Http message handlers in Web API

A message handler is a class that receives an HTTP request and returns an HTTP response. Message handlers derive from the abstract HttpMessageHandler class.

Typically, a series of message handlers are chained together. The first handler receives an HTTP request, does some processing, and gives the request to the next handler. At some point, the response is created and goes back up the chain. This pattern is called a delegating handler.


Server-Side Message Handlers
On the server side, the Web API pipeline uses some built-in message handlers:

HttpServer gets the request from the host.
HttpRoutingDispatcher dispatches the request based on the route.
HttpControllerDispatcher sends the request to a Web API controller.
You can add custom handlers to the pipeline. Message handlers are good for cross-cutting concerns that operate at the level of HTTP messages (rather than controller actions). For example, a message handler might:

Read or modify request headers.
Add a response header to responses.
Validate requests before they reach the controller.

https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/http-message-handlers