In most applications you store the state of the application in the database. If something needs to be changed you simply update values in a table.
When using event sourcing you'll take a different approach. All changes to application state are stored as a series of events. The key benefit here is that you now have a history of your database.
Let's make this idea more clear with a practical example. Imagine you are a bank with customers that have accounts. All these accounts have a balance. When money gets added or subtracted we could modify the balance. If we do that however, we would never know why the balance got to that number. If we were to store all the events we could calculate the balance.
Using the stored events we can do a few other interesting things. Image the managers of the bank are, after some years, interested a report of the average balance of each account in the past year. If you would have stored only the balance itself you wouldn't able to generate that report. But using the stored events you could generate such a report easily.
Getting started with event sourcing
There are a few cool options to start with event sourcing like prooph or EventSauce. These are very powerful packages. When you're new to event sourcing, they are also a bit hard to get started with.
I felt like there should be an easier entry point for Laravel users to get started with event sourcing. That's why I'm currently building a new package called laravel-event-projector. It's beautifully integrated with Laravel. It allows use to easily store events, create projections and reactors (you'll learn what those things are a bit further in this post), replaying events and much more.
In this post I'd like to give you an example on how to use it. In this repo on GitHub you'll find the finished app after following this post.
Let's install the package
laravel-event-projector can be installed into a Laravel app via Composer:
composer require spatie/laravel-event-projector
You need to publish and run the migrations to create the stored_events and projector_statuses_tables:
php artisan vendor:publish --provider="Spatie\EventProjector\EventProjectorServiceProvider" --tag="migrations"
php artisan migrate
With that out of the way we can start using the package.
Writing your first projection
Let's learn how to write a projector. A projector is simply a class that does some work when it hears some events come in. Typically it writes data (to the database or to a file on disk). We call that written data a projection.
Imagine you are a bank with customers that have accounts. All these accounts have a balance. When money gets added or subtracted we could modify the balance. If we do that however, we would never know why the balance got to that number. If we were to store all the events we could calculate the balance.
Creating a model
Here's a small migration to create a table that holds accounts:
use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;
class CreateAccountsTable extends Migration
{
public function up()
{
Schema::create('accounts', function (Blueprint $table) {
$table->increments('id');
$table->string('name');
$table->integer('balance')->default(0);
$table->timestamps();
});
}
}
The Account model itself could look like this:
use App\Events\AccountCreated;
use App\Events\AccountDeleted;
use App\Events\MoneyAdded;
use App\Events\MoneySubtracted;
use Illuminate\Database\Eloquent\Model;
class Account extends Model
{
protected $guarded = [];
public static function createWithAttributes(array $attributes)
{
event(new AccountCreated($attributes));
}
public function addMoney(int $amount)
{
event(new MoneyAdded($this->id, $amount));
return $this;
}
public function subtractMoney(int $amount)
{
event(new MoneySubtracted($this->id, $amount));
return $this;
}
public function close()
{
event(new AccountDeleted($this->id));
}
}
Defining events
Instead of calculating the balance we're simply firing off events. All these events should implement \Spatie\EventProjector\ShouldBeStored. This is an empty interface that signifies to our package that the event should be stored.
Let's take a look at all events used in the Account model.
namespace App\Events;
use Spatie\EventProjector\ShouldBeStored;
class AccountCreated implements ShouldBeStored
{
/** @var array */
public $accountAttributes;
public function __construct(array $accountAttributes)
{
$this->accountAttributes = $accountAttributes;
}
}
namespace App\Events;
use Spatie\EventProjector\ShouldBeStored;
class MoneyAdded implements ShouldBeStored
{
/** @var int */
public $accountId;
/** @var int */
public $amount;
public function __construct(int $accountId, int $amount)
{
$this->accountId = $accountId;
$this->amount = $amount;
}
}
namespace App\Events;
use Spatie\EventProjector\ShouldBeStored;
class MoneySubtracted implements ShouldBeStored
{
/** @var int */
public $accountId;
/** @var int */
public $amount;
public function __construct(int $accountId, int $amount)
{
$this->accountId = $accountId;
$this->amount = $amount;
}
}
namespace App\Events;
use Spatie\EventProjector\ShouldBeStored;
class AccountDeleted implements ShouldBeStored
{
/** @var int */
public $accountId;
public function __construct(int $accountId)
{
$this->accountId = $accountId;
}
}
Creating your first projector
A projector is a class that listens for events that were stored. When it hears an event that it is interested in, it can perform some work.
