In some cases the business has rules which imply a restart of the event stream for an aggregate since the past events are not relevant for the current state. A bank is often used as an example. A bank account has hundreds of transactions, but every bank makes a balance report at the end of the year. In this step the current account balance is persisted. This event is perfect to split the stream and start aggregating from this point.
Not only do some businesses require such an action, it also increases the performance for aggregates which would have a really long event stream.
In the background the library will mark all past events as archived and will not load them anymore for building the aggregate. It will only load the events from the split event and onwards. But subscriptions will still receive all events. So you can create projections which are based on the full event stream.
To use this feature you need to add the SplitStreamDecorator in the repository manager.
use Patchlevel\EventSourcing\Metadata\AggregateRoot\AggregateRootRegistry;
use Patchlevel\EventSourcing\Metadata\Event\EventMetadataFactory;
use Patchlevel\EventSourcing\Repository\DefaultRepositoryManager;
use Patchlevel\EventSourcing\Repository\MessageDecorator\SplitStreamDecorator;
use Patchlevel\EventSourcing\Store\Store;
/**
* @var AggregateRootRegistry $aggregateRootRegistry
* @var Store $store
* @var EventMetadataFactory $eventMetadataFactory
*/
$repositoryManager = new DefaultRepositoryManager(
$aggregateRootRegistry,
$store,
null,
null,
new SplitStreamDecorator($eventMetadataFactory),
);You can find out more about the message decorator.
You can use multiple decorators with the ChainMessageDecorator.
To use this feature you need to mark the event which should split the stream.
For that you can use the #[SplitStream] attribute.
use Patchlevel\EventSourcing\Attribute\Event;
use Patchlevel\EventSourcing\Attribute\SplitStream;
#[Event('bank_account.balance_reported')]
#[SplitStream]
final class BalanceReported
{
public function __construct(
public BankAccountId $bankAccountId,
public int $year,
public int $balanceInCents,
) {
}
}The event needs all data which is relevant for the aggregate, since all past events will not be loaded! Keep this in mind if you want to use this feature.
This impacts only the aggregate loaded by the repository. Subscriptions will still receive all events.
You can combine this feature with the snapshot feature to increase the performance even more.