You can use this protocol to implement a custom message payload transformation. For example you could implement a JSON message with a predefined structure by first implementing a struct for your payload:

defmodule Example.Payload do
  defstruct [a: "a", b: "b", c: nil]
end

and then providing an implementation of Lapin.Message.Payload for it:

defimpl Lapin.Message.Payload, for: Example.Payload do
  def content_type(_payload), do: "application/json"
  def encode(payload), do: Poison.encode(payload)
  def decode_into(payload, data), do: Poison.decode(data, as: payload)
end

Please note you will need to add the poison library as a dependency on in your project mix.exs for this to work.

Lapin will automatically encode and set the content-type property on publish.

To decode messages before consuming, implement the payload_for/2 callback of Lapin.Connection and return an instance of the payload to decode into.

defmodule Example.Connection do
  def payload_for(_channel, _message), do: %Example.Payload{}
end

The default implementation simply returns the unaltered binary data and sets the message content-type property to nil.