API Gateway Patterns

Content Overview

1. Ocelot Configuration

2. Custom Middleware

3. API Composition

4. Error Handling

A Simple Explanation

Imagine a hotel front desk:
An API Gateway is like the front desk of a hotel. Guests (clients) don’t go directly to their rooms (services); instead, they check in at the front desk, which handles requests, gives directions, answers questions, and enforces rules. The front desk makes sure every guest gets what they need, safely and efficiently, without ever seeing the complexity behind the scenes.

What is an API Gateway?
An API Gateway is a server that sits between clients (like browsers or mobile apps) and backend services. It receives all requests, routes them to the right service, applies security, combines data if needed, and returns a single, unified response.

Why Do API Gateways Exist?

Problem: Modern apps are built from many small services (microservices). If every client had to talk to every service directly, things would get messy, slow, and insecure.
Solution: The API Gateway acts as a single entry point, simplifying communication, improving security, and enabling advanced features like rate limiting, caching, and monitoring.

How does an API Gateway help?

Hides the complexity of backend services
Centralizes authentication and security
Enables request/response transformation
Aggregates data from multiple services
Handles errors and retries
Supports versioning and monitoring

The Absolute Basics: How API Gateways Work

Client sends a request to the API Gateway
Gateway checks authentication, applies rules, and routes the request
Gateway may call one or more backend services
Gateway combines responses (if needed) and sends a single response back to the client

Common Patterns:

Routing: Direct requests to the correct service
Aggregation (API Composition): Combine data from multiple services
Transformation: Change request/response formats
Authentication & Authorization: Enforce security policies
Rate Limiting: Prevent abuse by limiting requests
Caching: Store frequent responses for speed
Error Handling: Return friendly error messages

Practical Example: E-Commerce API Gateway

Scenario: An online store has separate services for products, orders, and users. The mobile app just wants to show a dashboard with all this info.

Client: Mobile app requests /dashboard
API Gateway: Receives the request, checks authentication
Gateway: Calls Product, Order, and User services
Gateway: Combines the results into a single response
Client: Receives a unified dashboard view

Sample Ocelot Configuration (ASP.NET Core):

{
  "Routes": [
    {
      "DownstreamPathTemplate": "/api/products",
      "UpstreamPathTemplate": "/products",
      "UpstreamHttpMethod": [ "Get" ],
      "AuthenticationOptions": {
        "AuthenticationProviderKey": "Bearer"
      }
    }
  ]
}

Real-World Use Cases

Mobile Backends: Provide a single API for mobile apps, hiding backend complexity
Microservices Aggregation: Combine results from many services into one response
Security Enforcement: Centralize authentication, authorization, and rate limiting
Legacy Modernization: Expose old systems through a modern API
API Monetization: Control and monitor usage for paid APIs
Multi-Channel Delivery: Serve web, mobile, and IoT clients from one entry point

Related Concepts to Explore

Reverse Proxy (NGINX, Envoy)
Service Mesh (Istio, Linkerd)
Backend-for-Frontend (BFF) Pattern
Microservices Architecture
Service Discovery
Load Balancing
Rate Limiting
Authentication & Authorization
Request/Response Transformation
API Versioning
Circuit Breaker Pattern
Caching Strategies
Monitoring & Observability
Ocelot (ASP.NET Core API Gateway)
Kong, Apigee, AWS API Gateway, Azure API Management

Summary

API Gateways are the front desk of modern distributed systems. They simplify client interactions, centralize security, and enable powerful features for scaling, monitoring, and evolving your APIs. If you want to build robust, secure, and flexible APIs, mastering API Gateway patterns is essential.

Isaac.