An API (Application Programming Interface) is a defined way for one piece of software to request something from another — a mobile app asking a server for a user’s order history, for example. Building the backend logic to fetch that data is only half the job. You also need to handle authentication, block abusive traffic, validate incoming requests, and route each request to the right piece of code — all before your actual business logic ever runs.

Amazon API Gateway is a fully managed service that handles exactly this “front door” layer. You define your API’s shape — its paths, methods, and what backend each one calls — and API Gateway takes care of receiving internet traffic, enforcing security rules, and routing valid requests onward, typically to services like AWS Lambda, EC2, or other HTTP endpoints.

An API is the top-level container. Inside it, resources represent URL paths (like /orders or /orders/{orderId}), and each resource can have one or more methods (GET, POST, PUT, DELETE) attached. Each method connects to an integration — the actual backend that handles the request, commonly a Lambda function, but it can also be another AWS service or any HTTP endpoint.

flowchart TB
    A[Client Application] -->|HTTPS Request| B[API Gateway Endpoint]
    B --> C[Authorizer - IAM, Cognito, or Lambda]
    C -->|Authorized| D[Resource and Method Routing]
    C -->|Denied| E[401 or 403 Response]
    D --> F[Integration - Lambda Function]
    D --> G[Integration - HTTP Backend]
    D --> H[Integration - Other AWS Service]
    F --> I[Response Mapping]
    G --> I
    H --> I
    I --> A
        

Everything you configure — resources, methods, integrations, authorizers — sits in a draft state until it is explicitly deployed to a stage. This separation between configuration and deployment is what allows you to build and test changes safely before they affect real traffic.

When a request arrives, API Gateway first matches it against your defined resources and methods to determine which integration should handle it. If an authorizer is attached, API Gateway invokes it first — this might mean validating a JSON Web Token against Amazon Cognito, checking an IAM signature, or calling a custom Lambda function that returns an allow or deny decision.

Once authorized, API Gateway can transform the incoming request using mapping templates, reshaping the request body or headers into whatever format your backend expects, and later reshaping the backend’s response back into whatever format the client expects. This transformation layer means your backend code does not need to know or care about the exact shape of the original HTTP request.

Every stage in this lifecycle can be observed independently, which is exactly why API Gateway pairs so naturally with detailed request logging and tracing — a slow API can usually be traced to one specific stage in this chain rather than treated as one big mystery.

The trade-off is centralized control versus a small added hop. You gain a single, consistent place to manage security, throttling, and routing across every API you expose, at the cost of one additional network hop between the client and your actual backend logic.

API Gateway scales automatically to handle sudden increases in traffic without any manual provisioning. Throttling protects both your API and its backend by limiting how many requests a client can send per second, preventing a single misbehaving consumer from overwhelming the system for everyone else.

For read-heavy endpoints, enabling the response cache lets API Gateway serve repeated identical requests without invoking the backend at all, similar in spirit to how a CDN caches static content. HTTP APIs, a newer and lighter-weight API type, also offer lower latency and lower cost than the original REST API type for many common use cases.

API Gateway is a fully managed, regionally distributed service, meaning it runs across multiple Availability Zones within a region without requiring you to configure or manage that redundancy yourself. There is no single server whose failure would take your API down.

Reliability at the API layer also depends on how integrations are configured — using retries and reasonable timeouts on the backend integration side, and designing Lambda functions or backend servers to handle transient failures gracefully, since API Gateway’s own availability is only one part of the overall reliability picture.

API Gateway supports several layered approaches to controlling who can call your API. IAM authorization uses AWS’s own identity and permission system, well-suited for service-to-service calls within AWS. Amazon Cognito authorizers validate user identity tokens, ideal for mobile and web application users. Lambda authorizers let you write fully custom authentication logic, such as validating a third-party token format.

Amazon CloudWatch automatically tracks metrics such as request count, latency, and error rates (4xx client errors and 5xx server errors) for every API and stage. Execution logs can be enabled to capture detailed information about each request’s journey through authorization, integration, and response mapping.

AWS X-Ray integration provides distributed tracing across the entire request path, showing exactly how much time was spent in API Gateway itself versus the backend integration versus any further downstream calls — invaluable for pinpointing where latency actually originates.

API Gateway supports three distinct API types. REST APIs offer the fullest feature set, including request validation, usage plans, and detailed transformation capabilities. HTTP APIs are a newer, lighter, lower-latency, lower-cost option covering the most common proxy use cases. WebSocket APIs support persistent, two-way connections, suited for chat applications or real-time notifications.

APIs can also be deployed as private APIs, reachable only from within a specified VPC, which is common for internal microservices that should never be exposed to the public internet at all.

A common and powerful pattern is the serverless API: API Gateway as the front door, Lambda functions as the backend logic, and DynamoDB or another managed database behind that — an entire application with zero servers to patch or scale manually. Another useful pattern is the API facade, where API Gateway presents a clean, stable public interface while quietly routing to multiple different internal backends or even legacy systems behind the scenes.