Using Asynchronous Processes in PHP.

For most programs written in PHP, its sole purpose is to execute a simple process consisting of multiple tasks, where the tasks must be executed in sequence such as data processing. We always have to tolerate the stop and wait aspect of synchronous programming. The synchronous style of code execution is referred to as blocking, this implies that tasks will be executed one by one. So what if we want to run tasks without them blocking each other, that means we need to have a non-blocking process? This approach would require applying asynchronous programming approaches in PHP, here tasks will be executed without depending on of each other.

A common approach to achieve a non-blocking execution in PHP is to implement queue processing. Tasks are persisted to a transport system e.g MySQL, Redis, Amazon SQS e.t.c, which is retrieved by a background worker and executed accordingly, thereby not blocking the main process in which it was created. A laravel application provides a queue mechanism that allows tasks in this case called jobs to be deferred for to a later time for processing.

Another approach would be to run all defined tasks in parallel. What we get out of this approach is that a particular task is done it can return control back to the main process immediately with a promise to execute code and notify us about the result later(e.g. callback). One might see little use case for the parallel processing approach; example use case could be performing image processing and making a get request to some external service.

Let’s see the difference between synchronous and asynchronous (parallel) process in PHP using a very simple use case.

Synchronous code

  foreach (range(1, 5) as $i) {
     $output = $i * 2;
     echo $output . "\n";
  }

Asynchronous code

    use Spatie\Async\Pool;

    $pool = Pool::create();

    foreach (range(1, 5) as $i) {
        $pool[] = async(function () use ($i) {
           $output = $i * 2;
           return $output;
        })->then(function (int $output) {
           echo $output . "\n";
        });
    }
    await($pool);

When we execute the first code we will get the output values in this order:

2
4
6
8
10

Retrying the execution, we will get the output in this same sequence above … hence each multiplication operation waits to execute before the next one. Next, running the second code block, let’s see what we get.

6
10
2
8
4

Retrying the execution for the second time:

2
6
4
10
8

One process happens to produce two different results. This exactly is what we get for utilising the asynchronous approach … our little tasks can be executed in a fashion they don’t block each other. Each multiplication task executes independently, some faster than others hence the disorderliness in the output. Also, notice our async function as a then method attached to it, this method is responsible for taking back control and it accepts a callback function as its parameter which can now perform extra operations with the received output.

The folks at Spatie made this nice spatie/async package, which helps in performing tasks in parallel. You can install the package via composer:

composer require spatie/async

The package provides a neat way to interact with the tasks created, that are to be executed in parallel. The Event Listeners on the tasks are described below:

• Performing another operation when the task is done as the callback is achievable with its then method.
• Error handling is easier to control when a particular task throws an exception using the catch method.
• Take for instance a task does not complete its operation, a timeout method allows one to handle such a scenario.

The event listeners are hooked to a task as shown below:

    $pool
        ->add(function () {
            // Task to be performed in a Parallel process
        })
        ->then(function ($output) {
            // On success, `$output` is returned by the process or callable you passed to           the queue.
        })
        ->catch(function ($exception) {
            // When an exception is thrown from within a process, it's caught and passed here.
        })
        ->timeout(function () {
            // Ohh No! A process took too long to finish. Let's do something 
        })
    ;

To learn more about the spatie/async package read this article from one of its contributors here and you can also refer to the GitHub repo.