export const meta = {
  title: `Configure Lambda resolvers`,
  description: `Quickly & easily configure AWS Lambda resolvers within your AWS AppSync API.`,
};

<MigrationAlert isLegacy url={"/cli/graphql/custom-business-logic"}/>

## @function

The `@function` directive allows you to quickly & easily configure AWS Lambda resolvers within your AWS AppSync API.

### Definition

```graphql
directive @function(name: String!, region: String) on FIELD_DEFINITION
```

### Usage

The `@function` directive allows you to quickly connect lambda resolvers to an AppSync API. You may deploy the AWS Lambda functions via the Amplify CLI, AWS Lambda console, or any other tool. To connect an AWS Lambda resolver, add the `@function` directive to a field in your `schema.graphql`.

Let's assume you have deployed an *echo* function with the following contents:

```js
exports.handler =  async function(event, context){
  return event.arguments.msg;
};
```

**If you deployed your function using the `function` category**

The Amplify CLI provides support for maintaining multiple environments out of the box. When you deploy a function via `amplify add function`, it will automatically add the environment suffix to your Lambda function name. For example if you create a function named **echofunction** using `amplify add function` in the **dev** environment, the deployed function will be named **echofunction-dev**. The `@function` directive allows you to use `${env}` to reference the current Amplify CLI environment.

```graphql
type Query {
  echo(msg: String): String @function(name: "echofunction-${env}")
}
```

**If you deployed your function without amplify**

If you deployed your API without amplify then you must provide the full Lambda function name. If you deployed the same function with the name **echofunction** then you would have:

```graphql
type Query {
  echo(msg: String): String @function(name: "echofunction")
}
```

**Example: Return custom data and run custom logic**

You can use the `@function` directive to write custom business logic in an AWS Lambda function. To get started, use
`amplify add function`, the AWS Lambda console, or other tool to deploy an AWS Lambda function with the following contents.

For example purposes assume the function is named `GraphQLResolverFunction`:

```js
const POSTS = [
  { id: 1, title: "AWS Lambda: How To Guide." },
  { id: 2, title: "AWS Amplify Launches @function and @key directives." },
  { id: 3, title: "Serverless 101" }
];
const COMMENTS = [
  { postId: 1, content: "Great guide!" },
  { postId: 1, content: "Thanks for sharing!" },
  { postId: 2, content: "Can't wait to try them out!" }
];

// Get all posts. Write your own logic that reads from any data source.
function getPosts() {
  return POSTS;
}

// Get the comments for a single post.
function getCommentsForPost(postId) {
  return COMMENTS.filter(comment => comment.postId === postId);
}

/**
 * Using this as the entry point, you can use a single function to handle many resolvers.
 */
const resolvers = {
  Query: {
    posts: ctx => {
      return getPosts();
    },
  },
  Post: {
    comments: ctx => {
      return getCommentsForPost(ctx.source.id);
    },
  },
}

// event
// {
//   "typeName": "Query", /* Filled dynamically based on @function usage location */
//   "fieldName": "me", /* Filled dynamically based on @function usage location */
//   "arguments": { /* GraphQL field arguments via $ctx.arguments */ },
//   "identity": { /* AppSync identity object via $ctx.identity */ },
//   "source": { /* The object returned by the parent resolver. E.G. if resolving field 'Post.comments', the source is the Post object. */ },
//   "request": { /* AppSync request object. Contains things like headers. */ },
//   "prev": { /* If using the built-in pipeline resolver support, this contains the object returned by the previous function. */ },
// }
exports.handler = async (event) => {
  const typeHandler = resolvers[event.typeName];
  if (typeHandler) {
    const resolver = typeHandler[event.fieldName];
    if (resolver) {
      return await resolver(event);
    }
  }
  throw new Error("Resolver not found.");
};
```

**Example: Get the logged in user from Amazon Cognito User Pools**

When building applications, it is often useful to fetch information for the current user. You can use the `@function` directive to quickly add a resolver that uses AppSync identity information to fetch a user from Amazon Cognito User Pools. First make sure you have added Amazon Cognito User Pools enabled via `amplify add auth` and a GraphQL API via `amplify add api` to an amplify project. Once you have created the user pool, get the **UserPoolId** from **amplify-meta.json** in the **backend/** directory of your amplify project. You will provide this value as an environment variable in a moment. Next, using the Amplify function category, AWS console, or some other tool, deploy an AWS Lambda function with the following contents.

