An API Gateway is a server that sits between clients and backend services, acting as a single entry point that receives, routes, and manages all API requests. Instead of clients calling many individual services directly, they call the gateway, which forwards each request to the appropriate service and returns the response. This centralizes and simplifies how APIs are exposed, especially in microservices architectures where an application may be composed of dozens or hundreds of small services.
The gateway handles cross-cutting concerns so that individual services do not have to. Common functions include request routing, load balancing, protocol translation, request/response transformation, caching, and aggregating results from multiple services into a single response. It also enforces operational policies such as rate limiting and throttling to protect backends from overload.
From a security perspective, the API gateway is a key control point. It commonly performs authentication (verifying the identity of the caller, for example via API keys, OAuth tokens, or JWTs), enforces coarse-grained authorization, terminates TLS, and centralizes logging and monitoring of API traffic. By consolidating these functions, the gateway provides consistent policy enforcement and visibility across all APIs behind it.
However, an API gateway is not a complete security solution on its own. It typically enforces perimeter-style checks and coarse policies, but it generally cannot understand the deeper business logic of each API or detect sophisticated abuse, broken object-level authorization, or business-logic attacks that unfold across sequences of legitimate-looking requests. Attackers who obtain valid credentials pass straight through the gateway. For this reason, gateways are best treated as one layer within a defense-in-depth strategy – valuable for routing, access management, and traffic control, but complemented by dedicated API security that inspects behavior and context to catch threats the gateway cannot see.