where to retrieve the items from
+ The keys of the items to retrieve from each table
The DynamoDB `BatchGetItem` limits apply and **no condition expression** can be provided.
The `BatchGetItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "BatchGetItem",
"tables" : {
"table1": {
"keys": [
## Item to retrieve Key
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item2 to retrieve Key
{
"foo" : ... typed value,
"bar" : ... typed value
}
],
"consistentRead": true|false,
"projection" : {
...
}
},
"table2": {
"keys": [
## Item3 to retrieve Key
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item4 to retrieve Key
{
"foo" : ... typed value,
"bar" : ... typed value
}
],
"consistentRead": true|false,
"projection" : {
...
}
}
}
}
```
The fields are defined as follows:
## BatchGetItem fields
### BatchGetItem fields list
** `version` **
The template definition version. Only `2018-05-29` is supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `BatchGetItem` DynamoDB operation, this must be set to `BatchGetItem`. This value is required.
** `tables` **
The DynamoDB tables to retrieve the items from. The value is a map where table names are specified as the keys of the map. At least one table must be provided. This `tables` value is required.
** `keys` **
List of DynamoDB keys representing the primary key of the items to retrieve. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md).
** `consistentRead` **
Whether to use a consistent read when executing a *GetItem* operation. This value is optional and defaults to *false*.
**`projection`**
A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see [Projections](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-projections). This field is optional.
Things to remember:
+ If an item has not been retrieved from the table, a *null* element appears in the data block for that table.
+ Invocation results are sorted per table, based on the order in which they were provided inside the request mapping template.
+ Each `Get` command inside a `BatchGetItem` is atomic, however, a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the *unprocessedKeys* block.
+ `BatchGetItem` is limited to 100 keys.
For the following example request mapping template:
```
{
"version": "2018-05-29",
"operation": "BatchGetItem",
"tables": {
"authors": [
{
"author_id": {
"S": "a1"
}
},
],
"posts": [
{
"author_id": {
"S": "a1"
},
"post_id": {
"S": "p2"
}
}
],
}
}
```
The invocation result available in `$ctx.result` is as follows:
```
{
"data": {
"authors": [null],
"posts": [
# Was retrieved
{
"author_id": "a1",
"post_id": "p2",
"post_title": "title",
"post_description": "description",
}
]
},
"unprocessedKeys": {
"authors": [
# This item was not processed due to an error
{
"author_id": "a1"
}
],
"posts": []
}
}
```
The `$ctx.error` contains details about the error. The keys **data**, **unprocessedKeys**, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been deleted appear in the **data** block. Items that haven’t been processed are marked as *null* inside the data block and are placed inside the **unprocessedKeys** block.
For a more complete example, follow the DynamoDB Batch tutorial with AppSync here [Tutorial: DynamoDB batch resolvers](tutorial-dynamodb-batch.md#aws-appsync-tutorial-dynamodb-batch).
# BatchDeleteItem
The `BatchDeleteItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `BatchWriteItem` request to DynamoDB to delete multiple items, potentially across multiple tables. For this request template, you must specify the following:
+ The table names where to delete the items from
+ The keys of the items to delete from each table
The DynamoDB `BatchWriteItem` limits apply and **no condition expression** can be provided.
The `BatchDeleteItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "BatchDeleteItem",
"tables" : {
"table1": [
## Item to delete Key
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item2 to delete Key
{
"foo" : ... typed value,
"bar" : ... typed value
}],
"table2": [
## Item3 to delete Key
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item4 to delete Key
{
"foo" : ... typed value,
"bar" : ... typed value
}],
}
}
```
The fields are defined as follows:
## BatchDeleteItem fields
### BatchDeleteItem fields list
** `version` **
The template definition version. Only `2018-05-29` is supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `BatchDeleteItem` DynamoDB operation, this must be set to `BatchDeleteItem`. This value is required.
** `tables` **
The DynamoDB tables to delete the items from. Each table is a list of DynamoDB keys representing the primary key of the items to delete. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). At least one table must be provided. The `tables` value is required.
Things to remember:
+ Contrary to the `DeleteItem` operation, the fully deleted item isn’t returned in the response. Only the passed key is returned.
+ If an item has not been deleted from the table, a *null* element appears in the data block for that table.
+ Invocation results are sorted per table, based on the order in which they were provided inside the request mapping template.
+ Each `Delete` command inside a `BatchDeleteItem` is atomic. However a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the *unprocessedKeys* block.
+ `BatchDeleteItem` is limited to 25 keys.
+ This operation **is not** supported when used with conflict detection. Using both at the same time may result in an error.
For the following example request mapping template:
```
{
"version": "2018-05-29",
"operation": "BatchDeleteItem",
"tables": {
"authors": [
{
"author_id": {
"S": "a1"
}
},
],
"posts": [
{
"author_id": {
"S": "a1"
},
"post_id": {
"S": "p2"
}
}
],
}
}
```
The invocation result available in `$ctx.result` is as follows:
```
{
"data": {
"authors": [null],
"posts": [
# Was deleted
{
"author_id": "a1",
"post_id": "p2"
}
]
},
"unprocessedKeys": {
"authors": [
# This key was not processed due to an error
{
"author_id": "a1"
}
],
"posts": []
}
}
```
The `$ctx.error` contains details about the error. The keys **data**, **unprocessedKeys**, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been deleted are present in the **data** block. Items that haven’t been processed are marked as *null* inside the data block and are placed inside the **unprocessedKeys** block.
For a more complete example, follow the DynamoDB Batch tutorial with AppSync here [Tutorial: DynamoDB batch resolvers](tutorial-dynamodb-batch.md#aws-appsync-tutorial-dynamodb-batch).
# BatchPutItem
The `BatchPutItem` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `BatchWriteItem` request to DynamoDB to put multiple items, potentially across multiple tables. For this request template, you must specify the following:
+ The table names where to put the items in
+ The full items to put in each table
The DynamoDB `BatchWriteItem` limits apply and **no condition expression** can be provided.
