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Resolver mapping template reference for Lambda

You can use Amazon AppSync functions and resolvers to invoke Lambda functions located in your account. You can shape your request payloads and the response from your Lambda functions before returning them to your clients. You can also use mapping templates to give hints to Amazon AppSync about the nature of the operation to be invoked. This section describes the different mapping templates for the supported Lambda operations.

Request mapping template

The Lambda request mapping template handles fields related to your Lambda function:

{
  "version": string,
  "operation": Invoke|BatchInvoke,
  "payload": any type,
  "invocationType": RequestResponse|Event
}

This is the JSON schema representation of the Lambda request mapping template when resolved:

{
  "definitions": {},
  "$schema": "https://json-schema.org/draft-06/schema#",
  "$id": "https://aws.amazon.com/appsync/request-mapping-template.json",
  "type": "object",
  "properties": {
    "version": {
      "$id": "/properties/version",
      "type": "string",
      "enum": [
        "2018-05-29"
      ],
      "title": "The Mapping template version.",
      "default": "2018-05-29"
    },
    "operation": {
      "$id": "/properties/operation",
      "type": "string",
      "enum": [
        "Invoke",
        "BatchInvoke"
      ],
      "title": "The Mapping template operation.",
      "description": "What operation to execute.",
      "default": "Invoke"
    },
    "payload": {},
    "invocationType": {
      "$id": "/properties/invocationType",
      "type": "string",
      "enum": [
        "RequestResponse",
        "Event"
      ],
      "title": "The Mapping template invocation type.",
      "description": "What invocation type to execute.",
      "default": "RequestResponse"
    }
  },
  "required": [
    "version",
    "operation"
  ],
  "additionalProperties": false
}

Here's an example that uses an invoke operation with its payload data being the getPost field from a GraphQL schema along with its arguments from the context:

{
  "version": "2018-05-29",
  "operation": "Invoke",
  "payload": {
    "field": "getPost",
    "arguments": $util.toJson($context.arguments)
  }
}

The entire mapping document is passed as the input to your Lambda function so that the previous example now looks like this:

{
  "version": "2018-05-29",
  "operation": "Invoke",
  "payload": {
    "field": "getPost",
    "arguments": {
      "id": "postId1"
    }
  }
}

Version

Common to all request mapping templates, the version defines the version that the template uses. The version is required and is a static value:

"version": "2018-05-29"

Operation

The Lambda data source lets you define two operations in the operation field: Invoke and BatchInvoke. The Invoke operation lets Amazon AppSync know to call your Lambda function for every GraphQL field resolver. BatchInvoke instructs Amazon AppSync to batch requests for the current GraphQL field. The operation field is required.

For Invoke, the resolved request mapping template matches the input payload of the Lambda function. Let's modify the example above:

{
  "version": "2018-05-29",
  "operation": "Invoke",
    "payload": {
      "arguments": $util.toJson($context.arguments)
    }
}

This is resolved and passed to the Lambda function, which could look something like this:

{
  "version": "2018-05-29",
  "operation": "Invoke",
    "payload": {
      "arguments": {
        "id": "postId1"
      }
    }
}

For BatchInvoke, the mapping template is applied to every field resolver in the batch. For conciseness, Amazon AppSync merges all the resolved mapping template payload values into a list under a single object matching the mapping template. The following example template shows the merge:

{
  "version": "2018-05-29",
  "operation": "BatchInvoke",
  "payload": $util.toJson($context)
}

This template is resolved into the following mapping document:

{
  "version": "2018-05-29",
  "operation": "BatchInvoke",
  "payload": [
    {...}, // context for batch item 1
    {...}, // context for batch item 2
    {...}  // context for batch item 3
  ]
}

Each element of the payload list corresponds to a single batch item. The Lambda function is also expected to return a list-shaped response matching the order of the items sent in the request:

[
  { "data": {...}, "errorMessage": null, "errorType": null }, // result for batch item 1
  { "data": {...}, "errorMessage": null, "errorType": null }, // result for batch item 2
  { "data": {...}, "errorMessage": null, "errorType": null }  // result for batch item 3
]

Payload

The payload field is a container used to pass any well-formed JSON to the Lambda function. If the operation field is set to BatchInvoke, Amazon AppSync wraps the existing payload values into a list. The payload field is optional.

