Snapshot Generator

Creates Snapshot infrastructure for optimizing event sourcing aggregate rebuilds.

When to Use

Scenario	Example
Long event streams	Aggregates with 100+ events in their history
Slow aggregate rebuild	Event replay taking too long for read operations
Frequent reads	Aggregates loaded many times per second
Large aggregates	Complex state requiring many events to reconstruct

Component Characteristics

Snapshot

• Immutable state capture of an aggregate at a point in time
• Properties: aggregateId, aggregateType, version, state (JSON), createdAt
• Supports fromArray() / toArray() for serialization
• Version must be >= 1

SnapshotStoreInterface

• save(Snapshot $snapshot): void — persist snapshot (upsert)
• load(string $aggregateId): ?Snapshot — retrieve latest snapshot
• delete(string $aggregateId): void — remove all snapshots for aggregate

SnapshotStrategy

• Configurable event threshold (default: 100)
• shouldTakeSnapshot(int $eventsSinceLastSnapshot): bool
• Determines when a new snapshot should be captured

AggregateSnapshotter

• Application service coordinating snapshot operations
• Loads aggregate from snapshot + remaining events
• Takes snapshots when strategy threshold is met

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Generation Process

Step 1: Generate Domain Components

Path: src/Domain/{BC}/Snapshot/

1. Snapshot.php — Immutable snapshot value object
2. SnapshotStoreInterface.php — Repository interface for snapshot persistence

Step 2: Generate Application Components

Path: src/Application/{BC}/Snapshot/

1. SnapshotStrategy.php — Configurable threshold strategy
2. AggregateSnapshotter.php — Application service for snapshot operations

Step 3: Generate Infrastructure Components

Path: src/Infrastructure/{BC}/Snapshot/

1. DoctrineSnapshotStore.php — Doctrine DBAL implementation of SnapshotStoreInterface
2. Database migration SQL for snapshots table

Step 4: Generate Tests

1. SnapshotTest.php — Construction, serialization, validation tests
2. SnapshotStrategyTest.php — Threshold behavior tests
3. AggregateSnapshotterTest.php — Load and take snapshot tests

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File Placement

Component	Path
Snapshot, SnapshotStoreInterface	src/Domain/{BC}/Snapshot/
SnapshotStrategy, AggregateSnapshotter	src/Application/{BC}/Snapshot/
DoctrineSnapshotStore	src/Infrastructure/{BC}/Snapshot/
Unit Tests	tests/Unit/Domain/{BC}/Snapshot/, tests/Unit/Application/{BC}/Snapshot/

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Naming Conventions

Component	Pattern	Example
Value Object	Snapshot	Snapshot
Store Interface	SnapshotStoreInterface	SnapshotStoreInterface
Strategy	SnapshotStrategy	SnapshotStrategy
Application Service	AggregateSnapshotter	AggregateSnapshotter
Infrastructure Store	DoctrineSnapshotStore	DoctrineSnapshotStore
Test	{ClassName}Test	SnapshotTest

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Quick Template Reference

Snapshot

final readonly class Snapshot
{
    public function __construct(
        public string $aggregateId,
        public string $aggregateType,
        public int $version,
        public string $state,
        public \DateTimeImmutable $createdAt
    ) {
        // version >= 1 validation
    }

    public static function fromArray(array $data): self;
    public function toArray(): array;
}

SnapshotStoreInterface

interface SnapshotStoreInterface
{
    public function save(Snapshot $snapshot): void;
    public function load(string $aggregateId): ?Snapshot;
    public function delete(string $aggregateId): void;
}

SnapshotStrategy

final readonly class SnapshotStrategy
{
    public function __construct(
        private int $eventThreshold = 100
    ) {}

    public function shouldTakeSnapshot(int $eventsSinceLastSnapshot): bool;
}

---

Usage Example

$snapshotter = new AggregateSnapshotter($snapshotStore, $strategy);

// Load aggregate from snapshot + remaining events
$result = $snapshotter->loadWithSnapshot($aggregateId, $eventStore);
$snapshot = $result['snapshot'];
$remainingEvents = $result['remainingEvents'];

$aggregate = $snapshot !== null
    ? Order::fromSnapshot($snapshot)
    : new Order($aggregateId);

foreach ($remainingEvents as $event) {
    $aggregate->apply($event);
}

// Take snapshot if threshold reached
$snapshotter->takeSnapshotIfNeeded(
    aggregateId: $aggregateId,
    aggregateType: 'Order',
    version: $aggregate->getVersion(),
    state: $aggregate->toSnapshot(),
    eventsSinceSnapshot: count($remainingEvents)
);

---

Database Schema

CREATE TABLE snapshots (
    id BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    aggregate_id VARCHAR(36) NOT NULL,
    aggregate_type VARCHAR(255) NOT NULL,
    version INT UNSIGNED NOT NULL,
    state JSON NOT NULL,
    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
    UNIQUE INDEX idx_snapshots_aggregate_id (aggregate_id),
    INDEX idx_snapshots_aggregate_type (aggregate_type)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

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Anti-patterns to Avoid

Anti-pattern	Problem	Solution
Snapshot Every Event	Storage bloat, no performance gain	Use threshold strategy (e.g., every 100 events)
Mutable Snapshots	State corruption, debugging nightmares	Make Snapshot immutable (final readonly)
Missing Version	Cannot determine event replay start point	Always track version in snapshot
No Cleanup	Unbounded storage growth	Implement retention policy, keep only latest
Tight Coupling	Snapshot tied to infrastructure	Domain interface, infrastructure implementation
Skipping Validation	Invalid snapshots persisted	Validate version >= 1, non-empty state

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References

For complete PHP templates and examples, see:

• references/templates.md — Snapshot, SnapshotStoreInterface, SnapshotStrategy, AggregateSnapshotter, DoctrineSnapshotStore templates
• references/examples.md — OrderAggregate integration, event sourcing examples and tests