The `BatchPutItem` mapping document has the following structure:
```
{
"version" : "2018-05-29",
"operation" : "BatchPutItem",
"tables" : {
"table1": [
## Item to put
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item2 to put
{
"foo" : ... typed value,
"bar" : ... typed value
}],
"table2": [
## Item3 to put
{
"foo" : ... typed value,
"bar" : ... typed value
},
## Item4 to put
{
"foo" : ... typed value,
"bar" : ... typed value
}],
}
}
```
The fields are defined as follows:
## BatchPutItem fields
### BatchPutItem fields list
** `version` **
The template definition version. Only `2018-05-29` is supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `BatchPutItem` DynamoDB operation, this must be set to `BatchPutItem`. This value is required.
** `tables` **
The DynamoDB tables to put the items in. Each table entry represents a list of DynamoDB items to insert for this specific table. At least one table must be provided. This value is required.
Things to remember:
+ The fully inserted items are returned in the response, if successful.
+ If an item hasn’t been inserted in the table, a *null* element is displayed in the data block for that table.
+ The inserted items are sorted per table, based on the order in which they were provided inside the request mapping template.
+ Each `Put` command inside a `BatchPutItem` is atomic, however, a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the *unprocessedKeys* block.
+ `BatchPutItem` is limited to 25 items.
+ This operation **is not** supported when used with conflict detection. Using both at the same time may result in an error.
For the following example request mapping template:
```
{
"version": "2018-05-29",
"operation": "BatchPutItem",
"tables": {
"authors": [
{
"author_id": {
"S": "a1"
},
"author_name": {
"S": "a1_name"
}
},
],
"posts": [
{
"author_id": {
"S": "a1"
},
"post_id": {
"S": "p2"
},
"post_title": {
"S": "title"
}
}
],
}
}
```
The invocation result available in `$ctx.result` is as follows:
```
{
"data": {
"authors": [
null
],
"posts": [
# Was inserted
{
"author_id": "a1",
"post_id": "p2",
"post_title": "title"
}
]
},
"unprocessedItems": {
"authors": [
# This item was not processed due to an error
{
"author_id": "a1",
"author_name": "a1_name"
}
],
"posts": []
}
}
```
The `$ctx.error` contains details about the error. The keys **data**, **unprocessedItems**, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been inserted are in the **data** block. Items that haven’t been processed are marked as *null* inside the data block and are placed inside the **unprocessedItems** block.
For a more complete example, follow the DynamoDB Batch tutorial with AppSync here [Tutorial: DynamoDB batch resolvers](tutorial-dynamodb-batch.md#aws-appsync-tutorial-dynamodb-batch).
# TransactGetItems
The `TransactGetItems` request mapping document lets you to tell the AWS AppSync DynamoDB resolver to make a `TransactGetItems` request to DynamoDB to retrieve multiple items, potentially across multiple tables. For this request template, you must specify the following:
+ The table name of each request item where to retrieve the item from
+ The key of each request item to retrieve from each table
The DynamoDB `TransactGetItems` limits apply and **no condition expression** can be provided.
The `TransactGetItems` mapping document has the following structure:
```
{
"version": "2018-05-29",
"operation": "TransactGetItems",
"transactItems": [
## First request item
{
"table": "table1",
"key": {
"foo": ... typed value,
"bar": ... typed value
},
"projection" : {
...
}
},
## Second request item
{
"table": "table2",
"key": {
"foo": ... typed value,
"bar": ... typed value
},
"projection" : {
...
}
}
]
}
```
The fields are defined as follows:
## TransactGetItems fields
### TransactGetItems fields list
** `version` **
The template definition version. Only `2018-05-29` is supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `TransactGetItems` DynamoDB operation, this must be set to `TransactGetItems`. This value is required.
** `transactItems` **
The request items to include. The value is an array of request items. At least one request item must be provided. This `transactItems` value is required.
** `table` **
The DynamoDB table to retrieve the item from. The value is a string of the table name. This `table` value is required.
** `key` **
The DynamoDB key representing the primary key of the item to retrieve. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md).
**`projection`**
A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see [Projections](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html#aws-appsync-resolver-mapping-template-reference-dynamodb-projections). This field is optional.
Things to remember:
+ If a transaction succeeds, the order of retrieved items in the `items` block will be the same as the order of request items.
+ Transactions are performed in an all-or-nothing way. If any request item causes an error, the whole transaction will not be performed and error details will be returned.
+ A request item being unable to be retrieved is not an error. Instead, a *null* element appears in the *items* block in the corresponding position.
+ If the error of a transaction is *TransactionCanceledException*, the `cancellationReasons` block will be populated. The order of cancellation reasons in `cancellationReasons` block will be the same as the order of request items.
+ `TransactGetItems` is limited to 100 request items.
For the following example request mapping template:
```
{
"version": "2018-05-29",
"operation": "TransactGetItems",
"transactItems": [
## First request item
{
"table": "posts",
"key": {
"post_id": {
"S": "p1"
}
}
},
## Second request item
{
"table": "authors",
"key": {
"author_id": {
"S": a1
}
}
}
]
}
```
If the transaction succeeds and only the first requested item is retrieved, the invocation result available in `$ctx.result` is as follows:
```
{
"items": [
{
// Attributes of the first requested item
"post_id": "p1",
"post_title": "title",
"post_description": "description"
},
// Could not retrieve the second requested item
null,
],
"cancellationReasons": null
}
```
If the transaction fails due to *TransactionCanceledException* caused by the first request item, the invocation result available in `$ctx.result` is as follows:
```
{
"items": null,
"cancellationReasons": [
{
"type":"Sample error type",
"message":"Sample error message"
},
{
"type":"None",
"message":"None"
}
]
}
```
The `$ctx.error` contains details about the error. The keys **items** and **cancellationReasons** are guaranteed to be present in `$ctx.result`.
For a more complete example, follow the DynamoDB Transaction tutorial with AppSync here [Tutorial: DynamoDB transaction resolvers](tutorial-dynamodb-transact.md#aws-appsync-tutorial-dynamodb-transact).
# TransactWriteItems
The `TransactWriteItems` request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a `TransactWriteItems` request to DynamoDB to write multiple items, potentially to multiple tables. For this request template, you must specify the following:
+ The destination table name of each request item
+ The operation of each request item to perform. There are four types of operations that are supported: *PutItem*, *UpdateItem*, *DeleteItem*, and *ConditionCheck*
+ The key of each request item to write
The DynamoDB `TransactWriteItems` limits apply.
