A plug is just a function that accepts a Plug.Conn.t and a configuration “thing” (I just want to say “object” but Elixir is not object oriented so I don’t know how to name it. A “term”?) That function can be wrapped in a separate module (something like a functor if you are familiar with C++).

If a function accepts the same arguments as above, changes the conn a little bit, and delegates the job to a plug, then the function is another plug.

In this way, you can chain plugs together, with each plug handles only a small task in the whole HTTP request handling, and the result of the chaining is a big plug. That’s what “composable” means. For example, in a conventional Phoenix application, you can find Plug.RequestId which attaches a unique random string to each request for the DevOps team to trace every request; Plug.Parsers for parsing the query strings and request bodies; Plug.MethodOverride for overriding the method according to the _method param; Plug.Session for loading sessions, and finally the MyApp.Router for dispatching each request to the correct controller action. (I omitted a few plugs) These plugs combine together into a single big plug.

If an Erlang/Elixir web server can chew and spit Plug.Conn.t, it can run any plug-based web application such as Phoenix application. If it can’t, well, probably someone will write or has already written an adapter for that server. So you can think of Plug as an adapter between your web application and the plug-aware web server, and that’s what “Connection adapters” mean.

@shijith.k I’ve actually wrote a small deep-dive into Plug and Cowboy and how they work under the hood which you can read here hope it helps!

• The BEAM is needed to run any Erlang/Elixir application.
• Cowboy is an Erlang application… (It would be the A in LAMP)
• Plug is just a library, that can be used by application.