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What makes Event Sourcing so interesting? I it is not the free, proven, audit trial, nor the possibility of reincarnating object in any particular state it had in the history. It is also not the great performance of add-only event store. It is testability.

Why Event Sourcing makes things testable? By splitting up logic responsible for two distinct things: calculating new state of the object and applying this state change. Normally, these two operations are mixed in one business logic method like this:

public class Customer
{
    public void PlaceOrder(Product product, int quantity)
    {
        CheckProductAvailability();
        CheckCustomerCreditLimit();
        CheckSomethingElse();
        decimal price = CalculateTotalPrice(product, quantity);
        Order o = new Order(product, quantity, price);
        _orders.Add(o);
    }
    //...
}

If you read this carefully, you will see that first 4 lines contain the business logic of state change calculation. The last 2 lines, on the other hand, contain no logic at all. Now, what’s the problem with this code’s testability?

The problem becomes visible, when I want to test how PlaceOrder behaves when adding fifth order. I have to call PlaceOrder four times to prepare state of the object. That is wrong, because my test code invokes a lot of logic during preparation phase. This means that a lot of things can go wrong and nobody likes tests which fail before the actual test code gets executed.

Let’s see if PlaceOrder can be corrected using SOLID principles. Does it have exactly one reason to change? It seems like it does. When order placement logic changes, the method changes, right? Let’s dig deeper. What about the last two lines? Do they change when the rest of method changes? Well, I don’t think so. Looks like they indeed do represent a different concern. So let’s refactor them to a separate method.

public class Customer
{
    public void PlaceOrder(Product product, int quantity)
    {
        CheckProductAvailability();
        CheckCustomerCreditLimit();
        CheckSomethingElse();
        decimal price = CalculateTotalPrice(product, quantity);
        ApplyPlaceOrder(product, quantity, price);
    }
    private void ApplyPlaceOrder(Product product, int quantity, decimal price)
    {
        Order o = new Order(product, quantity, price);
        _orders.Add(o);
    }
    //...
}

Now it looks better. At least from the have one reason to change principle. But I don’t see this improved testability at all. That is because although we split the responsibilities to separate methods, we left the methods tightly coupled to one another. Lets see what we can do about this.

public class Customer
{
    public void PlaceOrder(Product product, int quantity)
    {
        CheckProductAvailability();
        CheckCustomerCreditLimit();
        CheckSomethingElse();
        decimal price = CalculateTotalPrice(product, quantity);
        Apply(new OrderPlacedEvent(product, quantity, price));
    }
    private void OnOrderPlaced(OrderPlacedEvent @event)
    {
        Order o = new Order(@event.Product, @event.Quantity, @event.Price);
        _orders.Add(o);
    }
    //...
}

We introduced a concept of event as a way of encapsulating the state change we are about to apply. Now we can represent state changes by a series of events. The other thing we did was adding the Apply method. What is it? The Apply method is the API of Event Sourcing. It is a way of communicating hey, I, the aggregate root, want to apply the state change represented by this event! Apply invokes a method to apply state change. It can do this in a variety of ways: based on convention (like On… convention in this sample), an attribute or any other. The bottom line is, Apply applies the state change and it can be called externally, for example during tested state preparation.

[TestFixture]
public class CustomerTests
{
    [Test]
    public void When_placing_fifth_order()
    {
        Product product = new Product();
        //Arrange
        Customer c = new Customer();
        c.Apply(new OrderPlacedEvent(product, 1, 10));
        c.Apply(new OrderPlacedEvent(product, 1, 10));
        c.Apply(new OrderPlacedEvent(product, 1, 10));
        c.Apply(new OrderPlacedEvent(product, 1, 10));

        //Act
        c.PlaceOrder(product, 1);

        //Assert...
    }
}

You can see that we invoke no business logic during arrange phase. We only apply predefined, known to be right, state changes. The chance something bad will happen during this phase is minimal, as we certainly tested the OnOrderPlaced method in a different test, didn’t we?

This is what I call well tested code. PlaceOrder method is always tested in isolation, no matter if it is first or fifth order. One cannot overestimate the benefits of a testable codebase. If you happen to play (or work) with Domain-Driven Design, Event Sourcing can be beneficial for you. I see it as the missing piece of puzzle — the model is finally fully testable.