The `TransactWriteItems` mapping document has the following structure:
```
{
"version": "2018-05-29",
"operation": "TransactWriteItems",
"transactItems": [
{
"table": "table1",
"operation": "PutItem",
"key": {
"foo": ... typed value,
"bar": ... typed value
},
"attributeValues": {
"baz": ... typed value
},
"condition": {
"expression": "someExpression",
"expressionNames": {
"#foo": "foo"
},
"expressionValues": {
":bar": ... typed value
},
"returnValuesOnConditionCheckFailure": true|false
}
},
{
"table":"table2",
"operation": "UpdateItem",
"key": {
"foo": ... typed value,
"bar": ... typed value
},
"update": {
"expression": "someExpression",
"expressionNames": {
"#foo": "foo"
},
"expressionValues": {
":bar": ... typed value
}
},
"condition": {
"expression": "someExpression",
"expressionNames": {
"#foo":"foo"
},
"expressionValues": {
":bar": ... typed value
},
"returnValuesOnConditionCheckFailure": true|false
}
},
{
"table": "table3",
"operation": "DeleteItem",
"key":{
"foo": ... typed value,
"bar": ... typed value
},
"condition":{
"expression": "someExpression",
"expressionNames": {
"#foo": "foo"
},
"expressionValues": {
":bar": ... typed value
},
"returnValuesOnConditionCheckFailure": true|false
}
},
{
"table": "table4",
"operation": "ConditionCheck",
"key":{
"foo": ... typed value,
"bar": ... typed value
},
"condition":{
"expression": "someExpression",
"expressionNames": {
"#foo": "foo"
},
"expressionValues": {
":bar": ... typed value
},
"returnValuesOnConditionCheckFailure": true|false
}
}
]
}
```
## TransactWriteItems fields
### TransactWriteItems fields list
**The fields are defined as follows:**
** `version` **
The template definition version. Only `2018-05-29` is supported. This value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `TransactWriteItems` DynamoDB operation, this must be set to `TransactWriteItems`. This value is required.
** `transactItems` **
The request items to include. The value is an array of request items. At least one request item must be provided. This `transactItems` value is required.
For `PutItem`, the fields are defined as follows:
** `table` **
The destination DynamoDB table. The value is a string of the table name. This `table` value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `PutItem` DynamoDB operation, this must be set to `PutItem`. This value is required.
** `key` **
The DynamoDB key representing the primary key of the item to put. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
** `attributeValues` **
The rest of the attributes of the item to be put into DynamoDB. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This field is optional.
** `condition` **
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `PutItem` request overwrites any existing entry for that item. You can specify whether to retrieve the existing item back when condition check fails. For more information about transactional conditions, see [Transaction condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-transaction-condition-expressions.md). This value is optional.
For `UpdateItem`, the fields are defined as follows:
** `table` **
The DynamoDB table to update. The value is a string of the table name. This `table` value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `UpdateItem` DynamoDB operation, this must be set to `UpdateItem`. This value is required.
** `key` **
The DynamoDB key representing the primary key of the item to update. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
** `update` **
The `update` section lets you specify an update expression that describes how to update the item in DynamoDB. For more information about how to write update expressions, see the [DynamoDB UpdateExpressions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html). This section is required.
** `condition` **
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `UpdateItem` request updates the existing entry regardless of its current state. You can specify whether to retrieve the existing item back when condition check fails. For more information about transactional conditions, see [Transaction condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-transaction-condition-expressions.md). This value is optional.
For `DeleteItem`, the fields are defined as follows:
** `table` **
The DynamoDB table in which to delete the item. The value is a string of the table name. This `table` value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `DeleteItem` DynamoDB operation, this must be set to `DeleteItem`. This value is required.
** `key` **
The DynamoDB key representing the primary key of the item to delete. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
** `condition` **
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the `DeleteItem` request deletes an item regardless of its current state. You can specify whether to retrieve the existing item back when condition check fails. For more information about transactional conditions, see [Transaction condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-transaction-condition-expressions.md). This value is optional.
For `ConditionCheck`, the fields are defined as follows:
** `table` **
The DynamoDB table in which to check the condition. The value is a string of the table name. This `table` value is required.
** `operation` **
The DynamoDB operation to perform. To perform the `ConditionCheck` DynamoDB operation, this must be set to `ConditionCheck`. This value is required.
** `key` **
The DynamoDB key representing the primary key of the item to condition check. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a “typed value”, see [Type system (request mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This value is required.
** `condition` **
A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. You can specify whether to retrieve the existing item back when condition check fails. For more information about transactional conditions, see [Transaction condition expressions](aws-appsync-resolver-mapping-template-reference-dynamodb-transaction-condition-expressions.md). This value is required.
Things to remember:
+ Only keys of request items are returned in the response, if successful. The order of keys will be the same as the order of request items.
+ Transactions are performed in an all-or-nothing way. If any request item causes an error, the whole transaction will not be performed and error details will be returned.
+ No two request items can target the same item. Otherwise they will cause *TransactionCanceledException* error.
+ If the error of a transaction is *TransactionCanceledException*, the `cancellationReasons` block will be populated. If a request item’s condition check fails **and** you did not specify `returnValuesOnConditionCheckFailure` to be `false`, the item existing in the table will be retrieved and stored in `item` at the corresponding position of `cancellationReasons` block.
+ `TransactWriteItems` is limited to 100 request items.
+ This operation **is not** supported when used with conflict detection. Using both at the same time may result in an error.
