How do CQRS and event sourcing apply to Go microservice architecture?

CQRS (Command Query Responsibility Segregation) separates reads and writes into separate models. Event Sourcing stores every state change as an event — the current state is derived by replaying events. Both patterns appear in high-scale Go systems.

// CQRS: separate command and query handlers
// Command: mutates state, returns error
type CreateOrderCommand struct {
    UserID    string
    ProductID string
    Quantity  int
}

type OrderCommandHandler struct{ eventStore EventStore }

func (h *OrderCommandHandler) Handle(
    ctx context.Context, cmd CreateOrderCommand,
) error {
    event := OrderCreatedEvent{
        OrderID:   uuid.New().String(),
        UserID:    cmd.UserID,
        ProductID: cmd.ProductID,
        Quantity:  cmd.Quantity,
        CreatedAt: time.Now(),
    }
    return h.eventStore.Append(ctx, event.OrderID, event)
}

// Query: reads from a projection (read model)
type OrderQueryHandler struct{ readDB *sql.DB }

func (h *OrderQueryHandler) GetOrder(
    ctx context.Context, orderID string,
) (*OrderView, error) {
    // Read from denormalised view optimised for queries
    row := h.readDB.QueryRowContext(ctx,
        "SELECT id, status, total, user_name FROM order_views WHERE id=$1",
        orderID)
    var view OrderView
    return &view, row.Scan(&view.ID, &view.Status, &view.Total, &view.UserName)
}

// Event Sourcing: replay events to reconstruct state
type Order struct {
    ID      string
    Status  string
    Items   []OrderItem
    version int
}

func ReplayOrder(ctx context.Context, store EventStore, id string) (*Order, error) {
    events, err := store.Load(ctx, id)
    if err != nil { return nil, err }

    order := &Order{ID: id}
    for _, event := range events {
        order.apply(event)   // deterministically mutate state
        order.version++
    }
    return order, nil
}

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