Event Store¶
An event store is an append-only database that stores domain events as the primary source of truth. Instead of persisting the current state of an entity (as a traditional relational database does), the event store records every state-changing event in the order it occurred. The current state is derived by replaying all events for a given aggregate.
Event stores have three fundamental properties: events are immutable (never updated or deleted), ordered (within an aggregate stream), and complete (no information is lost). This makes them naturally auditable, debuggable, and replayable.
An event store differs from a general-purpose message queue: it is designed for long-term persistence and random-access replay, not just for transient message delivery. EventStoreDB is a purpose-built event store. Kafka is also used as an event store when configured with long retention periods.
In PHP event-sourcing implementations, the event store is typically an abstraction over a PostgreSQL table (with JSON event payloads), Kafka, or EventStoreDB. Libraries such as Prooph or Broadway provide event store interfaces.
<?php
// Simple append-only event store backed by PostgreSQL
class PostgresEventStore {
public function append(string $streamId, array $events, int $expectedVersion): void {
$this->pdo->beginTransaction();
foreach ($events as $i => $event) {
$stmt = $this->pdo->prepare(
'INSERT INTO events (stream_id, version, event_type, payload, occurred_at)
VALUES (:stream, :version, :type, :payload, NOW())'
);
$stmt->execute([
'stream' => $streamId,
'version' => $expectedVersion + $i + 1,
'type' => get_class($event),
'payload' => json_encode($event),
]);
}
$this->pdo->commit();
}
public function load(string $streamId, int $fromVersion = 0): array {
$stmt = $this->pdo->prepare(
'SELECT * FROM events WHERE stream_id = ? AND version > ? ORDER BY version'
);
$stmt->execute([$streamId, $fromVersion]);
return $stmt->fetchAll();
}
}
?>
See also EventStoreDB and Wikipedia: Event store.
Related : Event Sourcing, Event Replay, Event Streaming, Audit Trail, Domain Design Driven (DDD)
Related packages : prooph/event-store
