# Sync
<a name="js-aws-appsync-resolver-reference-dynamodb-sync"></a>

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 the following:
+ A filter to exclude results
+ How many items to return
+ Pagination Token
+ When your last `Sync` operation was started

The `Sync` request object has the following structure:

```
type DynamoDBSyncRequest = {
  operation: 'Sync';
  basePartitionKey?: string;
  deltaIndexName?: string;
  limit?: number;
  nextToken?: string;
  lastSync?: number;
  filter?: {
    expression: string;
    expressionNames?: { [key: string]: string };
    expressionValues?: { [key: string]: any };
  };
};
```

The fields are defined as follows:

## Sync fields
<a name="js-sync-list"></a>

### Sync fields list
<a name="js-sync-list-col"></a>

** `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](https://docs.aws.amazon.com/appsync/latest/devguide/js-resolver-reference-dynamodb.html#js-aws-appsync-resolver-reference-dynamodb-filter). 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 context result (`context.result`).

For more information about DynamoDB type conversion, see [Type system (response mapping)](https://docs.aws.amazon.com/appsync/latest/devguide/js-resolver-reference-dynamodb.html#js-aws-appsync-resolver-reference-dynamodb-typed-values-responses).

For more information about JavaScript resolvers, see [JavaScript resolvers overview](https://docs.aws.amazon.com/appsync/latest/devguide/resolver-reference-overview-js.html).

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 functions or 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
<a name="js-id14"></a>

The following example is a function request handler 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.

```
export function request(ctx) {
  const { nextToken, lastSync } = ctx.args;
  return { operation: 'Sync', limit: 100, nextToken, lastSync };
}
```