# Concurrency for Icicle
**Under development -- keep an eye out for things to come in the near future though!**

**True concurrency using native threading and multiprocessing for parallelizing code, *without* blocking.**

This library is a component for [Icicle](https://github.com/icicleio/icicle) that provides native threading, multiprocessing, process synchronization, shared memory, and task workers. Like other Icicle components, this library uses [Promises](https://github.com/icicleio/icicle/wiki/Promises) and [Generators](http://www.php.net/manual/en/language.generators.overview.php) for asynchronous operations that may be used to build [Coroutines](https://github.com/icicleio/icicle/wiki/Coroutines) to make writing asynchronous code more like writing synchronous code.

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This library provides a means of parallelizing code without littering your application with complicated lock checking and inter-process communication.

To be as flexible as possible, this library comes with a collection of non-blocking concurrency tools that can be used independently as needed, as well as an "opinionated" task API that allows you to assign units of work to a pool of worker threads or processes.

#### Requirements
- PHP 5.5+
- [pthreads](http://pthreads.org) for multithreading *or*
- System V-compatible Unix OS and PHP with `--enable-pcntl`

#### Benchmarks
A few benchmarks are provided for analysis and study. Can be used to back up implementation decisions, or to measure performance on different platforms or hardware.

    vendor/bin/athletic -p benchmarks -b vendor/autoload.php

## Installation
With [Composer](http://getcomposer.org). What did you expect?

    composer require icicleio/concurrent

To enable threading, you will need to compile pthreads from source, as this package depends on unstable and unreleased fixes in pthreads.

    git clone https://github.com/krakjoe/pthreads && cd pthreads
    git checkout master
    phpize
    ./configure
    make
    sudo make install

## Documentation
Concurrent can use either process forking or true threading to parallelize execution. Threading provides better performance and is compatible with Unix and Windows but requires ZTS (Zend thread-safe) PHP, while forking has no external dependencies but is only compatible with Unix systems. If your environment works meets neither of these requirements, this library won't work.

### Threads
Threading is a cross-platform concurrency method that is fast and memory efficient. Thread contexts take advantage of an operating system's multi-threading capabilities to run code in parallel. A spawned thread will run completely parallel to the parent thread, each with its own environment. Each thread is assigned a closure to execute when it is created, and the returned value is passed back to the parent thread. Concurrent goes for a "shared-nothing" architecture, so any variables inside the closure are local to that thread and can store any non-safe data.

You can spawn a new thread with the `Thread::spawn()` method:

```php
use Icicle\Concurrent\Threading\Thread;
use Icicle\Coroutine;
use Icicle\Loop;

Coroutine\create(function () {
    $thread = Thread::spawn(function () {
        print "Hello, World!\n";
    });

    yield $thread->join();
});

Loop\run();
```

You can wait for a thread to finish by calling `join()`. Joining does not block the parent thread and will asynchronously wait for the child thread to finish before resolving.

### Forks
For Unix-like systems, you can create parallel execution using fork contexts. Though not as efficient as multi-threading, in some cases forking can take better advantage of some multi-core processors than threads. Fork contexts use the `pcntl_fork()` function to create a copy of the current process and run alternate code inside the new process.

Spawning and controlling forks are quite similar to creating threads. To spawn a new fork, use the `Fork::spawn()` method:

```php
use Icicle\Concurrent\Forking\Fork;
use Icicle\Coroutine;
use Icicle\Loop;

Coroutine\create(function () {
    $fork = Fork::spawn(function () {
        print "Hello, World!\n";
    });

    yield $fork->join();
});

Loop\run();
```

Calling `join()` on a fork will asynchronously wait for the forked process to terminate, similar to the `pcntl_wait()` function.

#### Synchronization with channels
Threads and forks wouldn't be very useful if they couldn't be given any data to work on. The recommended way to share data between contexts is with a `Channel`. A channel is a low-level abstraction over local, non-blocking sockets, which can be used to pass messages and objects between two contexts. Channels are non-blocking and do not require locking. For example:

```php
use Icicle\Concurrent\Sync\Channel;
use Icicle\Concurrent\Threading\Thread;
use Icicle\Coroutine;
use Icicle\Loop;

Coroutine\create(function () {
    list($socketA, $socketB) = Channel::createSocketPair();
    $channel = new Channel($socketA);

    $thread = Thread::spawn(function ($socketB) {
        $channel = new Channel($socketB);
        yield $channel->send("Hello!");
    }, $socketB);

    $message = (yield $channel->receive());
    yield $thread->join();
});

Loop\run();
```

### Synchronization with parcels
Parcels are shared containers that allow you to store context-safe data inside a shared location so that it can be accessed by multiple contexts. To prevent race conditions, you still need to access a parcel's data exclusively, but Concurrent allows you to acquire a lock on a parcel asynchronously without blocking the context execution, unlike traditional mutexes.

## Development and contributing
Interested in contributing to Icicle? Please see our [contributing guidelines](https://github.com/icicleio/icicle/blob/master/CONTRIBUTING.md) in the [Icicle repository](https://github.com/icicleio/icicle).

Want to hack on the source? A [Vagrant](http://vagrantup.com) box is provided with the repository to give a common development environment for running concurrent threads and processes, and comes with a bunch of handy tools and scripts for testing and experimentation.

Starting up and logging into the virtual machine is as simple as

    vagrant up && vagrant ssh

Once inside the VM, you can install PHP extensions with [Pickle](https://github.com/FriendsOfPHP/pickle), switch versions with `newphp VERSION`, and test for memory leaks with [Valgrind](http://valgrind.org).