API Gateway Pattern

One door to rule them all

The Microservices Problem

When you have multiple microservices, clients need to know about each one:

Problems:

Clients need to know all services - Mobile apps must hardcode multiple service URLs, making updates difficult
Multiple round trips - Fetching related data requires multiple requests, increasing latency
Cross-cutting concerns duplicated (auth, rate limiting) - Each service implements its own authentication and rate limiting logic
Hard to change service locations - Moving services requires updating all clients

Solution: API Gateway - A single entry point that routes requests, handles cross-cutting concerns, and aggregates responses.

What is an API Gateway?

API Gateway is a single entry point that sits between clients and microservices. It acts as a reverse proxy that routes client requests to appropriate backend services, handles cross-cutting concerns like authentication and rate limiting, and can aggregate responses from multiple services.

The Core Concept

Instead of clients directly calling multiple microservices, they make requests to a single API Gateway. The gateway handles routing, security, monitoring, and other cross-cutting concerns, allowing backend services to focus on business logic. This pattern simplifies client code, centralizes security, and provides a unified interface to your microservices architecture.

Key Responsibilities

Request Routing - Route to correct service
Authentication - Verify who you are
Authorization - Check permissions
Rate Limiting - Control request volume
Load Balancing - Distribute load
Request Aggregation - Combine multiple calls
Protocol Translation - REST to gRPC, etc.
Monitoring - Logging, metrics

Core Functions

1. Request Routing

Route requests to appropriate services:

Routing Examples:

Request	Route To
`GET /api/users/123`	User Service
`POST /api/orders`	Order Service
`GET /api/products`	Product Service
`POST /api/payments`	Payment Service

2. Authentication & Authorization

Centralized security:

Benefits:

Services don’t need auth logic - Backend services can focus on business logic without implementing authentication
Consistent security - All requests go through the same security checks, ensuring uniform enforcement
Token validation once - Validate tokens at the gateway instead of in every service, improving performance

3. Rate Limiting

Control request volume per client:

4. Request Aggregation

Combine multiple service calls:

Without Gateway (Multiple Requests):

1
Client → User Service (get user)
2
Client → Order Service (get orders)
3
Client → Product Service (get products)
4
= 3 round trips

With Gateway (Aggregation):

1
Client → Gateway → [User, Order, Product Services] → Gateway → Client
2
= 1 round trip

Implementation Example

Simple API Gateway