# Module: Concurrent::Async

Defined in:
  lib/concurrent/async.rb

## Overview

A mixin module that provides simple asynchronous behavior to a class, turning it into a simple actor. Loosely based on Erlang's gen_server, but without supervision or linking.

A more feature-rich Actor is also available when the capabilities of `Async` are too limited.

    Feature:
      As a stateful, plain old Ruby class
      I want safe, asynchronous behavior
      So my long-running methods don't block the main thread

The `Async` module is a way to mix simple yet powerful asynchronous capabilities into any plain old Ruby object or class, turning each object into a simple Actor. Method calls are processed on a background thread. The caller is free to perform other actions while processing occurs in the background.

Method calls to the asynchronous object are made via two proxy methods: `async` (alias `cast`) and `await` (alias `call`). These proxy methods post the method call to the object's background thread and return a "future" which will eventually contain the result of the method call.

This behavior is loosely patterned after Erlang's `gen_server` behavior. When an Erlang module implements the `gen_server` behavior it becomes inherently asynchronous. The `start` or `start_link` function spawns a process (similar to a thread but much more lightweight and efficient) and returns the ID of the process. Using the process ID, other processes can send messages to the `gen_server` via the `cast` and `call` methods. Unlike Erlang's `gen_server`, however, `Async` classes do not support linking or supervision trees.

## Basic Usage

When this module is mixed into a class, objects of the class become inherently asynchronous. Each object gets its own background thread on which to post asynchronous method calls. Asynchronous method calls are executed in the background one at a time in the order they are received.

To create an asynchronous class, simply mix in the `Concurrent::Async` module:

    class Hello
      include Concurrent::Async

      def hello(name)
        "Hello, #{name}!"
      end
    end

When defining a constructor it is critical that the first line be a call to `super` with no arguments. The `super` method initializes the background thread and other asynchronous components.

    class BackgroundLogger
      include Concurrent::Async

      def initialize(level)
        super()
        @logger = Logger.new(STDOUT)
        @logger.level = level
      end

      def info(msg)
        @logger.info(msg)
      end
    end

Mixing this module into a class provides each object two proxy methods: `async` and `await`. These methods are thread safe with respect to the enclosing object. The former proxy allows methods to be called asynchronously by posting to the object's internal thread. The latter proxy allows a method to be called synchronously but does so safely with respect to any pending asynchronous method calls and ensures proper ordering. Both methods return a IVar which can be inspected for the result of the proxied method call. Calling a method with `async` will return a `:pending` `IVar` whereas `await` will return a `:complete` `IVar`.

    class Echo
      include Concurrent::Async

      def echo(msg)
        print "#{msg}\n"
      end
    end

    horn = Echo.new
    horn.echo('zero')      # synchronous, not thread-safe
                           # returns the actual return value of the method

    horn.async.echo('one') # asynchronous, non-blocking, thread-safe
                           # returns an IVar in the :pending state

    horn.await.echo('two') # synchronous, blocking, thread-safe
                           # returns an IVar in the :complete state

## Let It Fail

The `async` and `await` proxy methods have built-in error protection based on Erlang's famous "let it fail" philosophy. Instance methods should not be programmed defensively. When an exception is raised by a delegated method the proxy will rescue the exception, expose it to the caller as the `reason` attribute of the returned future, then process the next method call.

## Calling Methods Internally

External method calls should always use the `async` and `await` proxy methods. When one method calls another method, the `async` proxy should rarely be used and the `await` proxy should never be used.

When an object calls one of its own methods using the `await` proxy the second call will be enqueued behind the currently running method call. Any attempt to wait on the result will fail as the second call will never run until after the current call completes.

Calling a method using the `await` proxy from within a method that was itself called using `async` or `await` will irreversibly deadlock the object. Do not do this, ever.

## Instance Variables and Attribute Accessors

Instance variables do not need to be thread-safe so long as they are private. Asynchronous method calls are processed in the order they are received and are processed one at a time. Therefore private instance variables can only be accessed by one thread at a time. This is inherently thread-safe.

When using private instance variables within asynchronous methods, the best practice is to read the instance variable into a local variable at the start of the method then update the instance variable at the end of the method. This way, should an exception be raised during method execution the internal state of the object will not have been changed.

### Reader Attributes

The use of `attr_reader` is discouraged. Internal state exposed externally, when necessary, should be done through accessor methods. The instance variables exposed by these methods must be thread-safe, or they must be called using the `async` and `await` proxy methods. These two approaches are subtly different.

When internal state is accessed via the `async` and `await` proxy methods, the returned value represents the object's state at the time the call is processed, which may not be the state of the object at the time the call is made.

To get the state at the current time, irrespective of an enqueued method calls, a reader method must be called directly. This is inherently unsafe unless the instance variable is itself thread-safe, preferably using one of the thread-safe classes within this library. Because the thread-safe classes within this library are internally-locking or non-locking, they can be safely used from within asynchronous methods without causing deadlocks.

