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 function allows a condition expression to be specified in PutItem, UpdateItem, and DeleteItem request objects, and also a strategy to follow if the condition fails and the object was not updated.

Example 1

The following PutItem request object 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.

import { util } from '@aws-appsync/utils';
export function request(ctx) {
  const { foo, bar, ...values} = ctx.args
  return {
    operation: 'PutItem',
    key: util.dynamodb.toMapValues({foo, bar}),
    attributeValues: util.dynamodb.toMapValues(values),
  };
}

Example 2

The following PutItem object does have a condition expression that allows the operation succeed only if an item with the same key does not exist in DynamoDB.

import { util } from '@aws-appsync/utils';
export function request(ctx) {
  const { foo, bar, ...values} = ctx.args
  return {
    operation: 'PutItem',
    key: util.dynamodb.toMapValues({foo, bar}),
    attributeValues: util.dynamodb.toMapValues(values),
    condition: { expression: "attribute_not_exists(id)" }
  };
}

By default, if the condition check fails, the AWS AppSync DynamoDB function provides an error for the mutation.

However, the AWS AppSync DynamoDB function offers some additional features to help developers handle some common edge cases:

These are described in greater detail in the Handling a condition check failure section.

For more information about DynamoDB conditions expressions, see the DynamoDB ConditionExpressions documentation .

Specifying a condition

The PutItem, UpdateItem, and DeleteItem request objects 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:

type ConditionCheckExpression = {
  expression: string;
  expressionNames?: { [key: string]: string};
  expressionValues?: { [key: string]: any};
  equalsIgnore?: string[];
  consistentRead?: boolean;
  conditionalCheckFailedHandler?: {
    strategy: 'Custom' | 'Reject';
    lambdaArn?: string;
  };
};

The following fields specify the condition:

The remaining fields tell the AWS AppSync DynamoDB function how to handle a condition check failure:

Handling a condition check failure

When a condition check fails, the AWS AppSync DynamoDB function can pass on the error for the mutation and the current value of the object by using the util.appendError utility. However, the AWS AppSync DynamoDB function offers some additional features to help developers handle some common edge cases:

The flowchart for this process is:

Checking for the desired result

When the condition check fails, the AWS AppSync DynamoDB function 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:

If the current state of the object in DynamoDB is different from the expected result, the AWS AppSync DynamoDB function 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 function 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 function response handler looks like the following:

import { util } from '@aws-appsync/utils';
export function response(ctx) {
  const { version, ...values } = ctx.result;
  const result = { ...values, theVersion: version };
  if (ctx.error) {
    if (error) {
      return util.appendError(error.message, error.type, result, null);
    }
  }
  return result
}

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 function invokes a Lambda function to decide what to do next. The Lambda function chooses one of the following options:

The Lambda invocation request

The AWS AppSync DynamoDB function 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:

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 function should handle the failure. There are three options for handling the condition check failure:

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)
};