# Introduction **Routers** and **controllers** are considered the foundational building blocks of a Blueprint application. A controller is an application entity that provides the execution logic for different facets of the Blueprint application via actions. Each action in a controller is responsible for handling a request, which can include validating and sanitizing the request input, processing the request, and sending a response to the request. > A controller is an application entity that provides the execution logic for different facets of the Blueprint application via actions. Routers are the main access point to the Blueprint application. The router exposes the public facing interface (or application programming interface) via paths (or urls). For example, `/messages` is a path exposed by a router that clients can invoke. > Routers are the main access point to the Blueprint application. Each path in a router is connected to a single action on a controller. We call this a *route*. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://blueprint.onehilltech.com/developer-guide/routers-and-controllers/introduction.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.