Load Balancer vs API Gateway (can one replace other)
Source: Dev.to
In modern architectures, load balancers and API gateways are both critical components, but they solve different problems. Because they sometimes appear in the same request path, they are often confused or even treated as interchangeable.
Load Balancer Types
| Load Balancer | OSI Layer | What It Understands |
|---|---|---|
| L4 Load Balancer | Layer 4 (Transport) | TCP/UDP, IP, Port |
| L7 Load Balancer | Layer 7 (Application) | HTTP/HTTPS, headers, paths |
Key point: a load balancer primarily focuses on traffic distribution, not API logic.
API Gateway
| Component | OSI Layer |
|---|---|
| API Gateway | Layer 7 (Application) |
An API gateway deeply understands HTTP semantics, such as:
- Headers
- JSON payloads
- Authorization tokens
- Request paths
- API versions
Key Differences at a Glance
| Aspect | Load Balancer | API Gateway |
|---|---|---|
| Primary Purpose | Distribute traffic | Manage APIs |
| OSI Layer | L4 or L7 | L7 only |
| Authentication | ❌ (very limited) | ✅ Built‑in |
| Rate Limiting | ❌ | ✅ |
| API Versioning | ❌ | ✅ |
| Request Transformation | ❌ | ✅ |
| Protocol Awareness | TCP/HTTP | HTTP/REST/GraphQL |
| Backend Awareness | Servers | Microservices & APIs |
Why One Can’t Replace the Other
Load Balancer
- Analogy: a traffic police officer 🚦 deciding which server should handle the request.
- Layer 4 responsibilities: IP address, port number, TCP/UDP connections.
- Layer 7 responsibilities (when applicable):
- Distribute traffic across servers
- Path‑based routing (e.g.,
/api → server A) - Sticky sessions (send the same user to the same server)
- Health checks
- SSL termination
Note: sticky sessions only mean “send this user to the same server again.” They do not involve identity checking, token validation, or permission enforcement.
Load balancers are designed to be fast, lightweight, and simple. Adding heavy security logic would degrade performance and complicate scaling.
API Gateway
- Analogy: a security guard at the building entrance 🛂 controlling access to APIs.
- Core responsibilities:
- Authentication (JWT, OAuth, API keys)
- Authorization (what a user can do)
- Rate limiting & throttling
- API versioning (v1, v2, …)
- Request/response transformation
- Logging and monitoring
API gateways care about who is calling, how often, and what they are allowed to access. Authentication involves token validation, expiry checks, role/permission checks, and sometimes calls to external identity stores—far beyond merely reading HTTP headers.
Summary
- Load balancer: “Where should this request go?” – focuses on routing and basic health checks.
- API gateway: “Who is calling and what can they do?” – adds security, traffic shaping, and API‑specific features.
Because they operate at different layers of responsibility, a load balancer cannot fully replace an API gateway, even when both operate at Layer 7.