Building APIs with Amazon API Gateway

1. Building APIs with Amazon API Gateway

Welcome back. Last video we shaped the application: stateless, loosely coupled, resilient. Now we put a front door on it with Amazon API Gateway, controlling how every request is validated, transformed, and routed to your backend. Let's get started.

2. Three clients, one backend

Your backend serves a web app, a mobile app, and a partner integration, and each speaks a slightly different dialect. Without a front door, your backend code drowns in special cases. API Gateway gives you one clean boundary to validate, transform, and protect every caller.

3. What API Gateway does

Amazon API Gateway is a fully managed front door for your APIs. Clients talk to API Gateway, and it handles the cross-cutting concerns: routing requests, checking authorization, throttling abusive callers, and transforming payloads. Behind it sits your backend, which might be a Lambda function, AWS's way of running code without managing servers, an HTTP service, or another AWS service entirely. Because the gateway owns that boundary, you can change or protect the backend without changing how clients call you. That boundary is where a lot of deployment-time decisions live.

4. Integration types

API Gateway connects to a backend through an integration, and the type matters. Lambda proxy, meaning the gateway passes the request through untouched, hands the whole request to your function and returns what it sends back; simplest and most common. Lambda non-proxy lets you map request and response fields with templates, for more control. HTTP integration forwards to any HTTP endpoint. MOCK returns a response straight from the gateway with no backend, perfect for a health check or fixed value. Match the integration to how much transformation you need.

5. Request and response transformations

Sometimes the client speaks a different shape than your backend expects. Mapping templates fix that with the Velocity Template Language, or VTL. On the way in, a request template turns a query string or flat payload into the structure your Lambda wants. On the way out, a response template reshapes the backend's response into the contract clients expect. This keeps a clean public API even when the backend changes underneath. Mapping templates apply only to non-proxy integrations.

6. Request validation

The cheapest request to handle is the one you reject early. API Gateway can validate input before it ever invokes your backend, so malformed calls never cost you a Lambda invocation. You can require specific parameters, like a header or query string, and you can attach a model schema that defines the required JSON body shape. The validator runs in one of three modes: validate the body, validate the parameters, or validate both. Pushing validation to the gateway keeps your function code focused on real work.

7. Status codes and error contracts

Your clients deserve a predictable error contract, even when the backend misbehaves. API Gateway lets you override status codes, so a messy 502 from a backend, a backend failure, can become a clean, documented 503 for your clients. Gateway responses let you customize the body of common errors, like a 404, a missing resource, or a throttling 429, so every error looks consistent. The goal is that clients integrate against your contract, not against your backend's accidents. This is what makes an API feel stable across deployments.

8. Putting it together

Here is the full path. The gateway validates the request first; bad input is rejected immediately with a clean error. Valid input is reshaped by a request mapping template into the structure the backend expects. The backend does its work and returns a response, which a response mapping template reshapes into your public contract, with status code overrides keeping errors consistent. The client never sees the messy internals. That is API Gateway doing the heavy lifting at the boundary.

9. Let's practice!

Let's head to the exercises and build that front door.

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