Let's create your first projector. You can perform php artisan make:projector AccountBalanceProjector to create a projector in app\Projectors.
Here's an example projector that handles all the events mentioned above:
namespace App\Projectors;
use App\Account;
use App\Events\AccountCreated;
use App\Events\AccountDeleted;
use App\Events\MoneyAdded;
use Spatie\EventProjector\Projectors\Projector;
use Spatie\EventProjector\Projectors\ProjectsEvents;
class AccountBalanceProjector implements Projector
{
use ProjectsEvents;
/*
* Here you can specify which event should trigger which method.
*/
public $handlesEvents = [
AccountCreated::class => 'onAccountCreated',
MoneyAdded::class => 'onMoneyAdded',
MoneySubtracted::class => 'onMoneySubtracted',
AccountDeleted::class => 'onAccountDeleted',
];
public function onAccountCreated(AccountCreated $event)
{
Account::create($event->accountAttributes);
}
public function onMoneyAdded(MoneyAdded $event)
{
$account = Account::find($event->accountId);
$account->balance += $event->amount;
$account->save();
}
public function onMoneySubtracted(MoneyAdded $event)
{
$account = Account::find($event->accountId);
$account->balance -= $event->amount;
$account->save();
}
public function onAccountDeleted(AccountDeleted $event)
{
Account::find($event->accountId)->delete();
}
}
Registering your projector
The projector code up above will handle all the work that needs to be done for the given events. For the package to be able to locate this projector you should register it. The easiest way to register a projector is by calling addProjector on the EventProjectionist facade. Typically you would put this in a service provider of your own.
use \Spatie\EventProjector\Facades\EventProjectionist;
use \App\Projectors\AccountBalanceProjector;
...
EventProjectionist::addProjector(AccountBalanceProjector::class)
Let's fire off some events
With all this out of the way we can fire off some events.
Let's try adding an account with:
$account = Account::createWithAttributes(['name' => 'Luke']);
$anotherAccount = Account::createWithAttributes(['name' => 'Leia']);
And let's make some transactions on that account:
// do stuff like this
$account->addMoney(1000);
$anotherAccount->addMoney(500);
$account->subtractMoney(50);
...
If you take a look at the contents of the accounts table you should see some accounts together with their calculated balance. Sweet! In the stored_events you should see an entry per event that we fired.
Your second projector
Image that after a while someone at the bank wants to know which accounts have processed the most transactions. Because we stored all changes to the accounts in the events table we can easily get that info by creating another projector.
We are going to create another projector that stores the transaction count per account in a model. Bear in mind that you can easily use any other storage mechansim instead of a model. The projector doesn't care what you use.
Here's the migration and the model class that the projector is going to use:
use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;
class CreateTransactionCountsTable extends Migration
{
public function up()
{
Schema::create('transaction_counts', function (Blueprint $table) {
$table->increments('id');
$table->integer('account_id');
$table->integer('count')->default(0);
$table->timestamps();
});
}
}
namespace App;
use Illuminate\Database\Eloquent\Model;
class TransactionCount extends Model
{
public $guarded = [];
}
And here's the projector that is going to listen to the MoneyAdded and MoneySubtracted events
namespace App\Projectors;
use App\Events\MoneyAdded;
use App\Events\MoneySubtracted;
use App\TransactionCount;
use Spatie\EventProjector\Projectors\Projector;
use Spatie\EventProjector\Projectors\ProjectsEvents;
class TransactionCountProjector implements Projector
{
use ProjectsEvents;
public $handlesEvents = [
MoneyAdded::class => 'onMoneyAdded',
MoneySubtracted::class => 'onMoneySubtracted',
];
public function onMoneyAdded(MoneyAdded $event)
{
$transactionCounter = TransactionCount::firstOrCreate(['account_id' => $event->accountId]);
$transactionCounter->count +=1;
$transactionCounter->save();
}
public function onMoneySubtracted(MoneySubtracted $event)
{
$transactionCounter = TransactionCount::firstOrCreate(['account_id' => $event->accountId]);
$transactionCounter->count -=1;
$transactionCounter->save();
}
}
Let's not forget to register this projector:
// in a service provider of your own
EventProjectionist::addProjector(TransactionCountProjector::class)
If you've followed along you've already created some accounts and some events. To feed those past events to the projector we can simply perform this artisan command:
php artisan event-projector:replay-events
This command will take all events stored in the stored_events table and pass them to TransactionCountProjector. After the command completes you should see the transaction counts in the transaction_counts table.
