What is an API Gateway?
An API gateway is the single front door to your backend: every client request hits the gateway first, and it handles the concerns common to all of them — authentication, rate limiting, routing to the right service, TLS, caching, and logging — so the services behind it can focus on business logic instead of reimplementing the same plumbing over and over.
Why a single front door
As an application grows from one app into many services, a question arises: where does shared logic live? Every service needs to check auth tokens, enforce limits, terminate TLS, and emit logs. Copying that into each service is duplication and drift waiting to happen. The gateway pattern pulls those cross-cutting concerns into one layer that all traffic passes through, giving clients a single stable endpoint while the backend is free to change shape.
What it actually does
A typical gateway authenticates each request (validating an JWT or API key), applies rate limiting per client, routes the path to the correct upstream service, terminates TLS, and can cache responses, transform payloads, and aggregate calls. It is also the natural home for observability — every request flows through it, so it is where you measure latency, error rates, and traffic by consumer.
Gateway vs load balancer
These get confused, but they operate at different layers. A load balancer spreads raw traffic across identical instances; it does not know or care what an API route means. A gateway is API-aware — it understands routes, authenticates callers, and applies per-client policy. In practice they stack: the gateway makes a routing and policy decision, then hands the request to a load balancer that picks a healthy instance of the target service.
Gateway vs service mesh
A gateway governs north-south traffic — the requests entering your system from the outside. A service mesh governs east-west traffic — the calls services make to each other inside the cluster. Larger systems often run both: a gateway at the edge for public clients, and a mesh internally for service-to-service security, retries, and telemetry. They are complementary, not competing.
The risk of overcentralizing
Because everything funnels through it, the gateway is a single point of failure and a tempting dumping ground. Cram too much business logic into it and you have recreated a monolith with extra latency. The discipline is to keep the gateway focused on genuine cross-cutting concerns — auth, limits, routing, observability — and leave domain logic in the services. It also has to be highly available and horizontally scalable, because if it goes down, your whole API goes with it.
At QUANT LAB
We introduce a gateway when a system has earned one — multiple services, third-party consumers, or the need for consistent auth and limits across endpoints — and not before. Our API development work uses the gateway to centralize rate limiting, token validation, and TLS, with strict rules about what is allowed to live there. For early-stage products, we are just as happy to keep auth in a single REST API until the architecture actually needs a front door.
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