Resolver Mapping Template Reference for DynamoDB

The AWS AppSyncDynamoDB resolver enables you to use GraphQL to store and retrieve data in existing Amazon DynamoDB tables in your account. This resolver works by enabling you to map an incoming GraphQL request into a DynamoDB call, and then map the DynamoDB response back to GraphQL. This section describes the mapping templates for supported DynamoDB operations.

Topics

GetItem
PutItem
UpdateItem
DeleteItem
Query
Scan
BatchGetItem
BatchDeleteItem
BatchPutItem
Type System (Request Mapping)
Type System (Response Mapping)
Filters
Condition Expressions

GetItem

The GetItem request mapping document lets you tell the AWS AppSyncDynamoDB resolver to make a GetItem request to DynamoDB, and enables you to specify:

The key of the item in DynamoDB
Whether to use a consistent read or not

The GetItem mapping document has the following structure:


{
    "version" : "2017-02-28",
    "operation" : "GetItem",
    "key" : {
        "foo" : ... typed value,
        "bar" : ... typed value
    },
    "consistentRead" : true
}

The fields are defined as follows:

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 GetItemDynamoDB operation, this must be set to GetItem. This value is required.
key: The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a "typed value", see Type System (Request Mapping). This value is required.
consistentRead: Whether or not to perform a strongly consistent read with DynamoDB. This is optional, and defaults to false.

The item returned from DynamoDB is 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.

Example

Following is a mapping template for a GraphQL query getThing(foo: String!, bar: String!):


{
    "version" : "2017-02-28",
    "operation" : "GetItem",
    "key" : {
        "foo" : { "S" : "${context.arguments.foo}" },
        "bar" : { "S" : "${context.arguments.bar}" }
    },
    "consistentRead" : true
}

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

PutItem

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

The key of the item in DynamoDB
The full contents of the item (composed of key and attributeValues)
Conditions for the operation to succeed

The PutItem mapping document has the following structure:


{
    "version" : "2017-02-28",
    "operation" : "PutItem",
    "key": {
        "foo" : ... typed value,
        "bar" : ... typed value
    },
    "attributeValues" : {
        "baz" : ... typed value
    },
    "condition" : {
       ...
    }
}

The fields are defined as follows:

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 PutItemDynamoDB operation, this must be set to PutItem. This value is required.
key: The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a "typed value", see Type System (Request Mapping). This value is required.
attributeValues: The rest of the attributes of the item to be put into DynamoDB. For more information about how to specify a "typed value", see Type System (Request Mapping). This field is optional.
condition: A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the PutItem request overwrites any existing entry for that item. For more information about conditions, see Condition Expressions. This value is optional.

The item written to DynamoDB is 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.

Example 1

Following is a mapping template for a GraphQL mutation updateThing(foo: String!, bar: String!, name: String!, version: Int!).

If no item with the specified key exists, it's created. If an item already exists with the specified key, it's overwritten.


{
    "version" : "2017-02-28",
    "operation" : "PutItem",
    "key": {
        "foo" : { "S" : "${context.arguments.foo}" },
        "bar" : { "S" : "${context.arguments.bar}" }
    },
    "attributeValues" : {
        "name"    : { "S" : "${context.arguments.name}" },
        "version" : { "N" : ${context.arguments.version} }
    }
}

Example 2

Following is a mapping template for a GraphQL mutation updateThing(foo: String!, bar: String!, name: String!, expectedVersion: Int!).

This example checks to be sure the item currently in DynamoDB has the version field set to expectedVersion.


{
    "version" : "2017-02-28",
    "operation" : "PutItem",
    "key": {
        "foo" : { "S" : "${context.arguments.foo}" },
        "bar" : { "S" : "${context.arguments.bar}" }
    },
    "attributeValues" : {
        "name"    : { "S" : "${context.arguments.name}" },
        #set( $newVersion = $context.arguments.expectedVersion + 1 )
        "version" : { "N" : ${newVersion} }
    },
    "condition" : {
        "expression" : "version = :expectedVersion",
        "expressionValues" : {
            ":expectedVersion" : { "N" : ${context.arguments.expectedVersion} }
        }
    }
}

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

UpdateItem

The UpdateItem request mapping document enables you to tell the AWS AppSyncDynamoDB resolver to make an UpdateItem request to DynamoDB, and allows you to specify the following:

The key of the item in DynamoDB
An update expression describing how to update the item in DynamoDB
Conditions for the operation to succeed

The UpdateItem mapping document has the following structure:


{
    "version" : "2017-02-28",
    "operation" : "UpdateItem",
    "key": {
        "foo" : ... typed value,
        "bar" : ... typed value
    },
    "update" : {
        "expression" : "someExpression"
        "expressionNames" : {
           "#foo" : "foo"
       },
       "expressionValues" : {
           ":bar" : ... typed value
       }
    },
    "condition" : {
        ...
    }
}

The fields are defined as follows:

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 UpdateItemDynamoDB operation, this must be set to UpdateItem. This value is required.

key

The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about specifying a "typed value", see Type System (Request Mapping). This value is required.

update

The update section lets you specify an update expression that describes how to update the item in DynamoDB. For more information about how to write update expressions, see the DynamoDB UpdateExpressions documentation . This section is required.

The update section has three components:

expression: The update expression. This value is required.
expressionNames: The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the expression, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the expression.
expressionValues: The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a "typed value", see Type System (Request Mapping). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.

condition

A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the UpdateItem request updates the existing entry regardless of its current state. For more information about conditions, see Condition Expressions. This value is optional.

The item updated in DynamoDB is 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.

Example 1

Following is a mapping template for the GraphQL mutation upvote(id: ID!).

In this example, an item in DynamoDB has its upvotes and version fields incremented by 1.


