What Is an API Gateway Used For? Core Use Cases

An API gateway is used for centralizing everything that happens between a client and your backend services: authentication, rate limiting, request routing, logging, and traffic shaping. Instead of every service implementing its own auth checks, its own throttling, and its own logging, you put one layer in front of everything and handle those concerns once.

This matters most once you have more than one backend, more than one client type, or more than one team touching the same infrastructure. A single Node.js app talking to a single database doesn't need a gateway. A product with a mobile app, a web app, and three internal microservices does — because without one, you end up duplicating security logic across every service, or worse, forgetting to duplicate it somewhere.

The Core Jobs an API Gateway Does

1. Authentication and authorization

Instead of validating API keys or JWTs in every microservice, the gateway checks credentials once at the edge and passes a verified identity downstream. This is one of the most common reasons teams adopt a gateway in the first place — it removes an entire category of duplicated, easy-to-get-wrong code.

2. Rate limiting and quota enforcement

Gateways track usage per API key, per client, or per endpoint, and reject requests that exceed a limit. This protects backend services from being overwhelmed by a single misbehaving client and lets you offer different tiers of access (free vs. paid, internal vs. external) without touching application code.

3. Request routing

A gateway can route /users/ to one service and /orders/ to another, based on path, header, or method. As you split a monolith into services, this lets clients keep hitting one stable base URL while the routing underneath changes freely.

4. Protocol and format translation

Gateways commonly translate REST to gRPC, normalize response formats, or convert between JSON and XML. This is useful when backend services were built at different times with different conventions and you don't want that inconsistency visible to clients.

5. Logging, metrics, and observability

Because every request passes through the gateway, it's a natural place to capture latency, status codes, error rates, and usage volume per client — without instrumenting each service individually.

6. Caching

Gateways can cache responses for frequently requested, rarely changing data, cutting load on backend services and reducing response times for clients.

7. Security enforcement

TLS termination, IP allowlisting, request size limits, and basic input validation are commonly handled at the gateway so that malformed or malicious requests never reach application code.

A Concrete Example

Say you're building a product that lets users generate content through an LLM. Without a gateway, your frontend calls the model provider directly, which means your API key sits in client-side code, there's no per-user rate limiting, and you have no visibility into who's using what. With a gateway in front:

curl https://api.example.com/v1/generate \
  -H "Authorization: Bearer sub_live_abc123" \
  -H "Content-Type: application/json" \
  -d '{"prompt": "Summarize this document"}'

The gateway validates sub_live_abc123, checks it against a rate limit, logs the request, and forwards it to the actual model provider using credentials that never leave the server. The client never sees the upstream provider's key at all.

This is essentially what SubToAPI does for teams that want to expose Claude access as a proper API rather than passing around a single shared credential. Instead of every developer or service holding the same underlying access, you issue individual sub_live_... keys, each with its own usage tracking and permissions, and route everything through one gateway. Streaming, tool use, and usage metadata all come through the same interface — see the quickstart for how that looks in practice.

When You Actually Need One

A gateway earns its keep when at least one of these is true:

• You have multiple client applications (web, mobile, partner integrations) hitting the same backend
• You're splitting a monolith into services and need a stable public interface
• You need to issue scoped API keys to different users or teams without giving them your master credentials
• You need per-client rate limits, and your application code has no clean place to enforce them
• You want usage and error metrics without adding logging to every service individually

If none of these apply — say, a single internal script calling a single internal service — a gateway is probably overhead you don't need yet.

What It's Not Used For

It's worth being clear about the boundaries. An API gateway is not a substitute for:

• Business logic — routing and auth checks belong at the gateway; the actual work of processing a request belongs in your services
• A message queue — gateways handle synchronous request/response traffic, not asynchronous job processing
• A full API management platform — some gateways include developer portals, billing, and analytics dashboards, but the core gateway function is traffic control, not product management

Confusing these boundaries is a common cause of over-engineered gateway setups that try to do too much in one layer.

Getting Started Without Building Your Own

Building a gateway from scratch means handling TLS, key validation, rate limiting, and logging correctly under load — solvable, but not trivial, and easy to get subtly wrong on the security side. For teams that specifically need to turn shared Claude access into individual, trackable API keys with streaming and tool support, SubToAPI handles the gateway layer directly: sign up, generate a key, and start making requests documented in Messages, Streaming, and Tools. Plans start at €9/month with a free trial, so you can test whether a managed gateway fits before committing.

questions

Is an API gateway the same as a load balancer? No. A load balancer distributes traffic across identical service instances to spread load. A gateway does that plus authentication, rate limiting, routing by path or content, and logging — it's concerned with request handling logic, not just traffic distribution.

Do I need an API gateway for a small project? Usually not. If you have one client and one backend service, the added complexity of a gateway layer outweighs the benefit. It becomes worthwhile once you have multiple clients, multiple services, or need to issue scoped access to external users.

Can an API gateway replace authentication in my application? It can handle the verification step (checking a key or token is valid), but your application still needs to enforce authorization — deciding what an authenticated user is actually allowed to do once the request reaches your service.