concurrent-ruby/docs-source/erlang_actor.out.md

Examples

The simplest example is to use the actor as an asynchronous execution. Although, Promises.future { 1 + 1 } is better suited for that purpose.

actor = Concurrent::ErlangActor.spawn(:on_thread, name: 'addition') { 1 + 1 }
# => #<Concurrent::ErlangActor::Pid:0x000002 addition>
actor.terminated.value!                  # => 2

Let's send some messages and maintain some internal state which is what actors are good for.

actor = Concurrent::ErlangActor.spawn(:on_thread, name: 'sum') do
  sum = 0 # internal state
  # receive and sum the messages until the actor gets :done
  while true
    message = receive
    break if message == :done
    # if the message is asked and not only told, 
    # reply with a current sum
    reply sum += message   
  end
  sum
end
# => #<Concurrent::ErlangActor::Pid:0x000003 sum>

The actor can be either told a message asynchronously, or asked. The ask method will block until actor replies.

# tell returns immediately returning the actor 
actor.tell(1).tell(1)
# => #<Concurrent::ErlangActor::Pid:0x000003 sum>
# blocks, waiting for the answer 
actor.ask 10                             # => 12
# stop the actor
actor.tell :done
# => #<Concurrent::ErlangActor::Pid:0x000003 sum>
actor.terminated.value!                  # => 12

Actor types

There are two types of actors. The type is specified when calling spawn as a first argument, Concurrent::ErlangActor.spawn(:on_thread, ... or Concurrent::ErlangActor.spawn(:on_pool, ....

The main difference is in how receive method returns.

• :on_thread it blocks the thread until message is available, then it returns or calls the provided block first.

• However, :on_pool it has to free up the thread on the receive call back to the pool. Therefore the call to receive ends the execution of current scope. The receive has to be given block or blocks that act as a continuations and are called when there is message available.

Let's have a look at how the bodies of actors differ between the types:

ping = Concurrent::ErlangActor.spawn(:on_thread) { reply receive }
# => #<Concurrent::ErlangActor::Pid:0x000004>
ping.ask 42                              # => 42

It first calls receive, which blocks the thread of the actor. When it returns the received message is passed an an argument to reply, which replies the same value back to the ask method. Then the actor terminates normally, because there is nothing else to do.

However when running on pool a block with code which should be evaluated after the message is received has to be provided.

ping = Concurrent::ErlangActor.spawn(:on_pool) { receive { |m| reply m } }
# => #<Concurrent::ErlangActor::Pid:0x000005>
ping.ask 42                              # => 42

It starts by calling receive which will remember the given block for later execution when a message is available and stops executing the current scope. Later when a message becomes available the previously provided block is given the message and called. The result of the block is the final value of the normally terminated actor.

The direct blocking style of :on_thread is simpler to write and more straight forward however it has limitations. Each :on_thread actor creates a Thread taking time and resources. There is also a limited number of threads the Ruby process can create so you may hit the limit and fail to create more threads and therefore actors.

Since the :on_pool actor runs on a poll of threads, its creations is faster and cheaper and it does not create new threads. Therefore there is no limit (only RAM) on how many actors can be created.

To simplify, if you need only few actors :on_thread is fine. However if you will be creating hundreds of actors or they will be short-lived :on_pool should be used.

Erlang behaviour

The actor matches Erlang processes in behaviour. Therefore it supports the usual Erlang actor linking, monitoring, exit behaviour, etc.

actor = Concurrent::ErlangActor.spawn(:on_thread) do
  spawn(link: true) do # equivalent of spawn_link in Erlang
    terminate :err # equivalent of exit in Erlang    
  end
  trap # equivalent of process_flag(trap_exit, true) 
  receive  
end
# => #<Concurrent::ErlangActor::Pid:0x000006>
actor.terminated.value!
# => #<Concurrent::ErlangActor::Exit:0x000007
#     @from=#<Concurrent::ErlangActor::Pid:0x000008>,
#     @link_terminated=true,
#     @reason=:err>

TODO

• More erlang behaviour examples
• Back pressure with bounded mailbox
• _op methods
• types of actors