Command Bus in Laravel

Laravel 5.0 was released in early 2015. It was a radical departure from Laravel 4 (so much so, Laravel 5.0 was originally Laravel 4.3, but “bumped” the version when it became much more than a “minor” update).

One of the features introduced in Laravel 5.0 was a command bus (laravel.com/docs/5.0/bus), but by version 5.1 it disappeared, instead supplanted by queued jobs. Except it didn’t.

“Jobs” in Laravel 5.1+ is merely a name change. The command bus component still exists under the hood in Laravel and is what jobs use to dispatch themselves. When you create and dispatch a synchronous (non-queued) job, you’re essentially dispatching a self-handling command. Laravel by default assumes jobs are self-handling, but it is possible to separate a “command” and its handler in separate classes if you prefer.

Creating a command and handler class

Laravel’s queues documentation seems to use a “process podcast” example throughout, so I’ll stick with that.

Command class

The command class for processing a podcast doesn’t look too different to the job example in the queues documentation; it just doesn’t have a handle() method:

<?php

namespace App\Commands;

use App\Podcast;
use Illuminate\Foundation\Bus\Dispatchable;

class ProcessPodcast
{
    use Dispatchable;

    public $podcast;

    public function __construct(Podcast $podcast)
    {
        $this->podcast = $podcast;
    }
}

We define the class and pass it parameters (which are then stored on the object as public properties). Because dependencies are type-hinted, the command class almost acts as a data transfer object (DTO): you’ll only be able to instantiate the command class if you pass valid arguments to it.

We’ve removed the queue-related trait and interface as we want commands to be handled synchronously (immediately, rather than queued) but have kept the Dispatchable trait for convenience.

Handler class

The handler class would be where you would implement your handle() method:

<?php

namespace App\Handlers\Commands;

use App\AudioProcessor;
use App\Commands\ProcessPodcast;

class ProcessPodcastHandler
{
    protected $processor;

    public function __construct(AudioProcessor $processor)
    {
        $this->processor = $processor;
    }

    public function handle(ProcessPodcast $command)
    {
        $this->processor->process($command->podcast);
    }
}

Note: some things do change when implementing your handler in a separate class like this, though.

The handle() method now receives a single argument: the command class. In the Laravel documentation for queues, it says:

The handle method is called when the job is processed by the queue. Note that we are able to type-hint dependencies on the handle method of the job. The Laravel service container automatically injects these dependencies.

This is no longer the case in a command handler class now that the handle() method instead receives the command. Handler classes are resolved by the service container though, so you can just inject dependencies in its constructor instead like we have done with the AudioProcessor class.

Finally, inside the handler’s handle() method, we use the processor instead we injected in the handler’s constructor to process the podcast passed in via the command class.

Mapping commands to handlers

If you try and dispatch the ProcessPodcast command now it won’t work, as we have not told Laravel how to handle it. To do this, we have to define a mapping. A good place to do this is in a service provider.

If you have a small number of commands, you can define all of your command and handler mappings in a single service provider:

<?php

namespace App\Providers;

use App\Commands\ProcessPodcast;
use App\Handlers\Commands\ProcessPodcastHandler;
use Illuminate\Support\Facades\Bus;
use Illuminate\Support\ServiceProvider;

class BusServiceProvider extends ServiceProvider
{
    public function boot()
    {
        Bus::map([
            ProcessPodcast::class => ProcessPodcastHandler::class,
        ]);
    }
}

If you have a larger application and you have multiple service providers for components or modules in your application, then you can call Bus::map() in each of those service providers to add mapping’s for that particular module’s commands and handlers:

<?php

namespace App\Domain\Podcasts;

use App\Domain\Podcasts\Commands\ProcessPodcast;
use App\Domain\Podcasts\Handlers\Commands\ProcessPodcastHandler;
use Illuminate\Support\Facades\Bus;
use Illuminate\Support\ServiceProvider;

class PodcastServiceProvider extends ServiceProvider
{
    public function boot()
    {
        $this->registerCommandHandlers();
    }

    private function registerCommandHandlers()
    {
        Bus::map([
            ProcessPodcast::class => ProcessPodcastHandler::class,
        ]);
    }
}

Dispatching commands

Now that you have created your command and handler classes, and mapped them to each other, you can dispatch them from controllers, console commands etc:

<?php

namespace App\Http\Controllers;

use App\Commands\ProcessPodcast;
use App\Http\Requests\StorePodcastRequest;
use App\Podcast;

class PodcastController extends Controller
{
    public function store(StorePodcastRequest $request)
    {
        $podcast = Podcast::create($request->validated());

        // Dispatch the command to its handler
        ProcessPodcast::dispatch($podcast);
    }
}

Summary

So in closing, it is possible to have a command bus out of the box in Laravel without having to install any third-party packages or write additional code. The process is:

  • Create a job class, but remove the handle() method and queuing support.
  • Create a corresponding handler class.
  • Create a mapping for your new command and handler classes.
  • Dispatch the job and it will be handled synchronously.

Bonus: Pipelines

One additional feature of the bus component is that it has a notion of a “pipeline”. Pipelines can be thought of as middleware for commands, and each pipeline class in fact has a similar signature to a middleware class.

Pipelines can be defined anywhere in your application but again, it makes sense to define them in a service provider class:

<?php

namespace App\Providers;

use App\Bus\LogCommand;
use App\Bus\UseDatabaseTransactions;
use Illuminate\Support\Facades\Bus;
use Illuminate\Support\ServiceProvider;

class BusServiceProvider extends ServiceProvider
{
    public function boot()
    {
        Bus::pipeThrough([
            LogCommand::class,
            UseDatabaseTransactions::class,
        ]);
    }
}

A pipeline class has a handle() method that takes the command and the next class in the pipeline as its arguments:

<?php

namespace App\Bus;

use Closure;
use Illuminate\Support\Facades\DB;

class UseDatabaseTransactions
{
    public function handle($command, Closure $next)
    {
        return DB::transaction(function () use ($command, $next) {
            return $next($command);
        });
    }
}

Again, pipeline classes are resolved by Laravel’s service container so you can type-hint any dependencies in their constructors:

<?php

namespace App\Bus;

use Closure;
use Psr\Log\LoggerInterface;

class LogCommand
{
    protected $logger;

    public function __construct(LoggerInterface $logger)
    {
        $this->logger = $logger;
    }

    public function handle($command, Closure $next)
    {
        $result = $next($command);

        $this->logger->debug('Command handled: '.get_class($command));

        return $result;
    }
}