For the following example request mapping template:
```
{
"version": "2018-05-29",
"operation": "TransactWriteItems",
"transactItems": [
{
"table": "posts",
"operation": "PutItem",
"key": {
"post_id": {
"S": "p1"
}
},
"attributeValues": {
"post_title": {
"S": "New title"
},
"post_description": {
"S": "New description"
}
},
"condition": {
"expression": "post_title = :post_title",
"expressionValues": {
":post_title": {
"S": "Expected old title"
}
}
}
},
{
"table":"authors",
"operation": "UpdateItem",
"key": {
"author_id": {
"S": "a1"
},
},
"update": {
"expression": "SET author_name = :author_name",
"expressionValues": {
":author_name": {
"S": "New name"
}
}
},
}
]
}
```
If the transaction succeeds, the invocation result available in `$ctx.result` is as follows:
```
{
"keys": [
// Key of the PutItem request
{
"post_id": "p1",
},
// Key of the UpdateItem request
{
"author_id": "a1"
}
],
"cancellationReasons": null
}
```
If the transaction fails due to condition check failure of the `PutItem` request, the invocation result available in `$ctx.result` is as follows:
```
{
"keys": null,
"cancellationReasons": [
{
"item": {
"post_id": "p1",
"post_title": "Actual old title",
"post_description": "Old description"
},
"type": "ConditionCheckFailed",
"message": "The condition check failed."
},
{
"type": "None",
"message": "None"
}
]
}
```
The `$ctx.error` contains details about the error. The keys **keys** and **cancellationReasons** are guaranteed to be present in `$ctx.result`.
For a more complete example, follow the DynamoDB Transaction tutorial with AppSync here [Tutorial: DynamoDB transaction resolvers](tutorial-dynamodb-transact.md#aws-appsync-tutorial-dynamodb-transact).
# Type system (request mapping)
When using the AWS AppSync DynamoDB resolver to call your DynamoDB tables, AWS AppSync needs to know the type of each value to use in that call. This is because DynamoDB supports more type primitives than GraphQL or JSON (such as sets and binary data). AWS AppSync needs some hints when translating between GraphQL and DynamoDB, otherwise it would have to make some assumptions on how data is structured in your table.
For more information about DynamoDB data types, see the DynamoDB [Data type descriptors](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Programming.LowLevelAPI.html#Programming.LowLevelAPI.DataTypeDescriptors) and [Data types](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html#HowItWorks.DataTypes) documentation.
A DynamoDB value is represented by a JSON object containing a single key-value pair. The key specifies the DynamoDB type, and the value specifies the value itself. In the following example, the key `S` denotes that the value is a string, and the value `identifier` is the string value itself.
```
{ "S" : "identifier" }
```
Note that the JSON object cannot have more than one key-value pair. If more than one key-value pair is specified, the request mapping document isn’t parsed.
A DynamoDB value is used anywhere in a request mapping document where you need to specify a value. Some places where you need to do this include: `key` and `attributeValue` sections, and the `expressionValues` section of expression sections. In the following example, the DynamoDB String value `identifier` is being assigned to the `id` field in a `key` section (perhaps in a `GetItem` request mapping document).
```
"key" : {
"id" : { "S" : "identifier" }
}
```
**Supported Types**
AWS AppSync supports the following DynamoDB scalar, document, and set types:
**String type `S` **
A single string value. A DynamoDB String value is denoted by:
```
{ "S" : "some string" }
```
An example usage is:
```
"key" : {
"id" : { "S" : "some string" }
}
```
**String set type `SS` **
A set of string values. A DynamoDB String Set value is denoted by:
```
{ "SS" : [ "first value", "second value", ... ] }
```
An example usage is:
```
"attributeValues" : {
"phoneNumbers" : { "SS" : [ "+1 555 123 4567", "+1 555 234 5678" ] }
}
```
**Number type `N` **
A single numeric value. A DynamoDB Number value is denoted by:
```
{ "N" : 1234 }
```
An example usage is:
```
"expressionValues" : {
":expectedVersion" : { "N" : 1 }
}
```
**Number set type `NS` **
A set of number values. A DynamoDB Number Set value is denoted by:
```
{ "NS" : [ 1, 2.3, 4 ... ] }
```
An example usage is:
```
"attributeValues" : {
"sensorReadings" : { "NS" : [ 67.8, 12.2, 70 ] }
}
```
**Binary type `B` **
A binary value. A DynamoDB Binary value is denoted by:
```
{ "B" : "SGVsbG8sIFdvcmxkIQo=" }
```
Note that the value is actually a string, where the string is the base64-encoded representation of the binary data. AWS AppSync decodes this string back into its binary value before sending it to DynamoDB. AWS AppSync uses the base64 decoding scheme as defined by RFC 2045: any character that isn’t in the base64 alphabet is ignored.
An example usage is:
```
"attributeValues" : {
"binaryMessage" : { "B" : "SGVsbG8sIFdvcmxkIQo=" }
}
```
**Binary set type `BS` **
A set of binary values. A DynamoDB Binary Set value is denoted by:
```
{ "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ] }
```
Note that the value is actually a string, where the string is the base64-encoded representation of the binary data. AWS AppSync decodes this string back into its binary value before sending it to DynamoDB. AWS AppSync uses the base64 decoding scheme as defined by RFC 2045: any character that is not in the base64 alphabet is ignored.
An example usage is:
```
"attributeValues" : {
"binaryMessages" : { "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ] }
}
```
**Boolean type `BOOL` **
A Boolean value. A DynamoDB Boolean value is denoted by:
```
{ "BOOL" : true }
```
Note that only `true` and `false` are valid values.
An example usage is:
```
"attributeValues" : {
"orderComplete" : { "BOOL" : false }
}
```
**List type `L` **
A list of any other supported DynamoDB value. A DynamoDB List value is denoted by:
```
{ "L" : [ ... ] }
```
Note that the value is a compound value, where the list can contain zero or more of any supported DynamoDB value (including other lists). The list can also contain a mix of different types.
An example usage is:
```
{ "L" : [
{ "S" : "A string value" },
{ "N" : 1 },
{ "SS" : [ "Another string value", "Even more string values!" ] }
]
}
```
**Map type `M` **
Representing an unordered collection of key-value pairs of other supported DynamoDB values. A DynamoDB Map value is denoted by:
```
{ "M" : { ... } }
```
Note that a map can contain zero or more key-value pairs. The key must be a string, and the value can be any supported DynamoDB value (including other maps). The map can also contain a mix of different types.
