Testing (make yourself dangerous)

Event sourcing is unusually testable because:

• your “database writes” are events (data)
• your business logic is in aggregate methods (pure-ish)

You want tests that read like:

Given history, when command, then new events (or error)

Minimal test harness

public static class EsTest
{
  public static BankAccount Given(params IDomainEvent[] history)
  {
    var acc = new BankAccount();
    acc.LoadFromHistory(history);
    return acc;
  }

  public static IReadOnlyList<IDomainEvent> When(BankAccount acc, Action<BankAccount> act)
  {
    act(acc);
    return acc.DequeueUncommittedEvents();
  }
}

Example tests (xUnit-style)

using Xunit;

public sealed class BankAccountTests
{
  [Fact]
  public void Withdraw_rejects_if_insufficient_funds()
  {
    var acc = EsTest.Given(
      new AccountOpened(Guid.Parse("11111111-1111-1111-1111-111111111111"), "Jeevan"),
      new MoneyDeposited(10m)
    );

    var ex = Assert.Throws<InvalidOperationException>(() => acc.Withdraw(50m));
    Assert.Equal("Insufficient funds.", ex.Message);
  }

  [Fact]
  public void Withdraw_emits_event_when_valid()
  {
    var acc = EsTest.Given(
      new AccountOpened(Guid.Parse("11111111-1111-1111-1111-111111111111"), "Jeevan"),
      new MoneyDeposited(100m)
    );

    var emitted = EsTest.When(acc, a => a.Withdraw(30m));

    Assert.Collection(emitted,
      e => Assert.Equal(new MoneyWithdrawn(30m), e)
    );
  }

  [Fact]
  public void Close_prevents_further_deposits()
  {
    var acc = EsTest.Given(
      new AccountOpened(Guid.Parse("11111111-1111-1111-1111-111111111111"), "Jeevan"),
      new AccountClosed(DateTimeOffset.Parse("2025-01-01T00:00:00Z"))
    );

    var ex = Assert.Throws<InvalidOperationException>(() => acc.Deposit(10m));
    Assert.Equal("Account is closed.", ex.Message);
  }
}

Property-based tests (optional, high leverage)

If you want to level up fast:

• generate random sequences of deposits/withdrawals
• assert invariants always hold (balance never < 0, closed account never changes, etc.)

You can do this with FsCheck, but the core idea matters more than the tool.

Next

Now we tackle the hard part: running this in production over time.

Next: Operations + evolution