Invocation type

The Lambda data source allows you to define two invocation types: RequestResponse and Event. The invocation types are synonymous with the invocation types defined in the Lambda API. The RequestResponse invocation type lets Amazon AppSync call your Lambda function synchronously to wait for a response. The Event invocation allows you to invoke your Lambda function asynchronously. For more information on how Lambda handles Event invocation type requests, see Asynchronous invocation. The invocationType field is optional. If this field is not included in the request, Amazon AppSync will default to the RequestResponse invocation type.

For any invocationType field, the resolved request matches the input payload of the Lambda function. Let's modify the example above:

{
  "version": "2018-05-29",
  "operation": "Invoke",
  "invocationType": "Event"
  "payload": {
    "arguments": $util.toJson($context.arguments)
  }
}

This is resolved and passed to the Lambda function, which could look something like this:

{
  "version": "2018-05-29",
  "operation": "Invoke",
  "invocationType": "Event",
  "payload": {
    "arguments": {
      "id": "postId1"
    }
  }
}

When the BatchInvoke operation is used in conjunction with the Event invocation type field, Amazon AppSync merges the field resolver in the same way mentioned above, and the request is passed to your Lambda function as an asynchronous event with the payload being a list of values. We recommend that you disable resolver caching for Event invocation type resolvers because these would not be sent to Lambda if there were a cache hit.

Response mapping template

As with other data sources, your Lambda function sends a response to Amazon AppSync that must be converted to a GraphQL type.

The result of the Lambda function is set on the context object that is available via the Velocity Template Language (VTL) $context.result property.

If the shape of your Lambda function response exactly matches the shape of the GraphQL type, you can forward the response using the following response mapping template:

$util.toJson($context.result)

There are no required fields or shape restrictions that apply to the response mapping template. However, because GraphQL is strongly typed, the resolved mapping template must match the expected GraphQL type.

Lambda function batched response

If the operation field is set to BatchInvoke, Amazon AppSync expects a list of items back from the Lambda function. In order for Amazon AppSync to map each result back to the original request item, the response list must match in size and order. It's valid to have null items in the response list; $ctx.result is set to null accordingly.

Direct Lambda Resolvers

If you wish to circumvent the use of mapping templates entirely, Amazon AppSync can provide a default payload to your Lambda function and a default Lambda function response to a GraphQL type. You can choose to provide a request template, a response template, or neither, and Amazon AppSync handles it accordingly.

Direct Lambda request mapping template

When the request mapping template is not provided, Amazon AppSync will send the Context object directly to your Lambda function as an Invoke operation. For more information about the structure of the Context object, see Resolver mapping template context reference.

Direct Lambda response mapping template

When the response mapping template is not provided, Amazon AppSync does one of two things upon receiving your Lambda function's response. If you did not provide a request mapping template or if you provided a request mapping template with the version 2018-05-29, the response will be equivalent to the following response mapping template:

#if($ctx.error)
     $util.error($ctx.error.message, $ctx.error.type, $ctx.result)
 #end
 $util.toJson($ctx.result)

If you provided a template with the version 2017-02-28, the response logic functions equivalently to the following response mapping template:

$util.toJson($ctx.result)

Superficially, the mapping template bypass operates similarly to using certain mapping templates as shown in the preceding examples. However, behind the scenes, the evaluation of the mapping templates is circumvented entirely. Because the template evaluation step is bypassed, applications might experience less overhead and latency during the response in some scenarios compared to a Lambda function with a response mapping template that needs to be evaluated.

Custom error handling in Direct Lambda Resolver responses

You can customize error responses from Lambda functions that Direct Lambda Resolvers invoke by raising a custom exception. The following example demonstrates how to create a custom exception using JavaScript:

class CustomException extends Error {
  constructor(message) {
    super(message);
    this.name = "CustomException";
  }
}
 
throw new CustomException("Custom message");

When exceptions are raised, the errorType and errorMessage are the name and message, respectively, of the custom error that is thrown.

If errorType is UnauthorizedException, Amazon AppSync returns the default message ("You are not authorized to make this call.") instead of a custom message.

The following snippet is an example GraphQL response that demonstrates a custom errorType:

{
  "data": {
    "query": null
  },
  "errors": [
    {
      "path": [
        "query"
      ],
      "data": null,
      "errorType": "CustomException",
      "errorInfo": null,
      "locations": [
        {
          "line": 5,
          "column": 10,
          "sourceName": null
        }
      ],
      "message": "Custom Message"
    }
  ]
}

Direct Lambda Resolvers: Batching enabled

You can enable batching for your Direct Lambda Resolver by configuring the maxBatchSize on your resolver. When maxBatchSize is set to a value greater than 0 for a Direct Lambda resolver, Amazon AppSync sends requests in batches to your Lambda function in sizes up to maxBatchSize.

