Earl

Service objects for Crystal, aka Agents.

Crystal provides primitives for achieving concurrent applications, but doesn't have advanced layers for structuring applications. Earl tries to fill that gap with a simple object-based API that's easy to grasp and understand.

Is Earl for me?

• Your application has different, interconnected, objects that should always be alive, until they decide or are asked to stop.
• These different objects must communicate together.
• You feel that you spawn and loop and must rescue exceptions and restart objects too often.
• You need a pool of workers to dispatch work to.
• ...

If so, then Earl is for you.

Status

Earl is still in its infancy, but is fairly useable already.

If you believe Earl could help structure your application(s) please try it, and report any shortcomings and successes you had!

Usage

Add the earl shard to your dependencies then run shards install:

dependencies:
  earl:
    github: ysbaddaden/earl

For a formal depiction of the Earl library, you can read <SPEC.md>. For an informal introduction filled with examples, keep reading. For usage examples see the directory.

Getting Started

Agents

A simple agent is a class that includes Earl::Agent and implements a #call method. For example:

require "earl"

class Foo
  include Earl::Agent

  @count = 0

  def call
    while running?
      @count += 1
      sleep 1
    end
  end
end

Earl monitors the agent's state, and provides facilities to start and stop agents, to trap an agent crash or normal stop, as well as recycling them.

Communication (Earl::Mailbox) and broadcasting (Earl::Registry) are opt-in extensions, and introduced below.

Start Agents

You can start this agent in the current fiber with #start. This will block until the agent is stopped:

foo = Foo.new
foo.start

Alternatively you can call #spawn to start the agent in its own fiber, and return immediately:

foo = Foo.new
foo.spawn

do_something_else_concurrently

Depending on the context, it can be useful to block the current fiber. A library, for example, already spawned a dedicated fiber (e.g. HTTP::Server connections). Sometimes we need to start services in the background instead, and continue on.

Stop Agents

We can ask an agent to stop gracefully with #stop. Each agent must return quickly from the #call method when the agent's state changes. Hence the running? call in the Foo agent above to break out of the loop, for example.

foo.stop

When an agent is stopped its #terminate method hook is called, allowing the agent to act upon termination. For example notify other services, closing connections, or cleaning up.

Link & Trap Agents

When starting or spawning an agent A we can link another agent B to be notified when the agent A stopped or crashed (raised an unhandled exception). The linked agent B must implement the #trap(Agent, Exception?) method. If agent A crashed, then the unhandled exception is passed, otherwise it's nil. In all cases, the stopped/crashed agent is passed.

For example:

require "earl"

class A
  include Earl::Agent

  def call
    # ...
  end
end

class B
  include Earl::Agent

  def call
    # ...
  end

  def trap(agent, exception = nil)
    log.error("crashed with #{exception.message}") if exception
  end
end

a = A.new
b = B.new

a.start
b.start(link: a)

The Earl::Supervisor and Earl::Pool agents use links and traps to keep services alive for instance.

Recycle Agents

A stopped or crashed agent can be recycled to be restarted. Agents meant to be recycled must implement the #reset method, and return the agent's internal state to its pristine condition. A recycled agent must be indistinguishable from a created agent.

A recycled agent will return to the initial starting state, allowing it to restart. Earl::Supervisor, for example, expects the agents it monitors to properly reset themselves.

Agent Extensions

Mailbox

The Earl::Mailbox(M) module extends an agent with a Channel(M) along with methods to #send(M) a message to an agent and to receive them (concurrency safe).

The module merely wraps a Channel(M) but proposes a standard structure for agents to have an incoming mailbox of messages. All agents thus behave the same, and we can assume that an agent that expects to receive messages has a #send(M) method.

An agent's mailbox will be closed when the agent is asked to stop. An agent can simply loop over #receive? until it returns nil, without having to check for the agent's state.

See the Registry section below for an example.

Registry

The Earl::Registry(A, M) module will extend an agent to #register and #unregister agents of type A that can receive messages of type M. The agents to register must be capable to receive messages of type M —i.e. include Earl::Mailbox(M) or Earl::Artist(M)). When running, the agent can broadcast a message to all registered agents. It can also ask registered agents to stop.

For example, we can declare a Consumer agent that receives a count and prints it, until it's asked to stop:

class Consumer
  include Earl::Agent
  include Earl::Mailbox(Int32)

  def call
    while count = message.receive?
      p count
    end
  end
end

Now we can declare a producer that will broadcast numbers to registered consumers:

class Producer
  include Earl::Agent
  include Earl::Registry(Consumer, Int32)

  @count = 0

  def call
    while running?
      registry.send(@count += 1)
    end
  end

  def terminate
    registry.stop
  end
end

Now, we can create our producer and consumer agents, and register the consumers to the producer. We spawn the consumers that will start in their dedicated fiber. Last, we start the producer in the current fiber, that will block until we hit Ctrl+C to interrupt the program:

producer = Producer.new

a = Consumer.new
producer.register(a)
a.spawn

b = Consumer.new
producer.register(b)
b.spawn

Signal::INT.trap { producer.stop }
producer.start

The example registers consumers before starting the produce, but the registry is concurrency-safe. Consumers can be added and removed at any time.

Specific Agents

Supervisor

The Earl::Supervisor agent monitors other agents (including other supervisors). Monitored agents are spawned in their own fiber when the supervisor starts. If a monitored agent crashes it's recycled then restarted in its own fiber.

A supervisor can keep indefinitely running concurrent agents. It can also prevent the main thread from exiting.

For example, let's supervise the Producer example from the Registry section:

supervisor = Supervisor.new

producer = Producer.new
supervisor.monitor(producer)

a = Consumer.new
producer.register(a)
a.spawn

b = Consumer.new
producer.register(b)
b.spawn

Signal::INT.trap { supervisor.stop }
supervisor.start

Now if the producer crashes, it will be restarted. You can test this by adding a random raise "chaos monkey" into the Producer#call loop. The error will be logged, the producer restarted and the application continue running.

Pool

The Earl::Pool(A, M) agent spawns a fixed size list of agents of type A, to which we can dispatch messages (of type M). Messages are delivered to a single worker of the pool in an exactly-once manner. This is different from Earl::Registry that broadcasts a message to all registered agents.

Whenever a worker agent crashes, the pool will recycle and restart it. A worker can stop normally, but it should only do so when asked to stop.

Worker agents (of type A) must be capable to receive messages of type M. I.e. they include Earl::Mailbox(M) or Earl::Artist(M). They must also override their #reset method to properly reset an agent.

Note that Earl::Pool will replace the workers' mailbox. All workers then share a single Channel(M) for an exactly-once delivery of messages.

For example:

class Worker
  include Earl::Agent
  include Earl::Mailbox(String)

  def call
    while message = receive?
      p message
    end
  end
end

pool = Earl::Pool(Worker, String).new(capacity: 10)

spawn do
  5.times do |i|
    pool.send("message #{i}")
  end
  pool.stop
end

pool.start
# => message 1
# => message 2
# => message 3
# => message 4
# => message 5

Pools are regular agents, so we can have pools of pools, but we discourage such usage. It'll only increase the complexity of your application for little or no real benefit.

You can supervise pools with Earl::Supervisor. It can feel redundant because pools already monitor other agents, but it can be useful to only have a few supervisors to start (and stop).

Credits

• Author: Julien Portalier (@ysbaddaden)

Somewhat inspired by my very limited knowledge of Erlang OTP & Elixir.

License

Distributed under the Apache Software License 2.0. See LICENSE for details.