What Is an API Gateway? The Front Door of Microservices

In microservices architecture, you have dozens of services. Should the client talk to a different service each time? Of course not. That's where the API Gateway comes in — the single entry point for all requests.

Definition

An API Gateway is a middleware layer between the client (web/mobile app) and backend services. It receives all incoming requests, routes them to the correct service, and returns the response to the client.

Client (Web/Mobile)
        ↓
┌─────────────────────┐
│    API GATEWAY       │
│  - Authentication    │
│  - Rate Limiting     │
│  - Load Balancing    │
│  - Request Routing   │
└──────────┬──────────┘
     ↓     ↓     ↓
  Service Service Service
    A       B       C

Why Use an API Gateway?

1. Single Entry Point

Clients don't need to know the addresses of dozens of services. They access everything through a single URL.

2. Authentication & Authorization

Identity verification and authorization are handled centrally. No need for each service to implement its own auth logic.

3. Rate Limiting

Limits API requests to protect against DDoS attacks and excessive usage.

4. Request/Response Transformation

Creates responses in the format the client expects. Can aggregate responses from multiple backend services.

5. Load Balancing

Distributes requests across multiple service instances to balance the load.

6. Caching

Caches frequently requested data to reduce backend load and improve response times.

7. Monitoring & Logging

Centrally monitors, logs, and analyzes all API traffic.

Popular API Gateway Solutions

| Solution | Type | Advantage | Use Case | |----------|------|-----------|----------| | Kong | Open Source | Plugin ecosystem | General purpose | | AWS API Gateway | Managed | AWS integration | AWS ecosystem | | Nginx | Open Source | Performance | Reverse proxy + gateway | | Traefik | Open Source | Docker/K8s integration | Container environments | | Azure API Management | Managed | Enterprise features | Azure ecosystem | | Envoy | Open Source | Service mesh | With Istio |

API Gateway Patterns

Backend for Frontend (BFF)

A separate gateway for each client type (web, mobile, IoT):

Web App ──→ Web BFF ──→ Services
Mobile App ──→ Mobile BFF ──→ Services

The web app may need detailed data, while the mobile app needs less.

API Composition

Aggregating responses from multiple services into a single response:

Client: GET /user-dashboard

Gateway:
  → User Service: Profile info
  → Order Service: Recent orders
  → Notification Service: Unread notifications

Response: { profile, orders, notifications }

Gateway Offloading

Offloading common tasks (SSL termination, CORS, compression) to the gateway instead of services.

API Gateway vs Service Mesh

| Feature | API Gateway | Service Mesh | |---------|------------|-------------| | Location | Network edge | Between services | | Traffic | North-south (external→internal) | East-west (internal→internal) | | Focus | External API management | Inter-service communication | | Examples | Kong, AWS API GW | Istio, Linkerd |

Conclusion

An API Gateway is an essential component of microservices architecture. It provides security, performance, and manageability. However, for monolithic applications, it's often unnecessary — add it when you reach a scale that justifies the complexity.

Learn more about API Gateway design and microservices architecture in the Software Architecture 3.0 book.