# Yggdrasil for RabbitMQ

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`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])

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

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

  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: ""]

# With namespace
config :yggdrasil, RabbitMQOne,
  rabbitmq: [
    hostname: "",
    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.

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, "~> 5.0"}]

## 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