REST API vs HTTP API vs WebSocket API

Amazon API Gateway is a fully managed service that lets you create, publish, secure, and monitor APIs at any scale. It acts as the front door for your backend services—Lambda functions, EC2 instances, HTTP backends, or any AWS service. API Gateway handles traffic management, authorisation, throttling, monitoring, and API versioning, so your backend can focus on business logic rather than API infrastructure concerns.

REST API (original API Gateway product) provides the widest feature set: request/response transformation with mapping templates, per-method throttling, usage plans with API keys, response caching, WAF integration, X-Ray tracing, resource policies, and client certificate mutual TLS. REST APIs support all integration types: Lambda, HTTP, AWS service, Mock, and Lambda Proxy. Use REST API when you need advanced features like transformation, caching, or usage plans.

HTTP API was designed as a simpler, cheaper, faster alternative to REST API. It supports Lambda proxy and HTTP proxy integrations only—no AWS service integrations, no mock. Key advantages: up to 70% lower cost than REST API, lower latency, native OIDC and OAuth 2.0 JWT authorisation built in (no Lambda authoriser needed for common auth scenarios), and automatic deployment. If you don't need caching, usage plans, or request/response transformation, HTTP API is the better choice.

WebSocket API maintains persistent, bidirectional connections between clients and the server. Unlike HTTP (request-response), WebSocket allows the server to push messages to connected clients at any time without the client polling. API Gateway manages WebSocket connections and routes messages to Lambda functions based on route expressions. Use WebSocket APIs for real-time applications: chat apps, live dashboards, collaborative editing, gaming, stock tickers.

WebSocket APIs have three built-in routes: $connect (triggered when a client opens a connection), $disconnect (triggered when a connection closes), and $default (catches unmatched messages). You can add custom routes like sendmessage mapped to specific Lambda functions. Use the @connections management API to send messages back to connected clients from your Lambda function using the client's connection ID.

Key differences at a glance:

• REST API: full features (caching, usage plans, transforms, WAF), higher cost, supports all integration types
• HTTP API: Lambda/HTTP proxy only, 70% cheaper, built-in JWT auth, lower latency, no caching or usage plans
• WebSocket API: persistent bidirectional connections, server-push capable, billed per million messages and per minute of connection time

For the SAA-C03 exam: HTTP questions without real-time or advanced features → HTTP API. Real-time push → WebSocket. Complex API features → REST API.

API Gateway APIs are deployed to stages (e.g., dev, staging, prod). Each stage has its own URL, throttling settings, and can reference a specific deployment snapshot. Use stage variables (similar to environment variables) to parameterise backend endpoints per stage—for example, point the dev stage to a dev Lambda alias and prod to the prod alias without duplicating API configuration.

By default API Gateway URLs contain the API ID (e.g., abc123.execute-api.us-east-1.amazonaws.com). For production use, create a custom domain name backed by an ACM certificate and map it to your API and stage. Use base path mappings to host multiple APIs under one domain (e.g., api.example.com/orders → Orders API, api.example.com/users → Users API). Custom domain names require a Route 53 alias record pointing to the API Gateway endpoint.

REST APIs can be deployed in three endpoint types: Edge-Optimised (CloudFront front-end for global distribution, default), Regional (no CloudFront, lower latency for same-region clients or when you add your own CloudFront), and Private (only accessible from within your VPC via an interface VPC endpoint, for internal microservices). HTTP APIs support Edge-Optimised and Regional. WebSocket APIs support Regional and Private. Choose Regional for APIs consumed from the same region or when using custom CloudFront distributions.

API Gateway REST APIs support canary deployments on a stage. You can route a percentage of traffic to a canary deployment (new version) while the rest goes to the production deployment. Monitor error rates and latency on the canary; if stable, promote it to 100%. If problems arise, roll back by setting canary weight to 0. This is analogous to Lambda alias weighted routing and allows safe, gradual API updates without blue/green environment switching.

SAA-C03 exam question keywords to identify API type: 'cost-effective simple API' → HTTP API; 'real-time bidirectional' or 'server push' or 'chat' → WebSocket API; 'request transformation', 'usage plans', 'API key throttling', 'response caching' → REST API. When the question is simply 'expose Lambda as an HTTP endpoint with low cost', HTTP API wins. When the question involves complex API management features or partners, REST API is usually correct.

Test your understanding of AWS Solutions Architect (SAA-C03) concepts from this lesson.

In this lesson you learned: REST API provides the full API Gateway feature set (caching, transforms, usage plans, WAF) at a higher cost, HTTP API is 70% cheaper with built-in JWT auth and lower latency, ideal for Lambda/HTTP proxy scenarios without advanced features, and WebSocket API enables server-push real-time communication over persistent connections for chat, gaming, and live data applications. Next up we explore API Gateway integrations with Lambda, HTTP backends, and mock responses.