An example usage is:
```
{ "M" : {
"someString" : { "S" : "A string value" },
"someNumber" : { "N" : 1 },
"stringSet" : { "SS" : [ "Another string value", "Even more string values!" ] }
}
}
```
**Null type `NULL` **
A null value. A DynamoDB Null value is denoted by:
```
{ "NULL" : null }
```
An example usage is:
```
"attributeValues" : {
"phoneNumbers" : { "NULL" : null }
}
```
For more information about each type, see the [DynamoDB documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/HowItWorks.NamingRulesDataTypes.html) .
# Type system (response mapping)
When receiving a response from DynamoDB, AWS AppSync automatically converts it into GraphQL and JSON primitive types. Each attribute in DynamoDB is decoded and returned in the response mapping context.
For example, if DynamoDB returns the following:
```
{
"id" : { "S" : "1234" },
"name" : { "S" : "Nadia" },
"age" : { "N" : 25 }
}
```
Then the AWS AppSync DynamoDB resolver converts it into GraphQL and JSON types as:
```
{
"id" : "1234",
"name" : "Nadia",
"age" : 25
}
```
This section explains how AWS AppSync converts the following DynamoDB scalar, document, and set types:
**String type `S` **
A single string value. A DynamoDB String value is returned as a string.
For example, if DynamoDB returned the following DynamoDB String value:
```
{ "S" : "some string" }
```
AWS AppSync converts it to a string:
```
"some string"
```
**String set type `SS` **
A set of string values. A DynamoDB String Set value is returned as a list of strings.
For example, if DynamoDB returned the following DynamoDB String Set value:
```
{ "SS" : [ "first value", "second value", ... ] }
```
AWS AppSync converts it to a list of strings:
```
[ "+1 555 123 4567", "+1 555 234 5678" ]
```
**Number type `N` **
A single numeric value. A DynamoDB Number value is returned as a number.
For example, if DynamoDB returned the following DynamoDB Number value:
```
{ "N" : 1234 }
```
AWS AppSync converts it to a number:
```
1234
```
**Number set type `NS` **
A set of number values. A DynamoDB Number Set value is returned as a list of numbers.
For example, if DynamoDB returned the following DynamoDB Number Set value:
```
{ "NS" : [ 67.8, 12.2, 70 ] }
```
AWS AppSync converts it to a list of numbers:
```
[ 67.8, 12.2, 70 ]
```
**Binary type `B` **
A binary value. A DynamoDB Binary value is returned as a string containing the base64 representation of that value.
For example, if DynamoDB returned the following DynamoDB Binary value:
```
{ "B" : "SGVsbG8sIFdvcmxkIQo=" }
```
AWS AppSync converts it to a string containing the base64 representation of the value:
```
"SGVsbG8sIFdvcmxkIQo="
```
Note that the binary data is encoded in the base64 encoding scheme as specified in [RFC 4648](https://tools.ietf.org/html/rfc4648) and [RFC 2045](https://tools.ietf.org/html/rfc2045).
**Binary set type `BS` **
A set of binary values. A DynamoDB Binary Set value is returned as a list of strings containing the base64 representation of the values.
For example, if DynamoDB returned the following DynamoDB Binary Set value:
```
{ "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ] }
```
AWS AppSync converts it to a list of strings containing the base64 representation of the values:
```
[ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ]
```
Note that the binary data is encoded in the base64 encoding scheme as specified in [RFC 4648](https://tools.ietf.org/html/rfc4648) and [RFC 2045](https://tools.ietf.org/html/rfc2045).
**Boolean type `BOOL` **
A Boolean value. A DynamoDB Boolean value is returned as a Boolean.
For example, if DynamoDB returned the following DynamoDB Boolean value:
```
{ "BOOL" : true }
```
AWS AppSync converts it to a Boolean:
```
true
```
**List type `L` **
A list of any other supported DynamoDB value. A DynamoDB List value is returned as a list of values, where each inner value is also converted.
For example, if DynamoDB returned the following DynamoDB List value:
```
{ "L" : [
{ "S" : "A string value" },
{ "N" : 1 },
{ "SS" : [ "Another string value", "Even more string values!" ] }
]
}
```
AWS AppSync converts it to a list of converted values:
```
[ "A string value", 1, [ "Another string value", "Even more string values!" ] ]
```
**Map type `M` **
A key/value collection of any other supported DynamoDB value. A DynamoDB Map value is returned as a JSON object, where each key/value is also converted.
For example, if DynamoDB returned the following DynamoDB Map value:
```
{ "M" : {
"someString" : { "S" : "A string value" },
"someNumber" : { "N" : 1 },
"stringSet" : { "SS" : [ "Another string value", "Even more string values!" ] }
}
}
```
AWS AppSync converts it to a JSON object:
```
{
"someString" : "A string value",
"someNumber" : 1,
"stringSet" : [ "Another string value", "Even more string values!" ]
}
```
**Null type `NULL` **
A null value.
For example, if DynamoDB returned the following DynamoDB Null value:
```
{ "NULL" : null }
```
AWS AppSync converts it to a null:
```
null
```
# Filters
When querying objects in DynamoDB using the `Query` and `Scan` operations, you can optionally specify a `filter` that evaluates the results and returns only the desired values.
The filter mapping section of a `Query` or `Scan` mapping document has the following structure:
```
"filter" : {
"expression" : "filter expression"
"expressionNames" : {
"#name" : "name",
},
"expressionValues" : {
":value" : ... typed value
},
}
```
The fields are defined as follows:
** `expression` **
The query expression. For more information about how to write filter expressions, see the [DynamoDB QueryFilter](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/LegacyConditionalParameters.QueryFilter.html) and [DynamoDB ScanFilter](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/LegacyConditionalParameters.ScanFilter.html) documentation. This field must be specified.
** `expressionNames` **
The substitutions for expression attribute *name* placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the `expression`. The value must be a string that corresponds to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the `expression`.
** `expressionValues` **
The substitutions for expression attribute *value* placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the `expression`, and the value must be a typed value. For more information about how to specify a “typed value”, see [Type System (Request Mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the `expression`.
## Example
The following example is a filter section for a mapping template, where entries retrieved from DynamoDB are only returned if the title starts with the `title` argument.