{
    "version" : "2017-02-28",
    "operation" : "UpdateItem",
    "key" : {
        "id" : { "S" : "${context.arguments.id}" }
    },
    "update" : {
        "expression" : "ADD #votefield :plusOne, version :plusOne",
        "expressionNames" : {
            "#votefield" : "upvotes"
        },
        "expressionValues" : {
            ":plusOne" : { "N" : 1 }
        }
    }
}

Example 2

Following is a mapping template for a GraphQL mutation updateItem(id: ID!, title: String, author: String, expectedVersion: Int!).

This is a complex example that inspects the arguments and dynamically generates the update expression that only includes the arguments that have been provided by the client. For example, if title and author are omitted, they are not updated. If an argument is specified but its value is null, then that field is deleted from the object in DynamoDB. Finally, the operation has a condition, which verifies whether the item currently in DynamoDB has the version field set to expectedVersion:


{
    "version" : "2017-02-28",

    "operation" : "UpdateItem",

    "key" : {
        "id" : { "S" : "${context.arguments.id}" }
    },

    ## Set up some space to keep track of things we're updating **
    #set( $expNames  = {} )
    #set( $expValues = {} )
    #set( $expSet = {} )
    #set( $expAdd = {} )
    #set( $expRemove = [] )

    ## Increment "version" by 1 **
    $!{expAdd.put("version", ":newVersion")}
    $!{expValues.put(":newVersion", { "N" : 1 })}

    ## Iterate through each argument, skipping "id" and "expectedVersion" **
    #foreach( $entry in $context.arguments.entrySet() )
        #if( $entry.key != "id" && $entry.key != "expectedVersion" )
            #if( (!$entry.value) && ("$!{entry.value}" == "") )
                ## If the argument is set to "null", then remove that attribute from the item in DynamoDB **

                #set( $discard = ${expRemove.add("#${entry.key}")} )
                $!{expNames.put("#${entry.key}", "$entry.key")}
            #else
                ## Otherwise set (or update) the attribute on the item in DynamoDB **

                $!{expSet.put("#${entry.key}", ":${entry.key}")}
                $!{expNames.put("#${entry.key}", "$entry.key")}

                #if( $entry.key == "ups" || $entry.key == "downs" )
                    $!{expValues.put(":${entry.key}", { "N" : $entry.value })}
                #else
                    $!{expValues.put(":${entry.key}", { "S" : "${entry.value}" })}
                #end
            #end
        #end
    #end

    ## Start building the update expression, starting with attributes we're going to SET **
    #set( $expression = "" )
    #if( !${expSet.isEmpty()} )
        #set( $expression = "SET" )
        #foreach( $entry in $expSet.entrySet() )
            #set( $expression = "${expression} ${entry.key} = ${entry.value}" )
            #if ( $foreach.hasNext )
                #set( $expression = "${expression}," )
            #end
        #end
    #end

    ## Continue building the update expression, adding attributes we're going to ADD **
    #if( !${expAdd.isEmpty()} )
        #set( $expression = "${expression} ADD" )
        #foreach( $entry in $expAdd.entrySet() )
            #set( $expression = "${expression} ${entry.key} ${entry.value}" )
            #if ( $foreach.hasNext )
                #set( $expression = "${expression}," )
            #end
        #end
    #end

    ## Continue building the update expression, adding attributes we're going to REMOVE **
    #if( !${expRemove.isEmpty()} )
        #set( $expression = "${expression} REMOVE" )

        #foreach( $entry in $expRemove )
            #set( $expression = "${expression} ${entry}" )
            #if ( $foreach.hasNext )
                #set( $expression = "${expression}," )
            #end
        #end
    #end

    ## Finally, write the update expression into the document, along with any expressionNames and expressionValues **
    "update" : {
        "expression" : "${expression}"
        #if( !${expNames.isEmpty()} )
            ,"expressionNames" : $utils.toJson($expNames)
        #end
        #if( !${expValues.isEmpty()} )
            ,"expressionValues" : $utils.toJson($expValues)
        #end
    },

    "condition" : {
        "expression"       : "version = :expectedVersion",
        "expressionValues" : {
            ":expectedVersion" : { "N" : ${context.arguments.expectedVersion} }
        }
    }
}

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

DeleteItem

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

The key of the item in DynamoDB
Conditions for the operation to succeed

The DeleteItem mapping document has the following structure:


{
    "version" : "2017-02-28",
    "operation" : "DeleteItem",
    "key": {
        "foo" : ... typed value,
        "bar" : ... typed value
    },
    "condition" : {
        ...
    }
}

The fields are defined as follows:

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 DeleteItemDynamoDB operation, this must be set to DeleteItem. This value is required.
key: The key of the item in DynamoDB. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about specifying a "typed value", see Type System (Request Mapping). This value is required.
condition: A condition to determine if the request should succeed or not, based on the state of the object already in DynamoDB. If no condition is specified, the DeleteItem request deletes an item regardless of its current state. For more information about conditions, see Condition Expressions. This value is optional.

The item deleted from DynamoDB is 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.

Example 1

Following is a mapping template for a GraphQL mutation deleteItem(id: ID!). If an item exists with this ID, it's deleted.


{
    "version" : "2017-02-28",
    "operation" : "DeleteItem",
    "key" : {
        "id" : { "S" : "${context.arguments.id}" }
    }
}

Example 2

Following is a mapping template for a GraphQL mutation deleteItem(id: ID!, expectedVersion: Int!). If an item exists with this ID, it's deleted, but only if its version field set to expectedVersion:


{
    "version" : "2017-02-28",
    "operation" : "DeleteItem",
    "key" : {
        "id" : { "S" : "${context.arguments.id}" }
    },
    "condition" : {
        "expression"       : "attribute_not_exists(id) OR version = :expectedVersion",
        "expressionValues" : {
            ":expectedVersion" : { "N" : ${context.arguments.expectedVersion} }
        }
    }
}

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

Query

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

Key expression
Which index to use
Any additional filter
How many items to return
Whether to use consistent reads
query direction (forward or backward)
Pagination token

The Query mapping document has the following structure:


