Sync - AWS AppSync GraphQL
Sync fieldsExample

Sync

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

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. 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).

For more information about JavaScript resolvers, see JavaScript resolvers 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 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

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 };
}