```
"filter" : {
"expression" : "begins_with(#title, :title)",
"expressionNames" : {
"#title" : "title"
},
"expressionValues" : {
":title" : $util.dynamodb.toDynamoDBJson($context.arguments.title)
}
}
```
# Condition expressions
When you mutate objects in DynamoDB by using the `PutItem`, `UpdateItem`, and `DeleteItem` DynamoDB operations, you can optionally specify a condition expression that controls whether the request should succeed or not, based on the state of the object already in DynamoDB before the operation is performed.
The AWS AppSync DynamoDB resolver allows a condition expression to be specified in `PutItem`, `UpdateItem`, and `DeleteItem` request mapping documents, and also a strategy to follow if the condition fails and the object was not updated.
## Example 1
The following `PutItem` mapping document doesn’t have a condition expression. As a result, it puts an item in DynamoDB even if an item with the same key already exists, thereby overwriting the existing item.
```
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
"id" : { "S" : "1" }
}
}
```
## Example 2
The following `PutItem` mapping document does have a condition expression that allows the operation succeed only if an item with the same key does *not* exist in DynamoDB.
```
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
"id" : { "S" : "1" }
},
"condition" : {
"expression" : "attribute_not_exists(id)"
}
}
```
By default, if the condition check fails, the AWS AppSync DynamoDB resolver returns an error for the mutation. However, the AWS AppSync DynamoDB resolver offers some additional features to help developers handle some common edge cases:
+ If AWS AppSync DynamoDB resolver can determine that the current value in DynamoDB matches the desired result, it treats the operation as if it succeeded anyway.
+ Instead of returning an error, you can configure the resolver to invoke a custom Lambda function to decide how the AWS AppSync DynamoDB resolver should handle the failure.
These are described in greater detail in the [Handling a Condition Check Failure](#aws-appsync-resolver-mapping-template-reference-dynamodb-condition-handling) section.
For more information about DynamoDB conditions expressions, see the [DynamoDB ConditionExpressions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html) .
## Specifying a condition
The `PutItem`, `UpdateItem`, and `DeleteItem` request mapping documents all allow an optional `condition` section to be specified. If omitted, no condition check is made. If specified, the condition must be true for the operation to succeed.
A `condition` section has the following structure:
```
"condition" : {
"expression" : "someExpression"
"expressionNames" : {
"#foo" : "foo"
},
"expressionValues" : {
":bar" : ... typed value
},
"equalsIgnore" : [ "version" ],
"consistentRead" : true,
"conditionalCheckFailedHandler" : {
"strategy" : "Custom",
"lambdaArn" : "arn:..."
}
}
```
The following fields specify the condition:
** `expression` **
The update expression itself. For more information about how to write condition expressions, see the [DynamoDB ConditionExpressions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html) . This field must be specified.
** `expressionNames` **
The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the *expression*, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the *expression*.
** `expressionValues` **
The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a “typed value”, see [Type System (Request Mapping)](aws-appsync-resolver-mapping-template-reference-dynamodb-typed-values-request.md). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.
The remaining fields tell the AWS AppSync DynamoDB resolver how to handle a condition check failure:
** `equalsIgnore` **
When a condition check fails when using the `PutItem` operation, the AWS AppSync DynamoDB resolver compares the item currently in DynamoDB against the item it tried to write. If they are the same, it treats the operation as it if succeeded anyway. You can use the `equalsIgnore` field to specify a list of attributes that AWS AppSync should ignore when performing that comparison. For example, if the only difference was a `version` attribute, it treats the operation as if it succeeded. This field is optional.
** `consistentRead` **
When a condition check fails, AWS AppSync gets the current value of the item from DynamoDB using a strongly consistent read. You can use this field to tell the AWS AppSync DynamoDB resolver to use an eventually consistent read instead. This field is optional, and defaults to `true`.
** `conditionalCheckFailedHandler` **
This section allows you to specify how the AWS AppSync DynamoDB resolver treats a condition check failure after it has compared the current value in DynamoDB against the expected result. This section is optional. If omitted, it defaults to a strategy of `Reject`.
** `strategy` **
The strategy the AWS AppSync DynamoDB resolver takes after it has compared the current value in DynamoDB against the expected result. This field is required and has the following possible values:
** `Reject` **
The mutation fails, and an error is added to the GraphQL response.
** `Custom` **
The AWS AppSync DynamoDB resolver invokes a custom Lambda function to decide how to handle the condition check failure. When the `strategy` is set to `Custom`, the `lambdaArn` field must contain the ARN of the Lambda function to invoke.
** `lambdaArn` **
The ARN of the Lambda function to invoke that determines how the AWS AppSync DynamoDB resolver should handle the condition check failure. This field must only be specified when `strategy` is set to `Custom`. For more information about how to use this feature, see [Handling a Condition Check Failure](#aws-appsync-resolver-mapping-template-reference-dynamodb-condition-handling).
## Handling a condition check failure
By default, when a condition check fails, the AWS AppSync DynamoDB resolver returns an error for the mutation and the current value of the object in DynamoDB. However, the AWS AppSync DynamoDB resolver offers some additional features to help developers handle some common edge cases:
+ If AWS AppSync DynamoDB resolver can determine that the current value in DynamoDB matches the desired result, it treats the operation as if it succeeded anyway.
+ Instead of returning an error, you can configure the resolver to invoke a custom Lambda function to decide how the AWS AppSync DynamoDB resolver should handle the failure.
The flowchart for this process is:
![\[Flowchart showing process for transforming requests with mutation attempts and value checks.\]](http://docs.aws.amazon.com/appsync/latest/devguide/images/DynamoDB-condition-check-failure-handling.png)
### Checking for the desired result
When the condition check fails, the AWS AppSync DynamoDB resolver performs a `GetItem` DynamoDB request to get the current value of the item from DynamoDB. By default, it uses a strongly consistent read, however this can be configured using the `consistentRead` field in the `condition` block and compare it against the expected result:
+ For the `PutItem` operation, the AWS AppSync DynamoDB resolver compares the current value against the one it attempted to write, excluding any attributes listed in `equalsIgnore` from the comparison. If the items are the same, it treats the operation as successful and returns the item that was retrieved from DynamoDB. Otherwise, it follows the configured strategy.
