preHandler

Prehandler is a term used in software architecture to describe a function or component that runs before the main request handler in a processing pipeline. It typically lives in middleware stacks, API gateways, or event-driven systems, and its job is to perform early processing on an incoming request or message.

A prehandler may validate input, authenticate and authorize the caller, enforce rate limits, normalize data, attach metadata, and route or short-circuit requests based on conditions. By handling such cross-cutting concerns at the pre-processing stage, it helps keep the main handler focused on core business logic.

Implementation and behavior: A prehandler receives the same context as the main handler, can modify the request, enrich the context, or reject the request by returning an error before the main handler is invoked. If it approves, the pipeline proceeds with the (possibly augmented) context. Good prehandlers are deterministic, fast, and side-effect free when possible.

Design considerations: Order of prehandlers matters; failures should be clearly communicated; avoid expensive I/O; ensure security; provide a way to test individually and in integration.

Common tasks include:

- input validation

- authentication

- authorization

- rate limiting

- logging and tracing

- content-type checks

- request normalization

See also middleware, interceptor, hook, and pre-processing.