API Gateway Responsibilities

An API gateway is the controlled entry point for client traffic into a service estate. Instead of mobile apps, browsers, and partner systems calling many backend services directly, they send requests to the gateway first. The gateway then decides whether the request should be accepted, how it should be routed, and what cross-cutting policies apply before any business service sees it.

The first job is usually request admission. The gateway can parse the incoming HTTP request, check headers, enforce payload size limits, validate API keys or tokens, and reject malformed traffic early. Doing this at the edge protects downstream services from work they should never need to perform. It is cheaper to drop a bad request once than to let it fan out across half the platform.

The next job is authentication and authorisation integration. A gateway often works with an identity provider to validate JWTs, session cookies, OAuth tokens, or mTLS client certificates. It may also apply coarse-grained access rules, such as whether a caller can reach an admin route at all. Fine-grained business permissions usually still belong inside the service, but the gateway can remove a large amount of repeated security plumbing.

After the request is accepted, the gateway performs routing. That might be as simple as matching /orders/* to the orders service, or it might involve host-based routing, version-based routing, tenant-aware routing, or canary release rules. In some environments the gateway also handles service discovery, resolving a logical service name to one of many healthy instances.

Gateways often provide protocol translation and request shaping as well. A public REST request might be transformed into gRPC for an internal service. The gateway may inject tracing headers, normalise authentication data, or aggregate multiple downstream calls into one client-facing response. That can simplify clients, but it also makes the gateway more central and more complex.

Operational controls are another major reason gateways exist. They enforce rate limits, quotas, throttling, and sometimes WAF rules. They can short-circuit overloaded routes, open a circuit after repeated downstream failures, and return a controlled error instead of letting clients wait on hopeless timeouts. They are also a natural place for request logs, latency metrics, and response code breakdowns, which makes them useful during incident response.

That said, an API gateway is not a free win. It adds a network hop and becomes part of the critical path for almost every request. If it is misconfigured or overloaded, many services fail at once. Teams also make the mistake of pushing too much business logic into the gateway because it is convenient to centralise behaviour. Once that happens, the gateway stops being an edge component and starts becoming a brittle monolith in disguise.

A good API gateway therefore stays focused on cross-cutting concerns: authentication handoff, routing, policy enforcement, protocol adaptation, and observability. It should not own domain rules that properly belong to order processing, billing, or inventory. Used well, it reduces duplication and gives clients a stable front door. Used badly, it becomes another tightly coupled system that every release depends on.