{
    "version" : "2017-02-28",
    "operation" : "Query",
    "query" : {
        "expression" : "some expression",
        "expressionNames" : {
            "#foo" : "foo"
        },
        "expressionValues" : {
            ":bar" : ... typed value
        }
    }
    "index" : "fooIndex",
    "nextToken" : "a pagination token",
    "limit" : 10,
    "scanIndexForward" : true,
    "consistentRead" : false,
    "select" : "ALL_ATTRIBUTES",
    "filter" : {
        ...
    }
}

The fields are defined as follows:

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 QueryDynamoDB operation, this must be set to Query. This value is required.

query

The query section lets you specify a key condition expression that describes which items to retrieve from DynamoDB. For more information about how to write key condition expressions, see the DynamoDB KeyConditions documentation . This section must be specified.

expression: The query expression. This field must be specified.
expressionNames: The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the expression, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the expression.
expressionValues: The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a "typed value", see Type System (Request Mapping). This value is required. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.

filter

An additional 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.

nextToken

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

limit

The maximum number of items to evaluate (not necessarily the number of matching items). This field is optional.

scanIndexForward

A boolean indicating whether to query forwards or backwards. This field is optional, and defaults to true.

consistentRead

A boolean indicating whether to use consistent reads when querying DynamoDB. This field is optional, and defaults to false.

select

By default, the AWS AppSyncDynamoDB resolver only returns attributes that are projected into the index. If more attributes are required, you can set this field. 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.

The results from DynamoDB are automatically converted into GraphQL and JSON primitive types and are 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 query.
nextToken: If there might be more results, nextToken contains a pagination token that you can use in another request. Note that AWS AppSync encrypts and obfuscates the pagination token returned from DynamoDB. This prevents your table data from being inadvertently leaked to the caller. Also note that these pagination tokens cannot be used across different resolvers.
scannedCount: The number of items that matched the query condition expression, before a filter expression (if present) was applied.

Example

Following is a mapping template for a GraphQL query getPosts(owner: ID!).

In this example, a global secondary index on a table is queried to return all posts owned by the specified ID.


{
    "version" : "2017-02-28",
    "operation" : "Query",
    "query" : {
        "expression" : "ownerId = :ownerId",
        "expressionValues" : {
            ":ownerId" : { "S" : "${context.arguments.owner}" }
        }
    }
    "index" : "owner-index"
}

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

Scan

The Scan request mapping document lets you tell the AWS AppSyncDynamoDB 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" : {
        ...
    }
}

The fields are defined as follows:

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 ScanDynamoDB operation, this must be set to Scan. This value is required.

filter

An 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

limit

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

consistentRead

A boolean indicating 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 AppSyncDynamoDB 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.

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.

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

Following 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

Following 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" : { "S" : "${context.arguments.title}" }
        },
    }
}

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

BatchGetItem

The BatchGetItem request mapping document lets you tell the AWS AppSyncDynamoDB resolver to make a BatchGetItem request to DynamoDB to retrieve multiple items across potentially multiple tables. For this request template, you must specify the following:

The table names where to retrieve the items from
The keys of the items to retrieve from each table

The DynamoDBBatchGetItem limits apply and no condition expression can be provided.

The BatchGetItem mapping document has the following structure:


{
    "version" : "2018-05-29",
    "operation" : "BatchGetItem",
    "tables" : {
        "table1": {
           "keys": [
              ## Item to retrieve Key
              {
                   "foo" : ... typed value,
                   "bar" : ... typed value
              },
              ## Item2 to retrieve Key
              {
                   "foo" : ... typed value,
                   "bar" : ... typed value
              }
            ],
            "consistentRead": true|false
        },
        "table2": {
           "keys": [
              ## Item3 to retrieve Key
              {
                   "foo" : ... typed value,
                   "bar" : ... typed value
              },
              ## Item4 to retrieve Key
              {
                   "foo" : ... typed value,
                   "bar" : ... typed value
              }
            ],
            "consistentRead": true|false
        }
    }
}

The fields are defined as follows:

version

The template definition version. Only 2018-05-29 is supported. This value is required.

operation

The DynamoDB operation to perform. To perform the BatchGetItemDynamoDB operation, this must be set to BatchGetItem. This value is required.

tables

The DynamoDB tables to retrieve the items from. The value is a map where table names are specified as the keys of the map. At least one table must be provided. This value is required.

keys: List of DynamoDB keys representing the primary key of the items to retrieve. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a "typed value", see Type System (Request Mapping).
consistentRead: Whether to use a consistent read when executing a GetItem operation. This value is optional and defaults to false.

Things to remember:

If an item has not been retrieved from the table, a null element appears in the data block for that table.
Invocation results are sorted per table, based on the order they were provided inside the request mapping template.
Each Get command inside a BatchGetItem is atomic, however, a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the unprocessedKeys block.
BatchGetItem is limited to 100 keys.

For the following example request mapping template:


{
  "version": "2018-05-29",
  "operation": "BatchGetItem",
  "tables": {
    "authors": [
        {
          "author_id": {
            "S": "a1"
          }
        },
    ],
    "posts": [
        {
          "author_id": {
            "S": "a1"
          },
          "post_id": {
            "S": "p2"
          }
        }
    ],
  }
}

The invocation result available in $ctx.result is as follows:


{
   "data": {
     "authors": [null],
     "posts": [
        # Was retrieved
        {
          "author_id": "a1",
          "post_id": "p2",
          "post_title": "title",
          "post_description": "description",
        }
     ]
   },
   "unprocessedKeys": {
     "authors": [
        # This item was not processed due to an error
        {
          "author_id": "a1"
        }
      ],
     "posts": []
   }
}

The $ctx.error contains details about the error. The keys data, unprocessedKeys, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been deleted appear in the data block. Items that haven't been processed are marked as null inside the data block and are placed inside the unprocessedKeys block.

For a more complete example, follow the DynamoDB Batch tutorial with AppSync here Tutorial: DynamoDB Batch Resolvers.

