# Yggdrasil
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> *Yggdrasil* is an immense mythical tree that connects the nine worlds in
> Norse cosmology.
`Yggdrasil` manages subscriptions to channels/queues in several brokers with
the possibility to add more. Simple Redis, RabbitMQ and PostgreSQL adapters
are implemented. Message passing is done through `Phoenix.PubSub` adapters.
`Yggdrasil` also manages publishing pools to Redis, RabbitMQ and PostgreSQL
using `:pool_boy` library.
This library provides three functions
* For clients: `Yggdrasil.subscribe/1` and `Yggdrasil.unsubscribe/1`.
* For servers: `Yggdrasil.publish/2`.
And the channels can be defined with the structure `Yggdrasil.Channel`, i.e:
```elixir
channel = %Yggdrasil.Channel{
name: "redis_channel",
transformer: Yggdrasil.Transformer.Default,
adapter: Yggdrasil.Distributor.Adapter.Redis,
namespace: TestRedis
}
```
where `name` is the name of the channel understood by the adapter. In this
case the `adapter` used is `Yggdrasil.Distributor.Adapter.Redis` that
provides a basic fault tolerant subscription to Redis channel
`"redis_channel"`, so the channel should be a string. Also for that reason
this channel would be used only by subscribers so the `transformer` module
should have the function `decode/2` to decode the messages coming from Redis
to the subscribers. The default `transformer` module does nothing to the
incoming message, so it should be a string. If no `transformer` is supplied,
then it's used the default `transformer` `Yggdrasil.Default.Transformer`.
The `namespace` tells `Yggdrasil` which adapter configuration should be used,
i.e:
```elixir
use Mix.Config
config :yggdrasil, TestRedis,
redis: [host: "localhost"]
```
this allows you to have several connection configurations for the same
broker. By default, the `namespace` is `Yggdrasil` if no `namespace` is
supplied.
# General Example
It's possible to define custom transformers to _decode_ and _encode_ messages
_from_ and _to_ the brokers respectively i.e:
```elixir
defmodule QuoteTransformer do
use Yggdrasil.Transformer
alias Yggdrasil.Channel
def decode(%Channel{} = _channel, message) do
with {:ok, quoted} <- Code.string_to_quoted(message),
{encoded, _} <- Code.eval_quoted(quoted),
do: {:ok, encoded}
end
def encode(%Channel{} = _channel, data) do
encoded = inspect(data)
{:ok, encoded}
end
end
```
and using the RabbitMQ adapter the subscription channel would be:
```elixir
iex(1)> sub_chan = %Yggdrasil.Channel{
name: {"amq.topic", "quotes"},
adapter: Yggdrasil.Distributor.Adapter.RabbitMQ,
transformer: QuoteTransformer
}
```
where the `name` of the channel for this adapter is a tuple with the
exchange name and the routing key and the configuration doesn't have a
namespace i.e:
```elixir
use Mix.Config
config :yggdrasil,
rabbitmq: [host: "localhost"]
```
and the publication channel would be:
```elixir
iex(2)> pub_chan = %Yggdrasil.Channel{
name: {"amq.topic", "quotes"},
adapter: Yggdrasil.Publisher.Adapter.RabbitMQ,
transformer: QuoteTransformer
}
```
Using this in `IEx`:
```elixir
iex(3)> Yggdrasil.subscribe(sub_chan)
:ok
iex(4)> flush()
{:Y_CONNECTED,
%Yggdrasil.Channel{adapter: Yggdrasil.Distributor.Adapter.RabbitMQ,
name: {"amq.topic", "quotes"}, namespace: nil,
transformer: QuoteTransformer}}
iex(5)> Yggdrasil.publish(pub_chan, %{"answer" => 42})
:ok
iex(6)> flush()
{:Y_EVENT,
%Yggdrasil.Channel{adapter: Yggdrasil.Distributor.Adapter.RabbitMQ,
name: {"amq.topic", "quotes"}, namespace: nil,
transformer: QuoteTransformer},
%{"answer" => 42}
}
```
The available subscription adapters are:
* `Yggdrasil.Distributor.Adapter.Elixir` which uses `Phoenix.PubSub`
directly as broker. Channel names can be any Elixir term.
