Actor Model in Erlang

Traditional Concurrency

• Typically through the use of threads.  
  • Lack of atomicity
  • Locking primitives such as semaphores or mutexes are required.
  • Can create difficult to program and debug situations, such as deadlock.

Parallelized programming

• Moore's Law
• Use of parallelization becomes more and more attractive
• Amdahl's Law
  • A program is only parallelizable in terms of it's slowest part.

Actor Model

• Concurrency in actors is constrained only by the availability of hardware resources and by the logical-dependence inherent in the computation. - Gul Agha
• First proposed by Carl Hewitt in 1973
• Further solidified over many years by a number of other computer scientists and by Gul Agha in his dissertation on the Actor Model in 1985.

Actor Model

• Actors are a concurrency primitive that do not share resources with another actor.
• Actors share state/data via message passing.
• An Actor:
  • Has a mailbox and a behavior
  • Takes a message and can:
    • Send messages to other actors,
    • Create new actors
    • Return another actor describing its next behavior
  • Process one message at a time
• Actors come in systems
  • Everything is an actor

Actor Model Concepts

1. No shared state between actors
2. Functions asynchronously
3. All about message passing for communication
   1. Only way to share/change state is through this message passing

Enter Erlang...

• The actor model is inherent to Erlang and is built into the language and somewhat fundamental to understanding the distributed nature of Erlang.
• It was designed for the telephony industry, so some of the driving ideals such as reliability demanded that Erlang utilize a model that didn't incur potential for data loss, that is possible in the more traditional concurrency models discussed above.

Erlang...

• An Erlang program describes a series of functions.

• Each "thread" of execution is a lightweight process.

• It utilizes pattern matching to process messages.
  • ​Each function uses pattern matching to determine which function to execute.

The Erlang view of the world is that everything is a process and that processes can interact only by exchanging messages. - Joe Armstrong

Creating a process

• To create a process in Erlang, just call `spawn`, which will return a process id (pid)

-module(actor).
...
start() -> 
  spawn(actor, run, []).

1> c(actor).
2> Pid = actor:start().
3> Pid.
<0.80.0>

From the shell:

Erlang process

• Not a process in the OS definition.

• It's an Erlang process, so it runs in the Erlang VM.

• It runs in user space, not bound to the kernel, so is scheduled by the Erlang Scheduler.

Process (cont'd)

• And it's a lightweight process...how lightweight:

  • A newly spawned Erlang process uses 309 words of memory (a word is 4 or 8 bytes)

• Check how much memory our process(es) are using:

  •  

• And they are very quick to spawn...from Joe Armstrong:

  • Spawning 20,000 processes took an average of 3.0 microseconds/process of CPU time and 3.4 microseconds of elapsed (wall-clock) time.

{_,Bytes} = process_info(actor_file:start(), memory).
  {memory,2720}

Erlang scheduler

• Erlang uses a preemptive scheduler.
  • A preemptive scheduler works by running tasks, preempting, then resuming them based on a specific metric, such as priority, time or reduction.
  • Many other async languages, such as Node.js and Python's Twisted, utilize cooperative scheduling, which requires tasks to release themselves voluntarily.

Scheduler (cont'd)

• Erlang's preemptive scheduler uses a reduction count.
  • A reduction is typically a function call, garbage collection, message sending, etc.  
• Erlang's preemptive scheduler uses a reduction count.
  • A reduction is typically a function call, garbage collection, message sending, etc.
  • The reduction count for an Erlang process is 2000, which can go quite quickly.

Scheduler (cont'd)

• Processes do and can be flagged with priorities: 'max', 'high', 'normal', and 'low'.
• The scheduler makes Erlang a great choice for low-latency, parallelized programs as it can manage multi-tasking efficiently.

Further Reading

• Gul Agha, Actors: A Model of Concurrent Computation in Distributed Systems
• Ruben Vermeersch, Concurrency in Erlang & Scala: The Actor Model
• Fred Hebert, Learn You Some Erlang: for great good!
• Erlang/OTP System Documentation
• Joe Armstrong, Programming Erlang: Software for a Concurrent World
• Bruce Tate, Crossing borders: Concurrent programming with Erlang
• Hamidreza Soleimani, Erlang Scheduler Details and Why It Matters
• Jesper Louis Andersen, How Erlang does scheduling

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