BatchDeleteItem

The BatchDeleteItem request mapping document lets you tell the AWS AppSyncDynamoDB resolver to make a BatchWriteItem request to DynamoDB to delete multiple items across potentially multiple tables. For this request template, you must specify the following:

The table names where to delete the items from
The keys of the items to delete from each table

The DynamoDBBatchWriteItem limits apply and no condition expression can be provided.

The BatchDeleteItem mapping document has the following structure:


{
    "version" : "2018-05-29",
    "operation" : "BatchDeleteItem",
    "tables" : {
        "table1": [
        ## Item to delete Key
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        },
        ## Item2 to delete Key
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        }],
        "table2": [
        ## Item3 to delete Key
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        },
        ## Item4 to delete Key
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        }],
    }
}

The fields are defined as follows:

version: The template definition version. Only 2018-05-29 is supported. This value is required.
operation: The DynamoDB operation to perform. To perform the BatchDeleteItemDynamoDB operation, this must be set to BatchDeleteItem. This value is required.
tables: The DynamoDB tables to delete the items from. Each table is a list of DynamoDB keys representing the primary key of the items to delete. DynamoDB items may have a single hash key, or a hash key and sort key, depending on the table structure. For more information about how to specify a "typed value", see Type System (Request Mapping). At least one table must be provided. This value is required.

Things to remember:

Contrary to the DeleteItem operation, the fully deleted item isnt't returned in the response, only the passed key is returned.
If an item has not been deleted from the table, a null element appears in the data block for that table.
Invocation results are sorted per table, based on the order they were provided inside the request mapping template.
Each delete command inside a BatchDeleteItem is atomic, however a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the unprocessedKeys block.
BatchDeleteItem is limited to 25 keys.

For the following example request mapping template:


{
  "version": "2018-05-29",
  "operation": "BatchDeleteItem",
  "tables": {
    "authors": [
        {
          "author_id": {
            "S": "a1"
          }
        },
    ],
    "posts": [
        {
          "author_id": {
            "S": "a1"
          },
          "post_id": {
            "S": "p2"
          }
        }
    ],
  }
}

The invocation result available in $ctx.result is as follows:


{
   "data": {
     "authors": [null],
     "posts": [
        # Was deleted
        {
          "author_id": "a1",
          "post_id": "p2"
        }
     ]
   },
   "unprocessedKeys": {
     "authors": [
        # This key was not processed due to an error
        {
          "author_id": "a1"
        }
      ],
     "posts": []
   }
}

The $ctx.error contains details about the error. The keys data, unprocessedKeys, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been deleted are present in the data block. Items that haven't been processed are marked as null inside the data block and are placed inside the unprocessedKeys block.

For a more complete example, follow the DynamoDB Batch tutorial with AppSync here Tutorial: DynamoDB Batch Resolvers.

BatchPutItem

The BatchPutItem request mapping document lets you tell the AWS AppSyncDynamoDB resolver to make a BatchWriteItem request to DynamoDB to put multiple items across potentially multiple tables. For this request template, you must specify the following:

The table names where to put the items in
The full items to put in each table

The DynamoDBBatchWriteItem limits apply and no condition expression can be provided.

The BatchPutItem mapping document has the following structure:


{
    "version" : "2018-05-29",
    "operation" : "BatchPutItem",
    "tables" : {
        "table1": [
        ## Item to put
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        },
        ## Item2 to put
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        }],
        "table2": [
        ## Item3 to put
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        },
        ## Item4 to put
        {
             "foo" : ... typed value,
             "bar" : ... typed value
        }],
    }
}

The fields are defined as follows:

version: The template definition version. Only 2018-05-29 is supported. This value is required.
operation: The DynamoDB operation to perform. To perform the BatchDeleteItemDynamoDB operation, this must be set to BatchPutItem. This value is required.
tables: The DynamoDB tables to put the items in. Each table entry represents a list of DynamoDB items to insert for this specific table. At least one table must be provided. This value is required.

Things to remember:

The fully inserted items are returned in the response, if successful.
If an item hasn't been inserted in the table, a null element is displayed in the data block for that table.
The inserted items are sorted per table, based on the order they were provided inside the request mapping template.
Each put command inside a BatchPutItem is atomic, however, a batch can be partially processed. If a batch is partially processed due to an error, the unprocessed keys are returned as part of the invocation result inside the unprocessedKeys block.
BatchPutItem is limited to 25 items.

For the following example request mapping template:


{
  "version": "2018-05-29",
  "operation": "BatchPutItem",
  "tables": {
    "authors": [
        {
          "author_id": {
            "S": "a1"
          },
          "author_name": {
            "S": "a1_name"
          }
        },
    ],
    "posts": [
        {
          "author_id": {
            "S": "a1"
          },
          "post_id": {
            "S": "p2"
          },
          "post_title": {
            "S": "title"
          }
        }
    ],
  }
}

The invocation result available in $ctx.result is as follows:


{
   "data": {
     "authors": [
         null
     ],
     "posts": [
        # Was inserted
        {
          "author_id": "a1",
          "post_id": "p2",
          "post_title": "title"
        }
     ]
   },
   "unprocessedItems": {
     "authors": [
        # This item was not processed due to an error
        {
          "author_id": "a1",
          "author_name": "a1_name"
        }
      ],
     "posts": []
   }
}

The $ctx.error contains details about the error. The keys data, unprocessedItems, and each table key that was provided in the request mapping template are guaranteed to be present in the invocation result. Items that have been inserted are in the data block. Items that haven't been processed are marked as null inside the data block and are placed inside the unprocessedItems block.

For a more complete example, follow the DynamoDB Batch tutorial with AppSync here Tutorial: DynamoDB Batch Resolvers.

Type System (Request Mapping)

When using the AWS AppSyncDynamoDB resolver to call your DynamoDB tables, AWS AppSync needs to know the type of each value to use in that call. This is because DynamoDB supports more type primitives than GraphQL or JSON (such as sets and binary data). AWS AppSync needs some hints when translating between GraphQL and DynamoDB, otherwise it would have to make some assumptions on how data is structured in your table.