For example, if the `PutItem` request mapping document looked like the following:
```
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
"id" : { "S" : "1" }
},
"attributeValues" : {
"name" : { "S" : "Steve" },
"version" : { "N" : 2 }
},
"condition" : {
"expression" : "version = :expectedVersion",
"expressionValues" : {
":expectedVersion" : { "N" : 1 }
},
"equalsIgnore": [ "version" ]
}
}
```
And the item currently in DynamoDB looked like the following:
```
{
"id" : { "S" : "1" },
"name" : { "S" : "Steve" },
"version" : { "N" : 8 }
}
```
The AWS AppSync DynamoDB resolver would compare the item it tried to write against the current value, see that the only difference was the `version` field, but because it’s configured to ignore the `version` field, it treats the operation as successful and returns the item that was retrieved from DynamoDB.
+ For the `DeleteItem` operation, the AWS AppSync DynamoDB resolver checks to verify that an item was returned from DynamoDB. If no item was returned, it treats the operation as successful. Otherwise, it follows the configured strategy.
+ For the `UpdateItem` operation, the AWS AppSync DynamoDB resolver does not have enough information to determine if the item currently in DynamoDB matches the expected result, and therefore follows the configured strategy.
If the current state of the object in DynamoDB is different from the expected result, the AWS AppSync DynamoDB resolver follows the configured strategy, to either reject the mutation or invoke a Lambda function to determine what to do next.
### Following the “reject” strategy
When following the `Reject` strategy, the AWS AppSync DynamoDB resolver returns an error for the mutation.
For example, given the following mutation request:
```
mutation {
updatePerson(id: 1, name: "Steve", expectedVersion: 1) {
Name
theVersion
}
}
```
If the item returned from DynamoDB looks like the following:
```
{
"id" : { "S" : "1" },
"name" : { "S" : "Steve" },
"version" : { "N" : 8 }
}
```
And the response mapping template looks like the following:
```
{
"id" : $util.toJson($context.result.id),
"Name" : $util.toJson($context.result.name),
"theVersion" : $util.toJson($context.result.version)
}
```
The GraphQL response looks like the following:
```
{
"data": null,
"errors": [
{
"message": "The conditional request failed (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ConditionalCheckFailedException; Request ID: ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ)"
"errorType": "DynamoDB:ConditionalCheckFailedException",
...
}
]
}
```
Also, if any fields in the returned object are filled by other resolvers and the mutation had succeeded, they won’t be resolved when the object is returned in the `error` section.
### Following the “custom” strategy
When following the `Custom` strategy, the AWS AppSync DynamoDB resolver invokes a Lambda function to decide what to do next. The Lambda function chooses one of the following options:
+ `reject` the mutation. This tells the AWS AppSync DynamoDB resolver to behave as if the configured strategy was `Reject`, returning an error for the mutation and the current value of the object in DynamoDB as described in the previous section.
+ `discard` the mutation. This tells the AWS AppSync DynamoDB resolver to silently ignore the condition check failure and returns the value in DynamoDB.
+ `retry` the mutation. This tells the AWS AppSync DynamoDB resolver to retry the mutation with a new request mapping document.
**The Lambda invocation request**
The AWS AppSync DynamoDB resolver invokes the Lambda function specified in the `lambdaArn`. It uses the same `service-role-arn` configured on the data source. The payload of the invocation has the following structure:
```
{
"arguments": { ... },
"requestMapping": {... },
"currentValue": { ... },
"resolver": { ... },
"identity": { ... }
}
```
The fields are defined as follows:
** `arguments` **
The arguments from the GraphQL mutation. This is the same as the arguments available to the request mapping document in `$context.arguments`.
** `requestMapping` **
The request mapping document for this operation.
** `currentValue` **
The current value of the object in DynamoDB.
** `resolver` **
Information about the AWS AppSync resolver.
** `identity` **
Information about the caller. This is the same as the identity information available to the request mapping document in `$context.identity`.
A full example of the payload:
```
{
"arguments": {
"id": "1",
"name": "Steve",
"expectedVersion": 1
},
"requestMapping": {
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
"id" : { "S" : "1" }
},
"attributeValues" : {
"name" : { "S" : "Steve" },
"version" : { "N" : 2 }
},
"condition" : {
"expression" : "version = :expectedVersion",
"expressionValues" : {
":expectedVersion" : { "N" : 1 }
},
"equalsIgnore": [ "version" ]
}
},
"currentValue": {
"id" : { "S" : "1" },
"name" : { "S" : "Steve" },
"version" : { "N" : 8 }
},
"resolver": {
"tableName": "People",
"awsRegion": "us-west-2",
"parentType": "Mutation",
"field": "updatePerson",
"outputType": "Person"
},
"identity": {
"accountId": "123456789012",
"sourceIp": "x.x.x.x",
"user": "AIDAAAAAAAAAAAAAAAAAA",
"userArn": "arn:aws:iam::123456789012:user/appsync"
}
}
```
**The Lambda Invocation Response**
The Lambda function can inspect the invocation payload and apply any business logic to decide how the AWS AppSync DynamoDB resolver should handle the failure. There are three options for handling the condition check failure:
+ `reject` the mutation. The response payload for this option must have this structure:
```
{
"action": "reject"
}
```
This tells the AWS AppSync DynamoDB resolver to behave as if the configured strategy was `Reject`, returning an error for the mutation and the current value of the object in DynamoDB, as described in the section above.
+ `discard` the mutation. The response payload for this option must have this structure:
```
{
"action": "discard"
}
```
This tells the AWS AppSync DynamoDB resolver to silently ignore the condition check failure and returns the value in DynamoDB.
+ `retry` the mutation. The response payload for this option must have this structure:
```
{
"action": "retry",
"retryMapping": { ... }
}
```
This tells the AWS AppSync DynamoDB resolver to retry the mutation with a new request mapping document. The structure of the `retryMapping` section depends on the DynamoDB operation, and is a subset of the full request mapping document for that operation.
For `PutItem`, the `retryMapping` section has the following structure. For a description of the `attributeValues` field, see [PutItem](aws-appsync-resolver-mapping-template-reference-dynamodb-putitem.md).
```
{
"attributeValues": { ... },
"condition": {
"equalsIgnore" = [ ... ],
"consistentRead" = true
}
}
```
For `UpdateItem`, the `retryMapping` section has the following structure. For a description of the `update` section, see [UpdateItem](aws-appsync-resolver-mapping-template-reference-dynamodb-updateitem.md).
