Scan

The Scan request mapping document lets you tell the AWS AppSync DynamoDB resolver to make a Scan request to DynamoDB, and enables you to specify the following:

• A filter to exclude results

• Which index to use

• How many items to return

• Whether to use consistent reads

• Pagination token

• Parallel scans

The Scan mapping document has the following structure:

{
    "version" : "2017-02-28",
    "operation" : "Scan",
    "index" : "fooIndex",
    "limit" : 10,
    "consistentRead" : false,
    "nextToken" : "aPaginationToken",
    "totalSegments" : 10,
    "segment" : 1,
    "filter" : {
        ...
    },
    "projection" : {
        ...
    }
}

The fields are defined as follows:

Scan fields

Scan fields list

version

The template definition version. 2017-02-28 and 2018-05-29 are currently supported. This value is required.

operation

The DynamoDB operation to perform. To perform the Scan DynamoDB operation, this must be set to Scan. 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.

index

The name of the index to query. The DynamoDB query operation allows you to scan on Local Secondary Indexes and Global Secondary Indexes in addition to the primary key index for a hash key. If specified, this tells DynamoDB to query the specified index. If omitted, the primary key index is queried.

limit

The maximum number of items to evaluate at a single time. This field is optional.

consistentRead

A Boolean that indicates whether to use consistent reads when querying DynamoDB. This field is optional, and defaults to false.

nextToken

The pagination token to continue a previous query. This would have been obtained from a previous query. This field is optional.

select

By default, the AWS AppSync DynamoDB resolver only returns whatever attributes are projected into the index. If more attributes are required, then this field can be set. This field is optional. The supported values are:

ALL_ATTRIBUTES

Returns all of the item attributes from the specified table or index. If you query a local secondary index, DynamoDB fetches the entire item from the parent table for each matching item in the index. If the index is configured to project all item attributes, all of the data can be obtained from the local secondary index and no fetching is required.

ALL_PROJECTED_ATTRIBUTES

Allowed only when querying an index. Retrieves all attributes that have been projected into the index. If the index is configured to project all attributes, this return value is equivalent to specifying ALL_ATTRIBUTES.

SPECIFIC_ATTRIBUTES

Returns only the attributes listed in the projection's expression. This return value is equivalent to specifying the projection's expression without specifying any value for Select.

totalSegments

The number of segments to partition the table by when performing a parallel scan. This field is optional, but must be specified if segment is specified.

segment

The table segment in this operation when performing a parallel scan. This field is optional, but must be specified if totalSegments is specified.

projection

A projection that's used to specify the attributes to return from the DynamoDB operation. For more information about projections, see Projections. This field is optional.

The results returned by the DynamoDB scan are automatically converted into GraphQL and JSON primitive types and is available in the mapping context ($context.result).

For more information about DynamoDB type conversion, see Type system (response mapping).

For more information about response mapping templates, see Resolver mapping template overview.

The results have the following structure:

{
    items = [ ... ],
    nextToken = "a pagination token",
    scannedCount = 10
}

The fields are defined as follows:

items

A list containing the items returned by the DynamoDB scan.

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 resolvers.

scannedCount

The number of items that were retrieved by DynamoDB before a filter expression (if present) was applied.

Example 1

The following example is a mapping template for the GraphQL query: allPosts.

In this example, all entries in the table are returned.

{
    "version" : "2017-02-28",
    "operation" : "Scan"
}

Example 2

The following example is a mapping template for the GraphQL query: postsMatching(title: String!).

In this example, all entries in the table are returned where the title starts with the title argument.

{
    "version" : "2017-02-28",
    "operation" : "Scan",
    "filter" : {
        "expression" : "begins_with(title, :title)",
        "expressionValues" : {
            ":title" : $util.dynamodb.toDynamoDBJson($context.arguments.title)
        },
    }
}

For more information about the DynamoDB Scan API, see the DynamoDB API documentation.