For more information about DynamoDB data types, see the DynamoDBData Type Descriptors and Data Types documentation.

A DynamoDB value is represented by a JSON object containing a single key-value pair. The key specifies the DynamoDB type, and the value specifies the value itself. In the following example, the key S denotes that the value is a string, and the value identifier is the string value itself.


{ "S" : "identifier" }

Note that the JSON object cannot have more than one key-value pair. If more than one key-value pair is specified, the request mapping document isn't parsed.

A DynamoDB value is used anywhere in a request mapping document where you need to specify a value. Some places where you need to do this include: key and attributeValue sections, and the expressionValues section of expression sections. In the following example, the DynamoDB String value identifier is being assigned to the id field in a key section (perhaps in a GetItem request mapping document).


"key" : {
   "id" : { "S" : "identifier" }
}

Supported Types

AWS AppSync supports the following DynamoDB scalar, document and set types:

String type S

A single string value. A DynamoDB String value is denoted by:


{ "S" : "some string" }

An example usage is:


"key" : {
   "id" : { "S" : "some string" }
}

String set type SS

A set of string values. A DynamoDB String Set value is denoted by:


{ "SS" : [ "first value", "second value", ... ] }

An example usage is:


"attributeValues" : {
   "phoneNumbers" : { "SS" : [ "+1 555 123 4567", "+1 555 234 5678" ] }
}

Number type N

A single numeric value. A DynamoDB Number value is denoted by:


{ "N" : 1234 }

An example usage is:


"expressionValues" : {
   ":expectedVersion" : { "N" : 1 }
}

Number set type NS

A set of number values. A DynamoDB Number Set value is denoted by:


{ "NS" : [ 1, 2.3, 4 ... ] }

An example usage is:


"attributeValues" : {
   "sensorReadings" : { "NS" : [ 67.8, 12.2, 70 ] }
}

Binary type B

A binary value. A DynamoDB Binary value is denoted by:


{ "B" : "SGVsbG8sIFdvcmxkIQo=" }

Note that the value is actually a string, where the string is the Base64 encoded representation of the binary data. AWS AppSync decodes this string back into its binary value before sending it to DynamoDB. AWS AppSync uses the Base64 decoding scheme as defined by RFC 2045: any character that isn't in the Base64 alphabet is ignored.

An example usage is:


"attributeValues" : {
   "binaryMessage" : { "B" : "SGVsbG8sIFdvcmxkIQo=" }
}

Binary set type BS

A set of binary values. A DynamoDB Binary Set value is denoted by:


{ "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ] }

An example usage is:


"attributeValues" : {
   "binaryMessages" : { "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ] }
}

Boolean type BOOL

A boolean value. A DynamoDB Boolean value is denoted by:


{ "BOOL" : true }

Note that only true and false are valid values.

An example usage is:


"attributeValues" : {
   "orderComplete" : { "BOOL" : false }
}

List type L

A list of any other supported DynamoDB value. A DynamoDB List value is denoted by:


{ "L" : [ ... ] }

Note that the value is a compound value, where the list can contain zero or more of any supported DynamoDB value (including other lists). The list can also contain a mix of different types.

An example usage is:


{ "L" : [
      { "S"  : "A string value" },
      { "N"  : 1 },
      { "SS" : [ "Another string value", "Even more string values!" ] }
   ]
}

Map type M

Representing an unordered collection of key-value pairs of other supported DynamoDB values. A DynamoDB Map value is denoted by:


{ "M" : { ... } }

Note that a map can contain zero or more key-value pairs. The key must be a string, and the value can be any supported DynamoDB value (including other maps). The map can also contain a mix of different types.

An example usage is:


{ "M" : {
      "someString" : { "S"  : "A string value" },
      "someNumber" : { "N"  : 1 },
      "stringSet"  : { "SS" : [ "Another string value", "Even more string values!" ] }
   }
}

Null type NULL

A null value. A DynamoDB Null value is denoted by:


{ "NULL" : null }

An example usage is:


"attributeValues" : {
   "phoneNumbers" : { "NULL" : null }
}

For more information about each type, see the DynamoDB documentation .

Type System (Response Mapping)

When receiving a response from DynamoDB, AWS AppSync automatically converts it into GraphQL and JSON primitive types. Each attribute in DynamoDB is decoded and returned in the response mapping context.

For example, if DynamoDB returns the following:


{
    "id" : { "S" : "1234" },
    "name" : { "S" : "Nadia" },
    "age" : { "N" : 25 }
}

Then the AWS AppSyncDynamoDB resolver converts it into GraphQL and JSON types as:


{
    "id" : "1234",
    "name" : "Nadia",
    "age" : 25
}

This section explains how AWS AppSync converts the following DynamoDB scalar, document and set types:

String type S

A single string value. A DynamoDB String value is returned as a string.

For example, if DynamoDB returned the following DynamoDB String value:


{ "S" : "some string" }

AWS AppSync converts it to a string:


"some string"

String set type SS

A set of string values. A DynamoDB String Set value is returned as a list of strings.

For example, if DynamoDB returned the following DynamoDB String Set value:


{ "SS" : [ "first value", "second value", ... ] }

AWS AppSync converts it to a list of strings:


[ "+1 555 123 4567", "+1 555 234 5678" ]

Number type N

A single numeric value. A DynamoDB Number value is returned as a number.

For example, if DynamoDB returned the following DynamoDB Number value:


{ "N" : 1234 }

AWS AppSync converts it to a number:

Number set type NS

A set of number values. A DynamoDB Number Set value is returned as a list of numbers.

For example, if DynamoDB returned the following DynamoDB Number Set value:


{ "NS" : [ 67.8, 12.2, 70 ] }

AWS AppSync converts it to a list of numbers:


[ 67.8, 12.2, 70 ]

Binary type B

A binary value. A DynamoDB Binary value is returned as a string containing the Base64 representation of that value.

