What is the Event Sourcing pattern and when should it…

Event Sourcing is an architectural pattern where all changes to an application's state are stored as a sequence of immutable events. Each event represents a fact that occurred within the system. The current state of the system is reconstructed by "playing back" all events in chronological order.

Key principles:

Immutability: Events are not deleted or altered.
Sequence: Events are stored in strict chronological order.
State recovery: The current state is formed based on the history of events.

Advantages:

Complete history of changes: Allows tracking every action leading to the current state.
Debugging and auditing: Facilitates error cause analysis and user behavior analysis.
Time-travel debugging: Ability to "rollback" to any past moment.
Flexibility for projection changes: Easily create various data views based on a single event stream.
Passive integration: Other systems can subscribe to the event stream.

Disadvantages:

Implementation complexity: Requires a change in thinking and infrastructure for storing and processing events.
State recovery can be slow for a large number of events.
Difficulties with data deletion (GDPR, etc.).

Applications:

Complex domain models: Where change history and causal relationships are important.
Audit and logging systems: For detailed operation tracking.
Applications requiring "time machine": For example, financial systems or games.
High concurrency systems: Events are easily processed asynchronously.
Systems requiring flexible adaptation to new requirements: Easy to build new data views.

When to use:

When change history is a critical feature.
When detailed user or system action auditing is needed.
When the ability to "rollback" to previous states is invaluable.
When creating various "projections" of data for different purposes (e.g., read models).
When integration with other systems via event subscription is planned.

Example of an event structure:

{
    "event_id": "...",
    "aggregate_id": "...", // ID of the entity related to the event
    "event_type": "OrderPlaced",
    "occurred_at": "...",
    "payload": { // Data related to the event
        "order_id": "...",
        "user_id": "...",
        "amount": 100.00
    }
}

Example of loading an aggregate from events:

// Load events for the aggregate
$events = $eventStore->getEventsForAggregate($aggregateId);

// Create a new instance of the aggregate
$order = new Order();

// Apply events to the aggregate to restore its state
foreach ($events as $event) {
    $order->apply($event); // The apply method applies changes corresponding to the event
}

// Now $order is in its current state

What is the Event Sourcing pattern and when should it be used?