To be as flexible as possible, Concurrent 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.
Concurrent provides a generic interface for working with parallel tasks called "contexts". All contexts are capable of being executed in parallel from the main program code. Each context is assigned a closure to execute when it is created, and the returned value is passed back to the parent context. Concurrent goes for a "shared-nothing" architecture, so any variables inside the closure are local to that context and can store any non-safe data.
You can wait for a context to close by calling `join()`. Joining does not block the parent context and will asynchronously wait for the child context to finish before resolving.
```php
use Icicle\Concurrent\Threading\ThreadContext;
use Icicle\Coroutine;
use Icicle\Loop;
Coroutine\create(function () {
$thread = new ThreadContext(function () {
print "Hello, World!\n";
});
$thread->start();
yield $thread->join();
});
Loop\run();
```
#### Synchronization with channels
Contexts 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.
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.
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.