```
{
"update" : {
"expression" : "someExpression"
"expressionNames" : {
"#foo" : "foo"
},
"expressionValues" : {
":bar" : ... typed value
}
},
"condition": {
"consistentRead" = true
}
}
```
For `DeleteItem`, the `retryMapping` section has the following structure.
```
{
"condition": {
"consistentRead" = true
}
}
```
There is no way to specify a different operation or key to work on. The AWS AppSync DynamoDB resolver only allows retries of the same operation on the same object. Also, the `condition` section doesn’t allow a `conditionalCheckFailedHandler` to be specified. If the retry fails, the AWS AppSync DynamoDB resolver follows the `Reject` strategy.
Here is an example Lambda function to deal with a failed `PutItem` request. The business logic looks at who made the call. If it was made by `jeffTheAdmin`, it retries the request, updating the `version` and `expectedVersion` from the item currently in DynamoDB. Otherwise, it rejects the mutation.
```
exports.handler = (event, context, callback) => {
console.log("Event: "+ JSON.stringify(event));
// Business logic goes here.
var response;
if ( event.identity.user == "jeffTheAdmin" ) {
response = {
"action" : "retry",
"retryMapping" : {
"attributeValues" : event.requestMapping.attributeValues,
"condition" : {
"expression" : event.requestMapping.condition.expression,
"expressionValues" : event.requestMapping.condition.expressionValues
}
}
}
response.retryMapping.attributeValues.version = { "N" : event.currentValue.version.N + 1 }
response.retryMapping.condition.expressionValues[':expectedVersion'] = event.currentValue.version
} else {
response = { "action" : "reject" }
}
console.log("Response: "+ JSON.stringify(response))
callback(null, response)
};
```
# Transaction condition expressions
Transaction condition expressions are available in request mapping templates of all four types of operations in `TransactWriteItems`, namely, `PutItem`, `DeleteItem`, `UpdateItem`, and `ConditionCheck`.
For `PutItem`, `DeleteItem`, and `UpdateItem`, the transaction condition expression is optional. For `ConditionCheck`, the transaction condition expression is required.
## Example 1
The following transactional `DeleteItem` mapping document does not have a condition expression. As a result, it deletes the item in DynamoDB.
```
{
"version": "2018-05-29",
"operation": "TransactWriteItems",
"transactItems": [
{
"table": "posts",
"operation": "DeleteItem",
"key": {
"id": { "S" : "1" }
}
}
]
}
```
## Example 2
The following transactional `DeleteItem` mapping document does have a transaction condition expression that allows the operation succeed only if the author of that post equals a certain name.
```
{
"version": "2018-05-29",
"operation": "TransactWriteItems",
"transactItems": [
{
"table": "posts",
"operation": "DeleteItem",
"key": {
"id": { "S" : "1" }
}
"condition": {
"expression": "author = :author",
"expressionValues": {
":author": { "S" : "Chunyan" }
}
}
}
]
}
```
If the condition check fails, it will cause `TransactionCanceledException` and the error detail will be returned in `$ctx.result.cancellationReasons`. Note that by default, the old item in DynamoDB that made condition check fail will be returned in `$ctx.result.cancellationReasons`.
## Specifying a condition
The `PutItem`, `UpdateItem`, and `DeleteItem` request mapping documents all allow an optional `condition` section to be specified. If omitted, no condition check is made. If specified, the condition must be true for the operation to succeed. The `ConditionCheck` must have a `condition` section to be specified. The condition must be true for the whole transaction to succeed.
A `condition` section has the following structure:
```
"condition": {
"expression": "someExpression",
"expressionNames": {
"#foo": "foo"
},
"expressionValues": {
":bar": ... typed value
},
"returnValuesOnConditionCheckFailure": false
}
```
The following fields specify the condition:
** `expression` **
The update expression itself. For more information about how to write condition expressions, see the [DynamoDB ConditionExpressions documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html) . This field must be specified.
** `expressionNames` **
The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the *expression*, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the *expression*.
** `expressionValues` **
The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a “typed value”, see Type System (request mapping). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.
** `returnValuesOnConditionCheckFailure` **
Specify whether to retrieve the item in DynamoDB back when a condition check fails. The retrieved item will be in `$ctx.result.cancellationReasons[$index].item`, where `$index` is the index of the request item that failed the condition check. This value defaults to true.
# Projections
When reading objects in DynamoDB using the `GetItem`, `Scan`, `Query`, `BatchGetItem`, and `TransactGetItems` operations, you can optionally specify a projection that identifies the attributes that you want. The projection has the following structure, which is similar to filters:
```
"projection" : {
"expression" : "projection expression"
"expressionNames" : {
"#name" : "name",
}
}
```
The fields are defined as follows:
**`expression`**
The projection expression, which is a string. To retrieve a single attribute, specify its name. For multiple attributes, the names must be comma-separated values. For more information on writing projection expressions, see the [DynamoDB projection expressions](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ProjectionExpressions.html) documentation. This field is required.
**`expressionNames`**
The substitutions for expression attribute *name* placeholders in the form of key-value pairs. The key corresponds to a name placeholder used in the `expression`. The value must be a string that corresponds to the attribute name of the item in DynamoDB. This field is optional and should only be populated with substitutions for expression attribute name placeholders used in the `expression`. For more information about `expressionNames`, see the [DynamoDB documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ExpressionAttributeNames.html).
## Example 1
The following example is a projection section for a VTL mapping template in which only the attributes `author` and `id` are returned from DynamoDB:
```
"projection" : {
"expression" : "#author, id",
"expressionNames" : {
"#author" : "author"
}
}
```
**Tip**
You can access your GraphQL request selection set using [\$1context.info.selectionSetList](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-context-reference.html#aws-appsync-resolver-context-reference-info). This field allows you to frame your projection expression dynamically according to your requirements.
**Note**
While using projection expressions with the `Query` and `Scan` operations, the value for `select` must be `SPECIFIC_ATTRIBUTES`. For more information, see the [DynamoDB documentation](https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html#DDB-Query-request-Select).