Plug is a specification and set of connection adapters for composing web applications with functions in the Erlang VM.
Plug solves the problem of building modular web applications by providing a standard interface for handling HTTP requests and responses. It works by defining plugs—either module plugs with `init/1` and `call/2` functions or function plugs that take a connection and options—that transform a `Plug.Conn` struct representing the request-response cycle. These plugs can be composed together into pipelines, allowing developers to build web applications from small, reusable pieces. Plug also abstracts away differences between web servers by providing connection adapters for different Erlang VM-based servers, so the same application code can run on multiple backends.
Plug is suited for developers building web applications in Elixir who want a lightweight, composable foundation. It is particularly valuable if you need flexibility in choosing your web server, as it supports both Cowboy (Erlang-based) and Bandit (Elixir-based) through separate packages. The tool is widely used by higher-level frameworks like Phoenix to manage requests, responses, and WebSockets, making it a good choice if you want either a minimal foundation for custom applications or compatibility with the broader Elixir web ecosystem. Plug includes built-in features such as a router for defining routes and a connection upgrade API for WebSocket support, reducing the need for external dependencies in many cases.
The project maintains active development with regular updates to core functionality. The codebase shows ongoing refinement of the plug composition model and connection handling. Work continues on expanding adapter support and ensuring compatibility across different web server implementations. The project demonstrates sustained attention to both the specification layer and practical server integration concerns.