For example purposes assume the function is named `GraphQLResolverFunction`:

```js
/* Amplify Params - DO NOT EDIT
You can access the following resource attributes as environment variables from your Lambda function
var environment = process.env.ENV
var region = process.env.REGION
var authMyResourceNameUserPoolId = process.env.AUTH_MYRESOURCENAME_USERPOOLID

Amplify Params - DO NOT EDIT */

const { CognitoIdentityServiceProvider } = require('aws-sdk');
const cognitoIdentityServiceProvider = new CognitoIdentityServiceProvider();

/**
 * Get user pool information from environment variables.
 */
const COGNITO_USERPOOL_ID = process.env.AUTH_MYRESOURCENAME_USERPOOLID;
if (!COGNITO_USERPOOL_ID) {
  throw new Error(`Function requires environment variable: 'COGNITO_USERPOOL_ID'`);
}
const COGNITO_USERNAME_CLAIM_KEY = 'cognito:username';

/**
 * Using this as the entry point, you can use a single function to handle many resolvers.
 */
const resolvers = {
  Query: {
    echo: ctx => {
      return ctx.arguments.msg;
    },
    me: async ctx => {
      var params = {
        UserPoolId: COGNITO_USERPOOL_ID, /* required */
        Username: ctx.identity.claims[COGNITO_USERNAME_CLAIM_KEY], /* required */
      };
      try {
        // Read more: https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/CognitoIdentityServiceProvider.html#adminGetUser-property
        return await cognitoIdentityServiceProvider.adminGetUser(params).promise();
      } catch (e) {
        throw new Error(`NOT FOUND`);
      }
    }
  },
}

// event
// {
//   "typeName": "Query", /* Filled dynamically based on @function usage location */
//   "fieldName": "me", /* Filled dynamically based on @function usage location */
//   "arguments": { /* GraphQL field arguments via $ctx.arguments */ },
//   "identity": { /* AppSync identity object via $ctx.identity */ },
//   "source": { /* The object returned by the parent resolver. E.G. if resolving field 'Post.comments', the source is the Post object. */ },
//   "request": { /* AppSync request object. Contains things like headers. */ },
//   "prev": { /* If using the built-in pipeline resolver support, this contains the object returned by the previous function. */ },
// }
exports.handler = async (event) => {
  const typeHandler = resolvers[event.typeName];
  if (typeHandler) {
    const resolver = typeHandler[event.fieldName];
    if (resolver) {
      return await resolver(event);
    }
  }
  throw new Error("Resolver not found.");
};
```

You can connect this function to your AppSync API deployed via Amplify using this schema:

```graphql
type Query {
  posts: [Post] @function(name: "GraphQLResolverFunction")
}
type Post {
  id: ID!
  title: String!
  comments: [Comment] @function(name: "GraphQLResolverFunction")
}
type Comment {
  postId: ID!
  content: String
}
```

This simple lambda function shows how you can write your own custom logic using a language of your choosing. Try enhancing the example with your own data and logic.

> When deploying the function, make sure your function has access to the auth resource. You can run the `amplify update function` command for the CLI to automatically supply an environment variable named `AUTH_<RESOURCE_NAME>_USERPOOLID` to the function and associate corresponding CRUD policies to the execution role of the function.

