API Gateway Request and Response Mapping for Serverless APIs
When building serverless APIs with AWS Lambda, Amazon API Gateway acts as the front door. A crucial aspect of this integration is managing how data flows between the client, API Gateway, and your Lambda function. This is achieved through Request and Response Mapping.
Understanding Request Mapping
Request mapping allows you to transform the incoming HTTP request from a client before it's sent to your backend integration, such as an AWS Lambda function. This transformation can involve selecting specific parts of the request (like headers, query parameters, or the request body), restructuring them, or even adding new elements.
Request mapping transforms incoming client requests for backend processing.
You can use mapping templates to extract, reshape, and enrich data from HTTP requests (headers, query strings, body) before they reach your Lambda function. This ensures your Lambda function receives data in the format it expects.
API Gateway supports mapping templates using Apache Velocity Template Language (VTT) or JavaScript Object Notation (JSON) for request mapping. These templates define how to construct the payload that will be sent to your backend. For example, you might extract a user ID from a JWT token in the headers, combine it with a query parameter, and pass it as a single JSON object to your Lambda function. This decouples the client's request format from your Lambda function's expected input format.
Understanding Response Mapping
Response mapping is the inverse of request mapping. It allows you to transform the response received from your backend integration (e.g., AWS Lambda) before it's sent back to the client. This is useful for formatting the output, removing sensitive information, or adapting the response to meet the client's expectations.
Response mapping shapes the backend's output for the client.
Similar to request mapping, you can use mapping templates to modify the data returned by your Lambda function. This might include filtering out internal details, renaming fields, or structuring the data into a specific JSON format that the client application can easily consume.
Response mapping templates are also written in VTL or JSON. They take the output from your Lambda function (typically a JSON object) and transform it into the desired HTTP response. For instance, if your Lambda function returns a complex object with many internal fields, you might use a response mapping template to select only the essential data fields and present them to the client in a clean, predictable structure. This also helps in versioning your API responses independently of your backend logic.
Key Concepts and Benefits
| Feature | Request Mapping | Response Mapping |
|---|---|---|
| Purpose | Transform incoming request for backend | Transform backend response for client |
| Input | Client HTTP request (headers, body, params) | Backend integration output (e.g., Lambda JSON) |
| Output | Payload for backend integration | HTTP response for client |
| Use Cases | Data validation, enrichment, format adaptation | Data sanitization, formatting, versioning |
Mapping templates provide a powerful way to decouple your API's frontend from its backend, allowing for greater flexibility and maintainability.
Example Scenario: User Profile API
Consider an API endpoint to fetch a user's profile. The client might send a request like this:
Client Request: GET /users/123?fields=name
Your Lambda function might expect a payload like this:
Lambda Input Payload: { "userId": "123", "requestedFields": ["name", "email"], "authHeader": "Bearer <token>" }
A request mapping template in API Gateway can transform the incoming request into the expected Lambda input. This involves extracting the path parameter userId, the query string parameter fields and splitting it into an array, and the Authorization header. The template would construct a JSON object with these elements.
Text-based content
Library pages focus on text content
Similarly, if your Lambda function returns a detailed user object, a response mapping template can select only the
name
To transform the incoming client request before it reaches the backend integration (e.g., Lambda).
Apache Velocity Template Language (VTL) and JSON.
Learning Resources
Official AWS documentation detailing how to transform requests and responses in API Gateway, covering mapping templates and their capabilities.
A comprehensive reference for Apache Velocity Template Language (VTL) syntax and context variables available within API Gateway mapping templates.
A video tutorial explaining the concepts and practical application of mapping templates in AWS API Gateway for request and response manipulation.
A blog post from AWS that covers building serverless APIs, often touching upon the role of API Gateway and data transformation.
An in-depth article exploring the nuances of API Gateway mapping templates, including practical examples and best practices.
A community discussion on AWS re:Post featuring practical examples of request mapping for various scenarios.
Explains different integration types in API Gateway, which is foundational to understanding where mapping applies.
Provides context on JSONPath, a query language often used in conjunction with JSON data, which can be relevant for understanding data extraction in mapping.
A video tutorial demonstrating the end-to-end process of creating a serverless API, likely including mapping configurations.
A blog post focusing on more advanced uses and techniques for API Gateway mapping templates, offering practical tips.