VTL & JS Resolvers in AWS AppSync

Introduction

AWS AppSync is a managed GraphQL service that enables developers to build scalable applications. Within AppSync, resolvers are essential components that handle requests and responses for data operations. This lesson focuses on two types of resolvers: VTL (Velocity Template Language) and JavaScript resolvers.

VTL Resolvers

What is VTL?

VTL is a template language used to define the request and response mapping for resolvers in AWS AppSync. It is designed to be simple and efficient, allowing developers to manipulate data with minimal overhead.

Note: VTL is primarily used for simple use cases where data transformation is straightforward.

Creating a VTL Resolver

To create a VTL resolver, follow these steps:

Navigate to your AppSync API in the AWS Management Console.
Go to the Schema section and select the type you want to add a resolver for.
Click on Add Resolver and choose VTL as the resolver type.
Define the Request Mapping Template and Response Mapping Template using VTL syntax.

Example of VTL Request Mapping Template


{
  "version": "2018-05-29",
  "operation": "GetItem",
  "key": {
    "id": $util.dynamodb.toDynamoDBJson($ctx.args.id)
  }
}

JavaScript Resolvers

What are JavaScript Resolvers?

JavaScript resolvers allow you to write custom logic in JavaScript to handle data operations. This is useful for more complex scenarios where VTL may not be sufficient.

Tip: Use JavaScript resolvers when you need to perform complex data manipulations or integrate with external APIs.

Creating a JavaScript Resolver

Follow these steps to create a JavaScript resolver:

Open your AppSync API in the AWS Management Console.
Select the type for which you want to add a resolver.
Click on Add Resolver and select JavaScript as the resolver type.
Write your JavaScript code in the editor provided.

Example of JavaScript Resolver


exports.handler = async (event) => {
    const id = event.arguments.id;
    const result = await getDataFromDatabase(id);
    return result;
};

Comparison of VTL and JavaScript Resolvers

Feature	VTL Resolvers	JavaScript Resolvers
Complexity	Simple	Complex
Performance	High	Variable
Use Cases	Basic CRUD operations	Custom logic and integrations

Best Practices

Use VTL for simple data operations to maximize performance.
Choose JavaScript resolvers for complex data manipulations.
Always validate input data in your resolvers.
Monitor performance and error logs regularly.

FAQ

What is the primary advantage of using VTL?

VTL is optimized for performance and is straightforward for simple use cases, making it an efficient choice for basic CRUD operations.

When should I use JavaScript resolvers instead of VTL?

Use JavaScript resolvers when you need to incorporate complex logic, perform external API calls, or manage intricate data transformations.

Can I mix VTL and JavaScript resolvers in my AppSync API?

Yes, you can use both resolver types in your API, depending on the specific needs of each data operation.