Generally speaking, the best practice is to not expose internal state via reader methods. The best practice is to simply use the method's return value.

### Writer Attributes

Writer attributes should never be used with asynchronous classes. Changing the state externally, even when done in the thread-safe way, is not logically consistent. Changes to state need to be timed with respect to all asynchronous method calls which my be in-process or enqueued. The only safe practice is to pass all necessary data to each method as arguments and let the method update the internal state as necessary.

## Class Constants, Variables, and Methods

### Class Constants

Class constants do not need to be thread-safe. Since they are read-only and immutable they may be safely read both externally and from within asynchronous methods.

### Class Variables

Class variables should be avoided. Class variables represent shared state. Shared state is anathema to concurrency. Should there be a need to share state using class variables they must be thread-safe, preferably using the thread-safe classes within this library. When updating class variables, never assign a new value/object to the variable itself. Assignment is not thread-safe in Ruby. Instead, use the thread-safe update functions of the variable itself to change the value.

The best practice is to never use class variables with `Async` classes.

### Class Methods

Class methods which are pure functions are safe. Class methods which modify class variables should be avoided, for all the reasons listed above.

## An Important Note About Thread Safe Guarantees

> Thread safe guarantees can only be made when asynchronous method calls are not mixed with direct method calls. Use only direct method calls when the object is used exclusively on a single thread. Use only `async` and `await` when the object is shared between threads. Once you call a method using `async` or `await`, you should no longer call methods directly on the object. Use `async` and `await` exclusively from then on.

Examples:

    class Echo
      include Concurrent::Async

      def echo(msg)
        print "#{msg}\n"
      end
    end

    horn = Echo.new
    horn.echo('zero')      # synchronous, not thread-safe
                           # returns the actual return value of the method

    horn.async.echo('one') # asynchronous, non-blocking, thread-safe
                           # returns an IVar in the :pending state

    horn.await.echo('two') # synchronous, blocking, thread-safe
                           # returns an IVar in the :complete state

See Also:

- Actor
- "Actor Model" at Wikipedia
- Erlang gen_server
- "Let It Crash" at http://c2.com/

## Class Method Summary collapse

- .new(*args, &block) ⇒ Object

  Instanciate a new object and ensure proper initialization of the synchronization mechanisms.

## Instance Method Summary collapse

- #async ⇒ Concurrent::IVar (also: #cast)

  Causes the chained method call to be performed asynchronously on the object's thread.

- #await ⇒ Concurrent::IVar (also: #call)

  Causes the chained method call to be performed synchronously on the current thread.

## Class Method Details

### .new(*args, &block) ⇒ `Object`

Instanciate a new object and ensure proper initialization of the synchronization mechanisms.

Parameters:

- args (`Array<Object>`) —

  Zero or more arguments to be passed to the object's initializer.

- block (`Proc`) —

  Optional block to pass to the object's initializer.

Returns:

- (`Object`) —

  A properly initialized object of the asynchronous class.

    # File 'lib/concurrent/async.rb', line 239

## Instance Method Details

### #async ⇒ `Concurrent::IVar` Also known as: cast

Note:

The method call is guaranteed to be thread safe with respect to all other method calls against the same object that are called with either `async` or `await`. The mutable nature of Ruby references (and object orientation in general) prevent any other thread safety guarantees. Do NOT mix direct method calls with delegated method calls. Use only delegated method calls when sharing the object between threads.

Causes the chained method call to be performed asynchronously on the object's thread. The delegated method will return a future in the `:pending` state and the method call will have been scheduled on the object's thread. The final disposition of the method call can be obtained by inspecting the returned future.

Returns:

- (`Concurrent::IVar`) —

  the pending result of the asynchronous operation

Raises:

- (`NameError`) —

  the object does not respond to the requested method

- (`ArgumentError`) —

  the given `args` do not match the arity of the requested method

    408
    409
    410

    # File 'lib/concurrent/async.rb', line 408

    def async
      @__async_delegator__
    end

### #await ⇒ `Concurrent::IVar` Also known as: call

Note:

The method call is guaranteed to be thread safe with respect to all other method calls against the same object that are called with either `async` or `await`. The mutable nature of Ruby references (and object orientation in general) prevent any other thread safety guarantees. Do NOT mix direct method calls with delegated method calls. Use only delegated method calls when sharing the object between threads.

Causes the chained method call to be performed synchronously on the current thread. The delegated will return a future in either the `:fulfilled` or `:rejected` state and the delegated method will have completed. The final disposition of the delegated method can be obtained by inspecting the returned future.

Returns:

- (`Concurrent::IVar`) —

  the completed result of the synchronous operation

Raises:

- (`NameError`) —

  the object does not respond to the requested method

- (`ArgumentError`) —

  the given `args` do not match the arity of the requested method

    426
    427
    428

    # File 'lib/concurrent/async.rb', line 426

    def await
      @__await_delegator__
    end