After deploying your function, you can connect it to AppSync by defining some types and using the @function directive. Add this to your schema, to connect the
`Query.echo` and `Query.me` resolvers to your new function.

```graphql
type Query {
  me: User @function(name: "ResolverFunction")
  echo(msg: String): String @function(name: "ResolverFunction")
}
# These types derived from https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/CognitoIdentityServiceProvider.html#adminGetUser-property
type User {
  Username: String!
  UserAttributes: [Value]
  UserCreateDate: String
  UserLastModifiedDate: String
  Enabled: Boolean
  UserStatus: UserStatus
  MFAOptions: [MFAOption]
  PreferredMfaSetting: String
  UserMFASettingList: String
}
type Value {
  Name: String!
  Value: String
}
type MFAOption {
  DeliveryMedium: String
  AttributeName: String
}
enum UserStatus {
  UNCONFIRMED
  CONFIRMED
  ARCHIVED
  COMPROMISED
  UNKNOWN
  RESET_REQUIRED
  FORCE_CHANGE_PASSWORD
}
```

Next run `amplify push` and wait as your project finishes deploying. To test that everything is working as expected run `amplify api console` to open the GraphiQL editor for your API. You are going to need to open the Amazon Cognito User Pools console to create a user if you do not yet have any. Once you have created a user go back to the AppSync console's query page and click "Login with User Pools". You can find the **ClientId** in **amplify-meta.json** under the key **AppClientIDWeb**. Paste that value into the modal and login using your username and password. You can now run this query:

```graphql
query {
  me {
    Username
    UserStatus
    UserCreateDate
    UserAttributes {
      Name
      Value
    }
    MFAOptions {
      AttributeName
      DeliveryMedium
    }
    Enabled
    PreferredMfaSetting
    UserMFASettingList
    UserLastModifiedDate
  }
}
```

which will return user information related to the current user directly from your user pool.

### Structure of the function event

When writing lambda functions that are connected via the `@function` directive, you can expect the following structure for the AWS Lambda event object.

| Key  | Description  |
|---|---|
| typeName  | The name of the parent object type of the field being resolver.  |
| fieldName  | The name of the field being resolved.  |
| arguments  | A map containing the arguments passed to the field being resolved.  |
| identity  | A map containing identity information for the request. Contains a nested key 'claims' that will contains the JWT claims if they exist. |
| source  | When resolving a nested field in a query, the source contains parent value at runtime. For example when resolving `Post.comments`, the source will be the Post object.  |
| request   | The AppSync request object. Contains header information.  |
| prev | When using pipeline resolvers, this contains the object returned by the previous function. You can return the previous value for auditing use cases. |

Your function should follow the lambda handler guidelines for your specific language. See the developer guides from the
[AWS Lambda](https://docs.aws.amazon.com/lambda/latest/dg/welcome.html) documentation for your chosen language. If you choose to use structured types, your type should serialize
the AWS Lambda event object outlined above. For example, if using Golang, you should define a struct with the above fields.

### Calling functions in different regions

By default, you expect the function to be in the same region as the amplify project. If you need to call a function in a different (or static) region, you can provide the **region** argument.

```graphql
type Query {
  echo(msg: String): String @function(name: "echofunction", region: "us-east-1")
}
```

Calling functions in different AWS accounts is not supported via the @function directive but is supported by AWS AppSync.

### Chaining functions

The @function directive supports AWS AppSync pipeline resolvers. That means, you can chain together multiple functions such that they are invoked in series when your field's resolver is invoked. To create a pipeline resolver that calls out to multiple AWS Lambda functions in series, use multiple `@function` directives on the field.

```graphql
type Mutation {
  doSomeWork(msg: String): String @function(name: "worker-function") @function(name: "audit-function")
}
```

In the example above when you run a mutation that calls the `Mutation.doSomeWork` field, the **worker-function** will be invoked first then the **audit-function** will be invoked with an event that contains the results of the **worker-function** under the **event.prev.result** key. The **audit-function** would need to return **event.prev.result** if you want the result of **worker-function** to be returned for the field. Under the hood, Amplify creates an `AppSync::FunctionConfiguration` for each unique instance of `@function` in a document and a pipeline resolver containing a pointer to a function for each `@function` on a given field.

#### Generates

The `@function` directive generates these resources as necessary:

1. An AWS IAM role that has permission to invoke the function as well as a trust policy with AWS AppSync.
2. An AWS AppSync data source that registers the new role and existing function with your AppSync API.
3. An AWS AppSync pipeline function that prepares the lambda event and invokes the new data source.
4. An AWS AppSync resolver that attaches to the GraphQL field and invokes the new pipeline functions.

