Yggdrasil is an immense mythical tree that connects the nine worlds in Norse cosmology.

Yggdrasil is an agnostic publisher/subscriber. Some of the available adapters are the following:

• Redis (adapter's name :redis).
• RabbitMQ (adapter's name :rabbitmq).
• PostgreSQL (adapter's name :postgres).
• Ethereum (adapter's name :ethereum).
• GraphQL (adapter's name :graphql).

For more information on how to use them, check the respective repository.

Small Example

The following example uses the Elixir distribution to send the messages:

iex(1)> channel = %Yggdrasil.Channel{name: "channel"}
iex(2)> Yggdrasil.subscribe(channel)
iex(3)> flush()
{:Y_CONNECTED, %Yggdrasil.Channel{(...)}}

and to publish a for the subscribers:

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

When the subscriber wants to stop receiving messages, then it can unsubscribe from the channel:

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

Channels

The struct %Yggdrasil.Channel{} is used for subscription and message publishing e.g:

%Yggdrasil.Channel{
  name: term(),        # Depends on the adapter.
  adapter: atom(),     # Adapter's name.
  transformer: atom(), # Transformer's name.
  backend: atom(),     # Backend's name.
  namespace: atom()    # Adapter's configuration namespace.
}

Adapters

An adapter is a process that connects to a service and distributes its messages among the subscribers of a channel. The following repositories have some of the available adapters:

• RabbitMQ adapter: Fault-tolerant RabbitMQ adapter that handles exchange subscriptions and message distribution among subscribers. The name of the adapter is :rabbitmq.
• Redis adapter: Fault-tolerant Redis adapter that handles channel subscriptions and message distribution among subscribers. The name of the adapter is :redis.
• PostgreSQL adapter: Fault-tolerant Postgres adapter that handles channel subscriptions and message distribution among subscribers. The name of the adapter is :postgres.
• Ethereum adapter: Fault-tolerant Ethereum adapter that handles channel subscriptions to Solidity contracts. The name of the adapter is :ethereum.
• GraphQL adapter: Fault-tolerant adapter that bridges GraphQL subscriptions with Yggdrasil subscriptions in any adapter. The name of the adapter is :graphql.

For more information on how to use them, check the corresponding repository documentation.

Transformers

A transformer is the implementation of the behaviour Yggdrasil.Transformer. In essence implements two functions:

• decode/2 for decoding messages coming from the adapter.
• encode/2 for encoding messages going to the adapter

Yggdrasil has two implemented transformers:

• :default - Does nothing to the messages and it is the default transformer used if no transformer has been defined.
• :json - Transforms from Elixir maps to string JSONs and viceversa.

Backends

A backend is the implementation of the behaviour Yggdrasil.Backend. The module is in charge of distributing the messages with a certain format inside Yggdrasil. Currently, there is only one backend, :default, and it is used by default in :elixir adapter and the previously mentioned adapters :rabbitmq, :redis and :postgres.

The messages received by the subscribers when using :default backend are:

• {:Y_CONNECTED, %Yggdrasil.Channel{(...)}} when the connection with the adapter is established.
• {:Y_EVENT, %Yggdrasil.Channel{(...)}, term()} when a message is received from the adapter.
• {:Y_DISCONNECTED, %Yggdrasil.Channel{(...)}} when the connection with the adapter is finished due to disconnection or unsubscription.

Configuration

Yggdrasil works out of the box with no special configuration at all. However, it is possible to configure the publisher. Yggdrasil uses Phoenix.PubSub for message distribution and the following are the available options:

• pubsub_adapter - Phoenix.PubSub adapter (defaults to Phoenix.PubSub.PG2).
• pubsub_name - Name of the Phoenix.PubSub adapter (defaults to Yggdrasil.PubSub).
• pubsub_options - Options of the Phoenix.PubSub adapter (defaults to [pool_size: 1]).

The rest of the options are for configuring the publishers and process name registry:

• publisher_options - Poolboy options for publishing. Controls the amount of connections established with the adapter service (defaults to [size: 5, max_overflow: 10]).
• registry - Process name registry (defaults toExReg).

For more information about configuration using OS environment variables check the module Yggdrasil.Settings.

Installation

Yggdrasil is available as a Hex package. To install, add it to your dependencies in your mix.exs file:

def deps do
  [{:yggdrasil, "~> 4.1"}]
end

Relevant projects used

• ExReg: rich process name registry.
• Poolboy: A hunky Erlang worker pool factory.
• Jason: JSON parsing library.
• Credo: static code analysis tool for the Elixir language.

Author

Alexander de Sousa.

License

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