CQRS: Command/Query разделение, зачем и когда?

public class Account
{
    public decimal Balance { get; private set; }         // query — только читает
    public void Deposit(decimal amount) { Balance += amount; }   // command — только меняет

    // Нарушение CQS: и меняет, и возвращает — неочевидные побочные эффекты при «чтении»
    public decimal WithdrawAndGetBalance(decimal amount) { Balance -= amount; return Balance; }
}

public sealed record PlaceOrderCommand(Guid CustomerId, IReadOnlyList<OrderLine> Lines) : IRequest<Guid>;

public sealed class PlaceOrderHandler(ShopDbContext db, IClock clock) : IRequestHandler<PlaceOrderCommand, Guid>
{
    public async Task<Guid> Handle(PlaceOrderCommand cmd, CancellationToken ct)
    {
        var customer = await db.Customers.SingleAsync(c => c.Id == cmd.CustomerId, ct);
        var order = customer.PlaceOrder(cmd.Lines, clock.UtcNow);        // доменная логика и инварианты
        db.Orders.Add(order);
        await db.SaveChangesAsync(ct);
        return order.Id;
    }
}

public sealed record GetCustomerOrdersQuery(Guid CustomerId, int Page) : IRequest<IReadOnlyList<OrderListItem>>;

public sealed class GetCustomerOrdersHandler(IDbConnectionFactory connections) : IRequestHandler<GetCustomerOrdersQuery, IReadOnlyList<OrderListItem>>
{
    public async Task<IReadOnlyList<OrderListItem>> Handle(GetCustomerOrdersQuery q, CancellationToken ct)
    {
        await using var conn = await connections.OpenReadOnlyAsync(ct);        // можно на реплику
        var rows = await conn.QueryAsync<OrderListItem>("""
            SELECT o.id, o.created_at, o.status, SUM(i.qty * i.price) AS total, COUNT(*) AS items
            FROM orders o JOIN order_items i ON i.order_id = o.id
            WHERE o.customer_id = @CustomerId
            GROUP BY o.id
            ORDER BY o.created_at DESC
            LIMIT 20 OFFSET @Offset
            """, new { q.CustomerId, Offset = q.Page * 20 });
        return rows.AsList();
    }
}