# Yggdrasil for RabbitMQ
[](https://travis-ci.org/gmtprime/yggdrasil_rabbitmq) [](https://hex.pm/packages/yggdrasil_rabbitmq) [](https://hex.pm/packages/yggdrasil_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](https://github.com/bitwalker/distillery) releases)
## Small example
The following example uses RabbitMQ adapter to distribute messages e.g:
Given the following channel:
```elixir
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:
```elixir
iex(4)> Yggdrasil.publish(channel, "message")
iex(5)> flush()
{:Y_EVENT, %Yggdrasil.Channel{...}, "message"}
```
* Unsubscribe from it:
```elixir
iex(6)> Yggdrasil.unsubscribe(channel)
iex(7)> flush()
{:Y_DISCONNECTED, %Yggdrasil.Channel{...}}
```
And additionally, you can use `Yggdrasil` behaviour to build a subscriber:
```elixir
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:
```elixir
%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:
```elixir
# 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:
```elixir
def deps do
[{:yggdrasil_rabbitmq, "~> 5.0"}]
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.
