# Using DynamoDB sync operations on versioned data sources in AWS AppSync
<a name="aws-appsync-conflict-detection-and-sync-sync-operations"></a>

Versioned data sources support `Sync` operations that allow you to retrieve all the results from a DynamoDB table and then receive only the data altered since your last query (the delta updates). When AWS AppSync receives a request for a `Sync` operation, it uses the fields specified in the request to determine if the *Base* table or the *Delta* table should be accessed.
+ If the `lastSync` field is not specified, a `Scan` on the *Base* table is performed.
+ If the `lastSync` field is specified, but the value is before the `current moment - DeltaSyncTTL`, a `Scan` on the *Base* table is performed.
+ If the `lastSync` field is specified, and the value is on or after the `current moment - DeltaSyncTTL`, a `Query` on the *Delta* table is performed.

AWS AppSync returns the `startedAt` field to the response mapping template for all `Sync` operations. The `startedAt` field is the moment, in epoch milliseconds, when the `Sync` operation started that you can store locally and use in another request. 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.

For information about the format for `Sync` mapping templates, see [the mapping template reference](aws-appsync-resolver-mapping-template-reference-dynamodb-sync.md).