In a three-layer architecture, the Router layer is the first layer that contains the API routes of the application and is responsible for:

1. Parsing and validating the payload of incoming requests sent by the client in order to strip them away from any HTTP-specific properties.

2. Forwarding the parsed data to the Service layer responsible for handling the business logic of the application.

3. Translating the result of the call made to the Service layer into a valid HTTP response before sending it back to the client.

The request-response lifecycle

An incoming request usually goes through several processing stages in the Router layer before it is forwarded to the lower layers of the application, and a response is sent back to the client.

Here, I say usually, because in simpler cases, the request doesn't need to be processed, as it doesn't contain any additional data such as route parameters, query string parameters, or a message body.

In that case, the request-response lifecycle can be illustrated by the following schema.