Plug to help wrap headers and fields into the message body

This can be helpful if your broker doesn’t support message headers natively.

By default this plug will update the body of the message to:

%{
  "headers" => message.headers,
  "fields" => %{}, # message_id, correlation_id, etc.
  "body" => message.body
}

If you want a different wrapping structure for the message, you can pass the the :wrap_fn option. The wrap function should accept a message, the fields, the headers, and body. The return value should be a message.

Examples

iex> alias Conduit.Message
iex> defmodule MyPipeline do
iex>   use Conduit.Plug.Builder
iex>   plug Conduit.Plug.Wrap
iex> end
iex>
iex> message =
iex>   %Message{}
iex>   |> Message.put_correlation_id("1")
iex>   |> Message.put_header("foo", "bar")
iex>   |> Message.put_body(%{})
iex>   |> MyPipeline.run()
iex> message.body
%{
  "headers" => %{
    "foo" => "bar"
  },
  "fields" => %{
    "correlation_id" => "1"
  },
  "body" => %{}
}

iex> alias Conduit.Message
iex> defmodule MyOtherPipeline do
iex>   use Conduit.Plug.Builder
iex>   plug Conduit.Plug.Wrap, wrap_fn: fn message, fields, headers, body ->
iex>     body =
iex>       body
iex>       |> Map.put("meta", fields)
iex>       |> put_in(["meta", "headers"], headers)
iex>
iex>     Conduit.Message.put_body(message, body)
iex>   end
iex> end
iex>
iex> message =
iex>   %Message{}
iex>   |> Message.put_correlation_id("1")
iex>   |> Message.put_header("foo", "bar")
iex>   |> Message.put_body(%{})
iex>   |> MyOtherPipeline.run()
iex> message.body
%{
  "meta" => %{
    "correlation_id" => "1",
    "headers" => %{
      "foo" => "bar"
    },
  }
}