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.
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.
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.
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 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.
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:
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.
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).