Welcoming new events
Now that both of your projections have handled all events, try firing off some new events.
$yetAnotherAccount = Account::createWithAttributes(['name' => 'Yoda']);
And let's add some transactions to that account:
// do stuff like this
$yetAnotherAccount->addMoney(1000);
$yetAnotherAccount->subtractMoney(50);
You'll notice that both projectors are doing their jobs. The balance of the Account model is up to date and the data in the transaction_counts table gets updated.
Benefits of projectors and projections
The cool thing of projectors is that you can write them after events have happened. Image that someone at the bank wants to have a report of the average balance of each account of the past year. You would be able to write a new projector, replay all events and let that projector generate such a report.
Projections are very fast to query. Image that our application has processed millions of events. If you want to create a screen where you display the accounts with the most transactions you can easily query the transaction_counts table. This way you don't need to fire off some expensive query. The projector will keep the projections (the transaction_counts table) up to date.
Handling side effects using reactors
Now that you've written your first projector, let's learn how to handle side effects. With side effects we mean things like sending a mail, sending a notification, ... You only want to perform these actions when the original event happens. You don't want to do this work when replaying events.
A reactor is a class, that much like a projector, listens for incoming events. Unlike projectors however, reactors will not get called when events are replayed. Reactors only will get called when the original event fires.
Creating your first reactor
Let's create your first reactor. You can perform php artisan make:reactor BigAmountAddedReactor to create a projector in app\Reactors. We will make this reactor send a mail to the director of the bank whenever a a big amount of money is added to an account.
namespace App\Reactors;
use App\Account;
use App\Events\MoneyAdded;
use App\Mail\BigAmountAddedMail;
use Illuminate\Support\Facades\Mail;
class BigAmountAddedReactor
{
/*
* Here you can specify which event should trigger which method.
*/
public $handlesEvents = [
MoneyAdded::class => 'onMoneyAdded',
];
public function onMoneyAdded(MoneyAdded $event)
{
if ($event->amount < 900) {
return;
}
$account = Account::find($event->accountId);
Mail::to('director@bank.com')->send(new BigAmountAddedMail($account, $event->amount));
}
}
Registering your reactor
For the package to be able to locate the reactor you should register it. The easiest way to register a projector is by calling addReactor on the EventProjectionist facade. Typically you would put this in a service provider of your own.
use \Spatie\EventProjector\Facades\EventProjectionist;
use \App\Reactor\BigAmountAddedReactor;
...
EventProjectionist::addReactor(BigAmountAddedReactor::class)
Using the reactor
The reactor above will send an email to the director of the bank whenever an amount of 900 or more gets added to an account. Let's put the reactor to work.
$account = Account::createWithAttributes(['name' => 'Rey']);
$account->addMoney(1000);
A mail will be sent to the director.
If you truncate the accounts table and rebuild the contents with
php artisan event-projector:replay-events
no mail will be sent.
Closing thoughts
I hope you've enjoyed the tour of the package. Like mentioned above you can find an Laravel app with all examples of this post in this repo on GitHub.
In the coming days and weeks I'll work towards publishing a stable release of the package. It should already be pretty stable if you want to toy with it, but keep in mind that it's certainly not production ready yet. I might still make breaking changes. If you have any questions about it feel free to open an issue on the laravel-event-projector repo.
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