* `Yggdrasil.Distributor.Adapter.Redis` which uses `Redis` as broker.
Channel names should be strings. Namespaces are relevant to keep several
connection configurations.
* `Yggdrasil.Distributor.Adapter.RabbitMQ` which uses `RabbitMQ` as broker.
Channel names should be tuples with the name of the exchange as first
element and the routing key as second element. Namespaces are relevant to
keep several connection configurations.
* `Yggdrasil.Distributor.Adapter.Postgres` which uses `Postgres` as broker.
Channel names should be strings. Namespaces are relevant to keep several
connection configurations.
The available publishing adapters are:
* `Yggdrasil.Publisher.Adapter.Elixir` which uses `Phoenix.PubSub`
directly as broker. Channel names can be any Elixir term.
* `Yggdrasil.Publisher.Adapter.Redis` which uses `Redis` as broker.
Channel names should be strings. Namespaces are relevant to keep several
connection configurations.
* `Yggdrasil.Publisher.Adapter.RabbitMQ` which uses `RabbitMQ` as broker.
Channel names should be tuples with the name of the exchange as first
element and the routing key as second element. Namespaces are relevant to
keep several connection configurations.
* `Yggdrasil.Publisher.Adapter.Postgres` which uses `Postgres` as broker.
Channel names should be strings. Namespaces are relevant to keep several
connection configurations.
The only available transformer module is `Yggdrasil.Transformer.Default` and
does nothing to the messages.
# Configuration
There are several configuration options and all of them should be in the
general configuration for `Yggdrasil`:
* `pubsub_adapter` - `Phoenix.PubSub` adapter. By default
`Phoenix.PubSub.PG2` is used.
* `pubsub_name` - Name of the `Phoenix.PubSub` adapter. By default is
`Yggdrasil.PubSub`.
* `pubsub_options` - Options of the `Phoenix.PubSub` adapter. By default
are `[pool_size: 1]`.
* `publisher_options` - `Poolboy` options. By default are
`[size: 5, max_overflow: 10]`.
* `registry` - Process name registry. By default is used `ExReg`.
Also it can be specified the connection configurations with or without
namespace:
* `redis` - List of options of `Redix`:
+ `host` - Redis hostname.
+ `port` - Redis port.
+ `password` - Redis password.
* `rabbitmq` - List of options of `AMQP`:
+ `host` - RabbitMQ hostname.
+ `port` - RabbitMQ port.
+ `username` - Username.
+ `password` - Password.
+ `virtual_host` - Virtual host.
* `postgres` - List of options of `Postgrex`:
+ `hostname` - Postgres hostname.
+ `port` - Postgres port.
+ `username` - Postgres username.
+ `password` - Postgres password.
+ `database` - Postgres database name.
## Installation
`Yggdrasil` is available as a Hex package. To install, add it to your
dependencies in your `mix.exs` file:
```elixir
def deps do
[{:yggdrasil, "~> 3.0"}]
end
```
and ensure `Yggdrasil` is started before your application:
```elixir
def application do
[applications: [:yggdrasil]]
end
```
## Relevant projects used
* [`ExReg`](https://github.com/gmtprime/exreg): rich process name registry.
* [`Poolboy`](https://github.com/devinus/poolboy): A hunky Erlang worker pool
factory.
* [`VersionCheck`](https://github.com/gmtprime/version_check): Alerts of new
application versions in Hex.
* [`Redix.PubSub`](https://github.com/whatyouhide/redix_pubsub): Redis pubsub.
* [`AMQP`](https://github.com/pma/amqp): RabbitMQ pubsub.
* [`Postgrex`](https://github.com/elixir-ecto/postgrex): PostgreSQL pubsub.
* [`Connection`](https://github.com/fishcakez/connection): wrapper over
`GenServer` to handle connections.
* [`Credo`](https://github.com/rrrene/credo): static code analysis tool for
the Elixir language.
* [`InchEx`](https://github.com/rrrene/inch_ex): Mix task that gives you
hints where to improve your inline docs.
## Author
Alexander de Sousa.
## License
`Yggdrasil` is released under the MIT License. See the LICENSE file for further
details.
