Versioned data sources - Amazon AppSync
Versioned data source configurationDelta sync table loggingVersioned data source metadata
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Versioned data sources

Amazon AppSync currently supports versioning on DynamoDB data sources. Conflict Detection, Conflict Resolution, and Sync operations require a Versioned data source. When you enable versioning on a data source, Amazon AppSync will automatically:

  • Enhance items with object versioning metadata.

  • Record changes made to items with Amazon AppSync mutations to a Delta table.

  • Maintain deleted items in the Base table with a “tombstone” for a configurable amount of time.

Versioned data source configuration

When you enable versioning on a DynamoDB data source, you specify the following fields:

BaseTableTTL

The number of minutes to retain deleted items in the Base table with a “tombstone” - a metadata field indicating that the item has been deleted. You can set this value to 0 if you want items to be removed immediately when they are deleted. This field is required.

DeltaSyncTableName

The name of the table where changes made to items with Amazon AppSync mutations are stored. This field is required.

DeltaSyncTableTTL

The number of minutes to retain items in the Delta table. This field is required.

Delta sync table logging

Amazon AppSync currently supports Delta Sync Logging for mutations using PutItem, UpdateItem, and DeleteItem DynamoDB operations.

When an Amazon AppSync mutation changes an item in a versioned data source, a record of that change will be stored in a Delta table that is optimized for incremental updates. You can choose to use different Delta tables (e.g. one per type, one per domain area) for other versioned data sources or a single Delta table for your API. Amazon AppSync recommends against using a single Delta table for multiple APIs to avoid the collision of primary keys.

The schema required for this table is as follows:

ds_pk

A string value that is used as the partition key. It is constructed by concatenating the Base data source name and the ISO 8601 format of the date the change occurred (e.g. Comments:2019-01-01).

When the customPartitionKey flag from the VTL mapping template is set as the column name of the partition key (see Resolver Mapping Template Reference for DynamoDB in the Amazon AppSync Developer Guide), the format of ds_pk changes, and the string is constructed by appending it the value of the partition key in the new record in the Base table. For example, if the record in the Base table has a partition key value of 1a and a sort key value of 2b, the new value of the string will be: Comments:2019-01-01:1a.

ds_sk

A string value that is used as the sort key. It is constructed by concatenating the ISO 8601 format of the time the change occurred, the primary key of the item, and the version of the item. The combination of these fields guarantees uniqueness for every entry in the Delta table (e.g. for a time of 09:30:00 and an ID of 1a and version of 2, this would be 09:30:00:1a:2).

When the customPartitionKey flag from the VTL mapping template is set to the column name of the partition key (see Resolver Mapping Template Reference for DynamoDB in the Amazon AppSync Developer Guide), the format of ds_sk changes, and the string is constructed by replacing the value of the combination key with the value of the sort key in the Base table. Using the previous example above, if the record in the Base table has a partition key value of 1a and a sort key value of 2b, the new value of the string will be: 09:30:00:2b:3.

_ttl

A numeric value that stores the timestamp, in epoch seconds, when an item should be removed from the Delta table. This value is determined by adding the DeltaSyncTableTTL value configured on the data source to the moment when the change occurred. This field should be configured as the DynamoDB TTL Attribute.

The IAM role configured for use with the Base table must also contain permission to operate on the Delta table. In this example, the permissions policy for a Base table called Comments and a Delta table called ChangeLog is displayed:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "dynamodb:DeleteItem",
                "dynamodb:GetItem",
                "dynamodb:PutItem",
                "dynamodb:Query",
                "dynamodb:Scan",
                "dynamodb:UpdateItem"
            ],
            "Resource": [
                "arn:aws:dynamodb:us-east-1:000000000000:table/Comments",
                "arn:aws:dynamodb:us-east-1:000000000000:table/Comments/*",
                "arn:aws:dynamodb:us-east-1:000000000000:table/ChangeLog",
                "arn:aws:dynamodb:us-east-1:000000000000:table/ChangeLog/*"
            ]
        }
    ]
}

Versioned data source metadata

Amazon AppSync manages metadata fields on Versioned data sources on your behalf. Modifying these fields yourself may cause errors in your application or data loss. These fields include:

_version

A monotonically increasing counter that is updated any time that a change occurs to an item.

_lastChangedAt

A numeric value that stores the timestamp, in epoch milliseconds, when an item was last modified.

_deleted

A Boolean “tombstone” value that indicates that an item has been deleted. This can be used by applications to evict deleted items from local data stores.

_ttl

A numeric value that stores the timestamp, in epoch seconds, when an item should be removed from the underlying data source.

ds_pk

A string value that is used as the partition key for Delta tables.

ds_sk

A string value that is used as the sort key for Delta tables.

gsi_ds_pk

A string value attribute that's generated to support a global secondary index as a partition key. It will be included only if both the customPartitionKey and populateIndexFields flags are enabled in the VTL mapping template (see Resolver Mapping Template Reference for DynamoDB in the Amazon AppSync Developer Guide). If enabled, the value will be constructed by concatenating the Base data source name and the ISO 8601 format of the date at which the change occurred (e.g. if the Base table is named Comments, this record will be set as Comments:2019-01-01).

gsi_ds_sk

A string value attribute that's generated to support a global secondary index as a sort key. It will be included only if both the customPartitionKey and populateIndexFields flags are enabled in the VTL mapping template (see Resolver Mapping Template Reference for DynamoDB in the Amazon AppSync Developer Guide). If enabled, the value will be constructed by concatenating the ISO 8601 format of the time at which the change occurred, the partition key of the item in the Base table, the sort key of the item in the Base table, and the version of the item (e.g. for a time of 09:30:00, a partition key value of 1a, a sort key value of 2b, and version of 3, this would be 09:30:00:1a#2b:3).

These metadata fields will impact the overall size of items in the underlying data source. Amazon AppSync recommends reserving 500 bytes + Max Primary Key Size of storage for versioned data source metadata when designing your application. To use this metadata in client applications, include the _version, _lastChangedAt, and _deleted fields on your GraphQL types and in the selection set for mutations.