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The plug specification.
There are two kind of plugs: function plugs and module plugs.
#### Function plugs
A function plug is any function that receives a connection and a set of
options and returns a connection. Its type signature must be:
(Plug.Conn.t, Plug.opts) :: Plug.Conn.t
#### Module plugs
A module plug is an extension of the function plug. It is a module that must
* a `call/2` function with the signature defined above
* an `init/1` function which takes a set of options and initializes it.
The result returned by `init/1` is passed as second argument to `call/2`. Note
that `init/1` may be called during compilation and as such it must not return
pids, ports or values that are not specific to the runtime.
The API expected by a module plug is defined as a behaviour by the
`Plug` module (this module).
Here's an example of a function plug:
def json_header_plug(conn, opts) do
Here's an example of a module plug:
defmodule JSONHeaderPlug do
def init(opts) do
def call(conn, _opts) do
## The Plug pipeline
The `Plug.Builder` module provides conveniences for building plug