For example, if DynamoDB returned the following DynamoDB Binary value:


{ "B" : "SGVsbG8sIFdvcmxkIQo=" }

AWS AppSync converts it to a string containing the Base64 representation of the value:


"SGVsbG8sIFdvcmxkIQo="

Note that the binary data is encoded in the Base64 encoding scheme as specified in RFC 4648 and RFC 2045.

Binary set type BS

A set of binary values. A DynamoDB Binary Set value is returned as a list of strings containing the Base64 representation of the values.

For example, if DynamoDB returned the following DynamoDB Binary Set value:


{ "BS" : [ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ] }

AWS AppSync converts it to a list of strings containing the Base64 representation of the values:


[ "SGVsbG8sIFdvcmxkIQo=", "SG93IGFyZSB5b3U/Cg==" ... ]

Note that the binary data is encoded in the Base64 encoding scheme as specified in RFC 4648 and RFC 2045.

Boolean type BOOL

A boolean value. A DynamoDB Boolean value is returned as a boolean.

For example, if DynamoDB returned the following DynamoDB Boolean value:


{ "BOOL" : true }

AWS AppSync converts it to a boolean:


true

List type L

A list of any other supported DynamoDB value. A DynamoDB List value is returned as a list of values, where each inner value is also converted.

For example, if DynamoDB returned the following DynamoDB List value:


{ "L" : [
      { "S"  : "A string value" },
      { "N"  : 1 },
      { "SS" : [ "Another string value", "Even more string values!" ] }
   ]
}

AWS AppSync converts it to a list of converted values:


[ "A string value", 1, [ "Another string value", "Even more string values!" ] ]

Map type M

A key/value collection of any other supported DynamoDB value. A DynamoDB Map value is returned as a JSON object, where each key/value is also converted.

For example, if DynamoDB returned the following DynamoDB Map value:


{ "M" : {
      "someString" : { "S"  : "A string value" },
      "someNumber" : { "N"  : 1 },
      "stringSet"  : { "SS" : [ "Another string value", "Even more string values!" ] }
   }
}

AWS AppSync converts it to a JSON object:


{
   "someString" : "A string value",
   "someNumber" : 1,
   "stringSet"  : [ "Another string value", "Even more string values!" ]
}

Null type NULL

A null value.

For example, if DynamoDB returned the following DynamoDB Null value:


{ "NULL" : null }

AWS AppSync converts it to a null:


null

Filters

When querying objects in DynamoDB using the Query and Scan operations, you can optionally specify a filter that evaluates the results and returns only the desired values.

The filter mapping section of a Query or Scan mapping document has the following structure:


"filter" : {
    "expression" : "filter expression"
    "expressionNames" : {
        "#name" : "name",
    },
    "expressionValues" : {
        ":value" : ... typed value
    },
}

The fields are defined as follows:

expression: The query expression. For more information about how to write filter expressions, see the DynamoDB QueryFilter and DynamoDB ScanFilter documentation. This field must be specified.
expressionNames: The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the expression, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the expression.
expressionValues: The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a "typed value", see Type System (Request Mapping). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.

Example

Following is a filter section for a mapping template, where entries retrieved from DynamoDB are only returned if the title starts with the title argument.


"filter" : {
    "expression" : "begins_with(#title, :title)",
    "expressionNames" : {
        "#title" : "title"
    }
    "expressionValues" : {
        ":title" : { "S" : "${context.arguments.title}" }
    }
}

Condition Expressions

When you mutate objects in DynamoDB by using the PutItem, UpdateItem, and DeleteItemDynamoDB operations, you can optionally specify a condition expression that controls whether the request should succeed or not, based on the state of the object already in DynamoDB before the operation is performed.

The AWS AppSyncDynamoDB resolver allows a condition expression to be specified in PutItem, UpdateItem, and DeleteItem request mapping documents, and also a strategy to follow if the condition fails and the object was not updated.

Example 1

The following PutItem mapping document doesn't have a condition expression. As a result, it puts an item in DynamoDB even if an item with the same key already exists, thereby overwriting the existing item.


{
   "version" : "2017-02-28",
   "operation" : "PutItem",
   "key" : {
      "id" : { "S" : "1" }
   }
}

Example 2

The following PutItem mapping document does have a condition expression that allows the operation succeed only if an item with the same key does not exist in DynamoDB.


{
   "version" : "2017-02-28",
   "operation" : "PutItem",
   "key" : {
      "id" : { "S" : "1" }
   },
   "condition" : {
      "expression" : "attribute_not_exists(id)"
   }
}

By default, if the condition check fails, the AWS AppSyncDynamoDB resolver returns an error for the mutation and the current value of the object in DynamoDB in a data field in the error section of the GraphQL response. However, the AWS AppSyncDynamoDB resolver offers some additional features to help developers handle some common edge cases:

If AWS AppSyncDynamoDB resolver can determine that the current value in DynamoDB matches the desired result, it treats the operation as if it succeeded anyway.
Instead of returning an error, you can configure the resolver to invoke a custom Lambda function to decide how the AWS AppSyncDynamoDB resolver should handle the failure.

These are described in greater detail in the Handling a Condition Check Failure section.

For more information about DynamoDB conditions expressions, see the DynamoDB ConditionExpressions documentation .

Specifying a Condition

The PutItem, UpdateItem, and DeleteItem request mapping documents all allow an optional condition section to be specified. If omitted, no condition check is made. If specified, the condition must be true for the operation to succeed.

A condition section has the following structure:


"condition" : {
    "expression" : "someExpression"
    "expressionNames" : {
        "#foo" : "foo"
    },
    "expressionValues" : {
        ":bar" : ... typed value
    },
    "equalsIgnore" : [ "version" ],
    "consistentRead" : true,
    "conditionalCheckFailedHandler" : {
        "strategy" : "Custom",
        "lambdaArn" : "arn:..."
    }
}

The following fields specify the condition:

expression: The update expression itself. For more information about how to write condition expressions, see the DynamoDB ConditionExpressions documentation . This field must be specified.
expressionNames: The substitutions for expression attribute name placeholders, in the form of key-value pairs. The key corresponds to a name placeholder used in the expression, and the value must be a string corresponding to the attribute name of the item in DynamoDB. This field is optional, and should only be populated with substitutions for expression attribute name placeholders used in the expression.
expressionValues: The substitutions for expression attribute value placeholders, in the form of key-value pairs. The key corresponds to a value placeholder used in the expression, and the value must be a typed value. For more information about how to specify a "typed value", see Type System (request mapping). This must be specified. This field is optional, and should only be populated with substitutions for expression attribute value placeholders used in the expression.

