Decoupling, Sync or Async, and API Gateway

What it is

Two components are tightly coupled when one calls the other directly and waits for the answer. If the callee is slow, down or changed, the caller breaks, and both must scale together.

Loosely coupled components talk through a buffer or a broker. The sender hands off a message and moves on, so each side can fail, scale and deploy on its own.

The second decision is synchronous or asynchronous. A synchronous caller blocks until it gets a result. An asynchronous caller gets an acknowledgment and the work finishes later.

How it works

Choosing the decoupling service

A queue absorbs spikes. If a consumer is slow, messages wait instead of failing the producer. Because SQS can deliver a message more than once, consumers must be idempotent, meaning that processing the same message twice gives the same result.

Synchronous or asynchronous

When a request needs more time than a caller will wait, return an HTTP 202 (accepted) with a job ID, do the work through a queue, and let the client poll or receive a notification. In a REST API, the Lambda integration can be made asynchronous by setting the integration request header X-Amz-Invocation-Type to 'Event'. API Gateway can also call SQS directly as an AWS service integration, so no function is needed in the middle.

Shaping a REST API in API Gateway

A REST API method has four stages: method request, integration request, integration response and method response. You change data at the two integration steps.

• Lambda proxy integration passes the whole request to the function as one event. The function must return statusCode, headers and body, and the body must be a string. No mapping templates are used.
• Non proxy integration lets you transform. A mapping template, written in Velocity Template Language (VTL), rewrites the request before the backend and the response before the client.
• HTTP APIs are the simpler, cheaper type. They have no VTL templates and use parameter mapping instead.

{
  "orderId": "$input.params('id')",
  "quantity": $input.json('$.qty')
}

This template builds a backend payload from a path parameter and a body field.

Enforcing validation rules

Attach a model (a JSON Schema) to the method request and choose a request validator. The validator can check the body, and required query string parameters and headers. An invalid call gets a 400 response from API Gateway, and your function is never invoked. Use gateway responses to customize those error bodies.

Maintaining the API

Changes to a REST API do not go live until you create a deployment to a stage. Stages such as dev and prod have their own URLs. Stage variables let one API point at different Lambda aliases or backends per stage. The default integration timeout for a REST API is 29 seconds.

How the exam asks about this

• "A web app calls an API that takes several minutes to finish. Users must not wait." -> Accept the request, put the work on an SQS queue (or invoke Lambda asynchronously), and return 202.
• "Reject requests with a missing field before they reach Lambda, with the least code." -> A model plus a request validator on the method.
• "A legacy backend expects a different JSON shape, and you cannot change it." -> A mapping template on a non proxy integration.