ActionController

ActionController is a core component of the Ruby on Rails web framework that provides the controller layer in the Model-View-Controller architecture. Controllers are implemented as classes that inherit from ActionController::Base (or from ActionController::API in API-only applications) and define public methods called actions. They receive HTTP requests, coordinate with models, and decide how to respond, typically by rendering a view or returning data in another format.

Key responsibilities include handling request parameters, managing session and flash data, and producing responses. ActionController supports rendering templates (HTML, and other formats), as well as inline content, redirects, and explicit HTTP status codes. It also enables format negotiation through helpers like respond_to and respond_with, allowing responses in HTML, JSON, XML, and more. A rich callback system—before_action, around_action, and after_action—lets developers run code around action execution, while rescue_from provides centralized exception handling.

Security and parameter handling are supported via strong parameters, with methods such as params.require and params.permit to filter input data. ActionController integrates with the routing layer to map URLs to actions and with the view layer (ActionView) to render templates. It also supports layout selection, partial rendering, and various render options.

Variants include ActionController::Base for standard applications, ActionController::API for API-only apps that omit view-related features, and ActionController::Metal for a minimal, highly customized subset. ActionController is part of the Rails stack in the Action Pack gem, working with ActionView, ActionDispatch, and routing to handle the complete request-response cycle.