Yggdrasil for RabbitMQ

Yggdrasil for RabbitMQ is a publisher/subscriber that:

• It's easy to use and configure.
• It's fault tolerant: recovers disconnected subscriptions.
• It has reconnection support: configurable exponential backoff.
• It has OS environment variable configuration support (useful for Distillery releases)

Small example

The following example uses RabbitMQ adapter to distribute messages e.g:

Given the following channel:

iex> channel = [name: {"amq.topic", "routing.key"}, adapter: :rabbitmq]

You can:

• Subscribe to it:

  iex> Yggdrasil.subscribe(channel)
  iex> flush()
  {:Y_CONNECTED, %Yggdrasil.Channel{...}}

• Publish messages to it:

  iex(4)> Yggdrasil.publish(channel, "message")
  iex(5)> flush()
  {:Y_EVENT, %Yggdrasil.Channel{...}, "message"}

• Unsubscribe from it:

iex(6)> Yggdrasil.unsubscribe(channel)
iex(7)> flush()
{:Y_DISCONNECTED, %Yggdrasil.Channel{...}}

And additionally, you can use Yggdrasil behaviour to build a subscriber:

defmodule Subscriber do
  use Yggdrasil

  def start_link do
    channel = [name: {"amq.topic", "routing.key"}, adapter: :rabbitmq]
    Yggdrasil.start_link(__MODULE__, [channel])
  end

  @impl true
  def handle_event(_channel, message, _) do
    IO.inspect message
    {:ok, nil}
  end
end

The previous Subscriber will print every message that comes from the RabbitMQ exchange "amq.topic" and routing key "routing.key".

RabbitMQ adapter

The RabbitMQ adapter has the following rules:

• The adapter name is identified by the atom :rabbitmq.
• The channel name must be a tuple with the exchange and the routing key.
• The transformer must encode to a string. By default, Yggdrasil provides two transformers: :default (default) and :json.
• Any backend can be used (by default is :default).

The following is an example of a valid channel for both publishers and subscribers:

%Yggdrasil.Channel{
  name: {"amq.topic", "my.routing.key"},
  adapter: :rabbitmq,
  transformer: :json
}

The previous channel expects to:

• Subscribe to or publish to the exchange amq.topic and using the routing key my.routing.key.
• The adapter is :rabbitmq, so it will connect to RabbitMQ using the appropriate adapter.
• The transformer expects valid JSONs when decoding (consuming from a subscription) and maps or keyword lists when encoding (publishing).

Note: Though the struct Yggdrasil.Channel is used, Keyword lists and maps are also accepted as channels as long as they contain the required keys.

RabbitMQ configuration

This adapter supports the following list of options:

Option	Default	Description
hostname	"localhost"	RabbitMQ hostname.
port	5672	RabbitMQ port.
username	"guest"	RabbitMQ username.
password	"guest"	RabbitMQ password.
virtual_host	"/"	Virtual host.
heartbeat	10 seconds	Heartbeat of the connections.
max_retries	3	Amount of retries where the backoff time is incremented.
slot_size	10	Max amount of slots when adapters are trying to reconnect.
subscriber_connections	1	Amount of subscriber connections.
publisher_connections	1	Amount of publisher connections.

Note: Concurrency is handled by creating channels on the present connections instead of creating several connections for every subscriber/publisher.

For more information about the available options check Yggdrasil.Settings.RabbitMQ.

The following shows a configuration with and without namespace:

# Without namespace
config :yggdrasil,
  rabbitmq: [hostname: "rabbitmq.zero"]

# With namespace
config :yggdrasil, RabbitMQOne,
  rabbitmq: [
    hostname: "rabbitmq.one",
    port: 1234
  ]

All the available options are also available as OS environment variables. It's possible to even separate them by namespace e.g:

Given two namespaces, the default one and Rabbit.One, it's possible to load the hostname from the OS environment variables as follows:

• $YGGDRASIL_RABBITMQ_HOSTNAME for the default namespace.
• $RABBIT_ONE_YGGDRASIL_RABBITMQ_HOSTNAME for Rabbit.One.

In general, the namespace will go before the name of the variable.

Installation

Using this adapter with Yggdrasil is a matter of adding the available hex package to your mix.exs file e.g:

def deps do
  [{:yggdrasil_rabbitmq, "~> 6.0"}]
end

Running the tests

A docker-compose.yml file is provided with the project. If you don't have a RabbitMQ server, but you do have Docker installed, then you can run:

$ docker-compose up --build

And in another shell run:

$ mix deps.get
$ mix test

Author

Alexander de Sousa.

License

Yggdrasil is released under the MIT License. See the LICENSE file for further details.