The remaining fields tell the AWS AppSyncDynamoDB resolver how to handle a condition check failure:

equalsIgnore

When a condition check fails when using the PutItem operation, the AWS AppSyncDynamoDB resolver compares the item currently in DynamoDB against the item it tried to write. If they are the same, it treats the operation as it if succeeded anyway. You can use the equalsIgnore field to specify a list of attributes that AWS AppSync should ignore when performing that comparison. For example, if the only difference was a version attribute, it treats the operation as it if succeeded. This field is optional.

consistentRead

When a condition check fails, AWS AppSync gets the current value of the item from DynamoDB using a strongly consistent read. You can use this field to tell the AWS AppSyncDynamoDB resolver to use an eventually consistent read instead. This field is optional, and defaults to true.

conditionalCheckFailedHandler

This section allows you to specify how the AWS AppSyncDynamoDB resolver treats a condition check failure after it has compared the current value in DynamoDB against the expected result. This section is optional. If omitted, it defaults to a strategy of Reject.

strategy

The strategy the AWS AppSyncDynamoDB resolver takes after it has compared the current value in DynamoDB against the expected result. This field is required and has the following possible values:

Reject: The mutation fails, and an error for the mutation and the current value of the object in DynamoDB in a data field in the error section of the GraphQL response.
Custom: The AWS AppSyncDynamoDB resolver invokes a custom Lambda function to decide how to handle the condition check failure. When the strategy is set to Custom, the lambdaArn field must contain the ARN of the Lambda function to invoke.

lambdaArn

The ARN of the Lambda function to invoke that determines how the AWS AppSyncDynamoDB resolver should handle the condition check failure. This field must only be specified when strategy is set to Custom. For more information about how to use this feature, see Handling a Condition Check Failure.

Handling a Condition Check Failure

By default, when a condition check fails, the AWS AppSyncDynamoDB resolver returns an error for the mutation and the current value of the object in DynamoDB in a data field in the error section of the GraphQL response. However, the AWS AppSyncDynamoDB resolver offers some additional features to help developers handle some common edge cases:

If AWS AppSyncDynamoDB resolver can determine that the current value in DynamoDB matches the desired result, it treats the operation as if it succeeded anyway.
Instead of returning an error, you can configure the resolver to invoke a custom Lambda function to decide how the AWS AppSyncDynamoDB resolver should handle the failure.

The flowchart for this process is:

Checking for the Desired Result

When the condition check fails, the AWS AppSyncDynamoDB resolver performs a GetItemDynamoDB request to get the current value of the item from DynamoDB. By default, it uses a strongly consistent read, however this can be configured using the consistentRead field in the condition block and compare it against the expected result:

For the PutItem operation, the AWS AppSyncDynamoDB resolver compares the current value against the one it attempted to write, excluding any attributes listed in equalsIgnore from the comparison. If the items are the same, it treats the operation as successful and returns the item that was retrieved from DynamoDB. Otherwise, it follows the configured strategy.

For example, if the PutItem request mapping document looked like the following:
```
{
   "version" : "2017-02-28",
   "operation" : "PutItem",
   "key" : {
      "id" : { "S" : "1" }
   },
   "attributeValues" : {
      "name" : { "S" : "Steve" },
      "version" : { "N" : 2 }
   },
   "condition" : {
      "expression" : "version = :expectedVersion",
      "expressionValues" : {
          ":expectedVersion" : { "N" : 1 }
      },
      "equalsIgnore": [ "version" ]
   }
}
```
And the item currently in DynamoDB looked like the following:
```
{
   "id" : { "S" : "1" },
   "name" : { "S" : "Steve" },
   "version" : { "N" : 8 }
}
```
The AWS AppSyncDynamoDB resolver would compare the item it tried to write against the current value, see that the only difference was the version field, but because it's configured to ignore the version field, it treats the operation as successful and returns the item that was retrieved from DynamoDB.
For the DeleteItem operation, the AWS AppSyncDynamoDB resolver checks to verify that an item was returned from DynamoDB. If no item was returned, it treats the operation as successful. Otherwise, it follows the configured strategy.
For the UpdateItem operation, the AWS AppSyncDynamoDB resolver does not have enough information to determine if the item currently in DynamoDB matches the expected result, and therefore follows the configured strategy.

If the current state of the object in DynamoDB is different from the expected result, the AWS AppSyncDynamoDB resolver follows the configured strategy, to either reject the mutation or invoke a Lambda function to determine what to do next.

Following the "Reject" Strategy

When following the Reject strategy, the AWS AppSyncDynamoDB resolver returns an error for the mutation, and the current value of the object in DynamoDB is also returned in a data field in the error section of the GraphQL response. The item returned from DynamoDB is put through the response mapping template to translate it into a format the client expects, and it is filtered by the selection set.

For example, given the following mutation request:


mutation {
    updatePerson(id: 1, name: "Steve", expectedVersion: 1) {
        Name
        theVersion
    }
}

If the item returned from DynamoDB looks like the following:


{
   "id" : { "S" : "1" },
   "name" : { "S" : "Steve" },
   "version" : { "N" : 8 }
}

And the response mapping template looks like the following:


{
   "id" : "${context.result.id}",
   "Name" : "${context.result.name}",
   "theVersion" : ${context.result.version}
}

The GraphQL response looks like the following:


{
  "data": null,
  "errors": [
    {
      "message": "The conditional request failed (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ConditionalCheckFailedException; Request ID: ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ)"
      "errorType": "DynamoDB:ConditionalCheckFailedException",
      "data": {
        "Name": "Steve",
        "theVersion": 8
      },
      ...
    }
  ]
}

Also, if any fields in the returned object are filled by other resolvers and the mutation had succeeded, they won't be resolved when the object is returned in the error section.

