CQRS Knowledge Base

Quick reference for CQRS architecture patterns and PHP implementation guidelines.

Core Principles

Separation of Concerns

┌─────────────────────────────────────────────────────────────┐
│                      APPLICATION                            │
├─────────────────────────────────────────────────────────────┤
│   WRITE SIDE (Commands)     │     READ SIDE (Queries)       │
├─────────────────────────────┼───────────────────────────────┤
│ Command → Handler → Domain  │  Query → Handler → ReadModel  │
│ Changes state               │  Returns data, no side effects│
│ Uses Domain Model           │  Can bypass Domain Model      │
│ Single aggregate per cmd    │  Can join multiple sources    │
└─────────────────────────────┴───────────────────────────────┘

Rule: Commands WRITE, Queries READ. Never mix.

CQRS Components

Component	Purpose	Returns	Side Effects
Command	Request to change state	void or ID	Yes
CommandHandler	Executes command logic	void or ID	Yes
Query	Request for data	Data DTO	No
QueryHandler	Fetches and transforms data	Data DTO	No
CommandBus	Routes commands to handlers	Depends	N/A
QueryBus	Routes queries to handlers	Query result	N/A

Quick Checklists

Command Checklist

• Named as imperative verb + noun (CreateOrder, ConfirmPayment)
• Immutable (readonly class)
• Contains only data needed for operation
• Returns void or created ID (never entities)
• One command = one aggregate affected
• Validated before dispatch

Query Checklist

• Named as Get/Find/List + noun (GetOrderDetails, ListCustomers)
• Immutable (readonly class)
• Contains filtering/pagination params
• Handler has NO side effects
• Can use optimized read models
• Returns DTOs, not entities

Handler Checklist

• Single execute() or __invoke() method
• One handler per command/query
• CommandHandler can dispatch domain events
• QueryHandler never dispatches events
• No cross-aggregate transactions in single handler

Common Violations Quick Reference

Violation	Where to Look	Severity
Query with side effects	QueryHandler calling save()	Critical
Command returning data	CommandHandler returning entity	Critical
Mixed read/write in handler	Handler with both get and save	Critical
Business logic in handler	if/switch on domain state	Warning
Missing command validation	Command without invariants	Warning
Query using write DB	QueryHandler using EntityManager	Info

PHP 8.4 CQRS Patterns

Command

final readonly class CreateOrderCommand
{
    public function __construct(
        public CustomerId $customerId,
        /** @var array<OrderLineData> */
        public array $lines,
        public ?string $notes = null
    ) {
        if (empty($lines)) {
            throw new InvalidArgumentException('Order must have at least one line');
        }
    }
}

Command Handler

final readonly class CreateOrderHandler
{
    public function __construct(
        private OrderRepositoryInterface $orders,
        private EventDispatcherInterface $events
    ) {}

    public function __invoke(CreateOrderCommand $command): OrderId
    {
        $order = Order::create(
            id: OrderId::generate(),
            customerId: $command->customerId,
            lines: $command->lines
        );

        $this->orders->save($order);

        foreach ($order->releaseEvents() as $event) {
            $this->events->dispatch($event);
        }

        return $order->id();
    }
}

Query

final readonly class GetOrderDetailsQuery
{
    public function __construct(
        public OrderId $orderId
    ) {}
}

Query Handler

final readonly class GetOrderDetailsHandler
{
    public function __construct(
        private OrderReadModelInterface $readModel
    ) {}

    public function __invoke(GetOrderDetailsQuery $query): ?OrderDetailsDTO
    {
        return $this->readModel->findById($query->orderId);
    }
}

References

For detailed information, load these reference files:

• references/command-patterns.md — Command structure, naming, validation
• references/query-patterns.md — Query structure, read models
• references/handler-patterns.md — Handler patterns, async/sync
• references/bus-patterns.md — Command/Query bus implementations
• references/antipatterns.md — Common violations with detection patterns