Setting maxBatchSize to 0 on a Direct Lambda resolver turns off batching.

For more information on how batching with Lambda resolvers works, see Advanced use case: Batching.

Request mapping template

When batching is enabled and the request mapping template is not provided, Amazon AppSync sends a list of Context objects as a BatchInvoke operation directly to your Lambda function.

Response mapping template

When batching is enabled and the response mapping template is not provided, the response logic is equivalent to the following response mapping template:

#if( $context.result && $context.result.errorMessage )
      $utils.error($context.result.errorMessage, $context.result.errorType,
      $context.result.data)
#else
      $utils.toJson($context.result.data)
#end

The Lambda function must return a list of results in the same order as the list of Context objects that were sent. You can return individual errors by providing an errorMessage and errorType for a specific result. Each result in the list has the following format:

{
   "data" : { ... }, // your data
   "errorMessage" : { ... }, // optional, if included an error entry is added to the "errors" object in the AppSync response 
   "errorType" : { ... } // optional, the error type
}

Handling errors from Lambda

You can return an error for all results by throwing an exception or an error in your Lambda function. If the payload request or response size for your batch request is too large, Lambda returns an error. In that case, you should consider reducing your maxBatchSize or reducing the size of the response payload.

For information on handling individual errors, see Returning individual errors.

Sample Lambda functions

Using the schema below, you can create a Direct Lambda Resolver for the Post.relatedPosts field resolver and enable batching by setting maxBatchSize above 0:

schema {
    query: Query
    mutation: Mutation
}

type Query {
    getPost(id:ID!): Post
    allPosts: [Post]
}

type Mutation {
    addPost(id: ID!, author: String!, title: String, content: String, url: String): Post!
}

type Post {
    id: ID!
    author: String!
    title: String
    content: String
    url: String
    ups: Int
    downs: Int
    relatedPosts: [Post]
}

In the following query, the Lambda function will be called with batches of requests to resolve relatedPosts:

query getAllPosts {
  allPosts {
    id
    relatedPosts {
      id
    }
  }
}

A simple implementation of a Lambda function is provided below:

const posts = {
  1: {
    id: '1',
    title: 'First book',
    author: 'Author1',
    url: 'https://amazon.com/',
    content:
      'SAMPLE TEXT AUTHOR 1 SAMPLE TEXT AUTHOR 1 SAMPLE TEXT AUTHOR 1 SAMPLE TEXT AUTHOR 1 SAMPLE TEXT AUTHOR 1 SAMPLE TEXT AUTHOR 1',
    ups: '100',
    downs: '10',
  },
  2: {
    id: '2',
    title: 'Second book',
    author: 'Author2',
    url: 'https://amazon.com',
    content: 'SAMPLE TEXT AUTHOR 2 SAMPLE TEXT AUTHOR 2 SAMPLE TEXT',
    ups: '100',
    downs: '10',
  },
  3: { id: '3', title: 'Third book', author: 'Author3', url: null, content: null, ups: null, downs: null },
  4: {
    id: '4',
    title: 'Fourth book',
    author: 'Author4',
    url: 'https://www.amazon.com/',
    content:
      'SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4 SAMPLE TEXT AUTHOR 4',
    ups: '1000',
    downs: '0',
  },
  5: {
    id: '5',
    title: 'Fifth book',
    author: 'Author5',
    url: 'https://www.amazon.com/',
    content: 'SAMPLE TEXT AUTHOR 5 SAMPLE TEXT AUTHOR 5 SAMPLE TEXT AUTHOR 5 SAMPLE TEXT AUTHOR 5 SAMPLE TEXT',
    ups: '50',
    downs: '0',
  },
}

const relatedPosts = {
  1: [posts['4']],
  2: [posts['3'], posts['5']],
  3: [posts['2'], posts['1']],
  4: [posts['2'], posts['1']],
  5: [],
}
exports.handler = async (event) => {
  console.log('event ->', event)
  // retrieve the ID of each post
  const ids = event.map((context) => context.source.id)
  // fetch the related posts for each post id
  const related = ids.map((id) => relatedPosts[id])

  // return the related posts; or an error if none were found
  return related.map((r) => {
    if (r.length > 0) {
      return { data: r }
    } else {
      return { data: null, errorMessage: 'Not found', errorType: 'ERROR' }
    }
  })
}