Following the "Custom" Strategy

When following the Custom strategy, the AWS AppSyncDynamoDB resolver invokes a Lambda function to decide what to do next. The Lambda function chooses one of the following options:

reject the mutation. This tells the AWS AppSyncDynamoDB resolver to behave as if the configured strategy was Reject, returning an error for the mutation and the current value of the object in DynamoDB as described in the previous section.
discard the mutation. This tells the AWS AppSyncDynamoDB resolver to silently ignore the condition check failure and returns the value in DynamoDB.
retry the mutation. This tells the AWS AppSyncDynamoDB resolver to retry the mutation with a new request mapping document.

The Lambda invocation request

The AWS AppSyncDynamoDB resolver invokes the Lambda function specified in the lambdaArn. It uses the same service-role-arn configured on the data source. The payload of the invocation has the following structure:


{
    "arguments": { ... },
    "requestMapping": {... },
    "currentValue": { ... },
    "resolver": { ... },
    "identity": { ... }
}

The fields are defined as follows:

arguments: The arguments from the GraphQL mutation. This is the same as the arguments available to the request mapping document in $context.arguments.
requestMapping: The request mapping document for this operation.
currentValue: The current value of the object in DynamoDB.
resolver: Information about the AWS AppSync resolver.
identity: Information about the caller. This is the same as the identity information available to the request mapping document in $context.identity.

A full example of the payload:


{
    "arguments": {
        "id": "1",
        "name": "Steve",
        "expectedVersion": 1
    },
    "requestMapping": {
        "version" : "2017-02-28",
        "operation" : "PutItem",
        "key" : {
           "id" : { "S" : "1" }
        },
        "attributeValues" : {
           "name" : { "S" : "Steve" },
           "version" : { "N" : 2 }
        },
        "condition" : {
           "expression" : "version = :expectedVersion",
           "expressionValues" : {
               ":expectedVersion" : { "N" : 1 }
           },
           "equalsIgnore": [ "version" ]
        }
    },
    "currentValue": {
        "id" : { "S" : "1" },
        "name" : { "S" : "Steve" },
        "version" : { "N" : 8 }
    },
    "resolver": {
        "tableName": "People",
        "awsRegion": "us-west-2",
        "parentType": "Mutation",
        "field": "updatePerson",
        "outputType": "Person"
    },
    "identity": {
        "accountId": "123456789012",
        "sourceIp": "x.x.x.x",
        "user": "AIDAAAAAAAAAAAAAAAAAA",
        "userArn": "arn:aws:iam::123456789012:user/appsync"
    }
}

The Lambda Invocation Response

The Lambda function can inspect the invocation payload and apply any business logic to decide how the AWS AppSyncDynamoDB resolver should handle the failure. There are three options for handling the condition check failure:

reject the mutation. The response payload for this option must have this structure:
```
{
    "action": "reject"
}
```
This tells the AWS AppSyncDynamoDB resolver to behave as if the configured strategy was Reject, returning an error for the mutation and the current value of the object in DynamoDB, as described in the section above.
discard the mutation. The response payload for this option must have this structure:
```
{
    "action": "discard"
}
```
This tells the AWS AppSyncDynamoDB resolver to silently ignore the condition check failure and returns the value in DynamoDB.
retry the mutation. The response payload for this option must have this structure:
```
{
    "action": "retry",
    "retryMapping": { ... }
}
```
This tells the AWS AppSyncDynamoDB resolver to retry the mutation with a new request mapping document. The structure of the retryMapping section depends on the DynamoDB operation, and is a subset of the full request mapping document for that operation.

For PutItem, the retryMapping section has the following structure. For a description of the attributeValues field, see PutItem.
```
{
    "attributeValues": { ... },
    "condition": {
        "equalsIgnore" = [ ... ],
        "consistentRead" = true
    }
}
```
For UpdateItem, the retryMapping section has the following structure. For a description of the update section, see UpdateItem.
```
{
    "update" : {
        "expression" : "someExpression"
        "expressionNames" : {
            "#foo" : "foo"
        },
        "expressionValues" : {
            ":bar" : ... typed value
        }
    },
    "condition": {
        "consistentRead" = true
    }
}
```
For DeleteItem, the retryMapping section has the following structure.
```
{
    "condition": {
        "consistentRead" = true
    }
}
```
There is no way to specify a different operation or key to work on. The AWS AppSyncDynamoDB resolver only allows retries of the same operation on the same object. Also, the condition section doesn't allow a conditionalCheckFailedHandler to be specified. If the retry fails, the AWS AppSyncDynamoDB resolver follows the Reject strategy.

Here is an example Lambda function to deal with a failed PutItem request. The business logic looks at who made the call. If it was made by jeffTheAdmin, it retries the request, updating the version and expectedVersion from the item currently in DynamoDB. Otherwise, it rejects the mutation.


exports.handler = (event, context, callback) => {
    console.log("Event: "+ JSON.stringify(event));

    // Business logic goes here.

    var response;
    if ( event.identity.user == "jeffTheAdmin" ) {
        response = {
            "action" : "retry",
            "retryMapping" : {
                "attributeValues" : event.requestMapping.attributeValues,
                "condition" : {
                    "expression" : event.requestMapping.condition.expression,
                    "expressionValues" : event.requestMapping.condition.expressionValues
                }
            }
        }
        response.retryMapping.attributeValues.version = { "N" : event.currentValue.version.N + 1 }
        response.retryMapping.condition.expressionValues[':expectedVersion'] = event.currentValue.version

    } else {
        response = { "action" : "reject" }
    }

    console.log("Response: "+ JSON.stringify(response))
    callback(null, response)
};

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