Pipeline resolvers (VTL)

AWS AppSync executes resolvers on a GraphQL field. In some cases, applications require executing multiple operations to resolve a single GraphQL field. With pipeline resolvers, developers can now compose operations called Functions and execute them in sequence. Pipeline resolvers are useful for applications that, for instance, require performing an authorization check before fetching data for a field.

A pipeline resolver is composed of a Before mapping template, an After mapping template, and a list of Functions. Each Function has a request and response mapping template that it executes against a data source. As a pipeline resolver delegates execution to a list of functions, it is therefore not linked to any data source. Unit resolvers and functions are primitives that execute operation against data sources. See the Resolver mapping template overview for more information.

Create A Pipeline Resolver

In the AWS AppSync console, go to the Schema page.

Save the following schema:

schema {
    query: Query
    mutation: Mutation
}

type Mutation {
    signUp(input: Signup): User
}

type Query {
    getUser(id: ID!): User
}

input Signup {
    username: String!
    email: String!
}

type User {
    id: ID!
    username: String
    email: AWSEmail
}

We are going to wire a pipeline resolver to the signUp field on the Mutation type. In the Mutation type on the right side, choose Attach resolver next to the signUp field. On the Create Resolver page, click on the Switch to Pipeline button. The page should now show 3 sections, a Before Mapping Template text area, a Functions section, and a After Mapping template text area.

Our pipeline resolver signs up a user by first validating the email address input and then saving the user in the system. We are going to encapsulate the email validation inside a validateEmail function, and the saving of the user inside a saveUser function. The validateEmail function executes first, and if the email is valid, then the saveUser function executes.

The execution flow will be as follow:

1. Mutation.signUp resolver request mapping template

2. validateEmail function

3. saveUser function

4. Mutation.signUp resolver response mapping template

Because we will probably reuse the validateEmail function in other resolvers on our API, we want to avoid accessing $ctx.args since these will change from one GraphQL field to another. Instead, we can use the $ctx.stash to store the email attribute from the signUp(input: Signup) input field argument.

BEFORE mapping template:

## store email input field into a generic email key
$util.qr($ctx.stash.put("email", $ctx.args.input.email))
{}

The console provides a default passthrough AFTER mapping template that will we use:

$util.toJson($ctx.result)

Create A Function

From the pipeline resolver page, on the Functions section, click on Create Function. It is also possible to create functions without going through the resolver page, to do this, in the AWS AppSync console, go to the Functions page. Choose the Create Function button. Let’s create a function that checks if an email is valid and comes from a specific domain. If the email is not valid, the function raises an error. Otherwise, it forwards whatever input it was given.

Select NONE data source on the function page, and fill in the validateEmail request mapping template:

#set($valid = $util.matches("^[a-zA-Z0-9_.+-]+@(?:(?:[a-zA-Z0-9-]+\.)?[a-zA-Z]+\.)?(myvaliddomain)\.com", $ctx.stash.email))
#if (!$valid)
    $util.error("$ctx.stash.email is not a valid email.")
#end
{
    "payload": { "email": $util.toJson(${ctx.stash.email}) }
}

and response mapping template:

$util.toJson($ctx.result)

We just created our validateEmail function. Repeat these steps to create the saveUser function with the following request and response mapping templates. For the sake of simplicity we use a NONE data source and pretend the user has been saved in the system after the function executes.

Request mapping template:

## $ctx.prev.result contains the signup input values. We could have also
## used $ctx.args.input.
{
    "payload": $util.toJson($ctx.prev.result)
}

and response mapping template:

## an id is required so let's add a unique random identifier to the output
$util.qr($ctx.result.put("id", $util.autoId()))
$util.toJson($ctx.result)

We just created our saveUser function.

Adding a Function to a Pipeline Resolver

Our functions should have been automatically added to the pipeline resolver we just created. If you happened to have created the functions through the console Functions page, you can click on Add Function on the resolver page to attach them. Add both validateEmail and saveUser functions to the resolver. The validateEmail function should be placed before the saveUser function. As you add more functions you can use the up and down arrows to reorganize the order of execution of your functions.

Executing a Query

In the AWS AppSync console, go to the Queries page. Enter the following query:

mutation {
  signUp(input: {
    email: "[e-mail]"
    username: "nadia"
  }) {
    id
    username
  }
}

This should return something like:

{
  "data": {
    "signUp": {
      "id": "256b6cc2-4694-46f4-a55e-8cb14cc5d7fc",
      "username": "nadia"
    }
  }
}

We have successfully signed up our user and validated the input email using a pipeline resolver. To follow a more complete tutorial focusing on pipeline resolvers, you can go to Tutorial: Pipeline Resolvers