write-specs-events

name	write-specs-events
description	Use this skill when asked to write tests or specs for event appending, event log behavior, constraint violations, or concurrency violations using EventScenario in a Cratis-based project. Produces infrastructure-free in-process specs using EventScenario and AppendResult Should* extensions.

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Writing Event Sequence Specs with EventScenario

Use EventScenario to test event-appending behavior in-process — no Chronicle server, database, or network required.

When to use this skill

• Testing that a command appends the correct event(s)
• Testing that a constraint correctly rejects a duplicate or conflicting append
• Testing AppendMany for batch event appends
• Testing concurrency or error conditions in event appending

For testing read model projections or reducers, use ReadModelScenario<TReadModel> instead.

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Package

dotnet add package Cratis.Chronicle.Testing

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Basic structure

// In your spec file (follows BDD Establish/Because/should_ pattern)
public class when_appending_<event_name> : Specification
{
    EventScenario _scenario;
    IAppendResult _result;

    void Establish() => _scenario = new EventScenario();

    async Task Because() =>
        _result = await _scenario.EventLog.Append(<eventSourceId>, new <EventType>(<args>));

    [Fact] void should_be_successful() => _result.ShouldBeSuccessful();
}

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Step 1 — Create the scenario

Always create a new EventScenario per test. It starts with an empty event log and sequence number zero.

var scenario = new EventScenario();

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Step 2 — Seed pre-existing state (optional)

Use the fluent Given builder when the act phase depends on prior state:

await scenario.Given
    .ForEventSource(authorId)
    .Events(new AuthorRegistered("Jane Smith"), new BookAdded("Clean Code"));

Chain multiple ForEventSource calls to seed different event sources:

await scenario.Given
    .ForEventSource(author1Id)
    .Events(new AuthorRegistered("Jane Smith"));

await scenario.Given
    .ForEventSource(author2Id)
    .Events(new AuthorRegistered("John Doe"));

Rules:

• Call Given before the act phase — seeded events get monotonically increasing sequence numbers.
• Do not put the act-under-test inside Given; only pre-existing state goes here.

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Step 3 — Append and assert

var result = await scenario.EventLog.Append(eventSourceId, new SomeEvent("value"));
result.ShouldBeSuccessful();

Use AppendMany for batch appends:

var result = await scenario.EventLog.AppendMany(cartId, [
    new ItemAddedToCart(itemId1),
    new ItemAddedToCart(itemId2)
]);
result.ShouldBeSuccessful();

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Assertion reference

Method	Asserts
result.ShouldBeSuccessful()	Operation succeeded with no violations or errors
result.ShouldBeFailed()	Operation failed (any violation or error)
result.ShouldHaveConstraintViolations()	At least one constraint violation is present
result.ShouldNotHaveConstraintViolations()	No constraint violations are present
result.ShouldHaveConstraintViolationFor("ConstraintName")	A violation for the named constraint is present
result.ShouldHaveConcurrencyViolations()	At least one concurrency violation is present
result.ShouldNotHaveConcurrencyViolations()	No concurrency violations are present
result.ShouldHaveErrors()	At least one error is present
result.ShouldNotHaveErrors()	No errors are present

All Should* methods throw AppendResultAssertionException on failure with a message that lists all violations and errors found.

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Example: happy path

public class when_registering_an_author : Specification
{
    EventScenario _scenario;
    IAppendResult _result;

    void Establish() => _scenario = new EventScenario();

    async Task Because() =>
        _result = await _scenario.EventLog.Append(AuthorId.New(), new AuthorRegistered("Jane Smith"));

    [Fact] void should_be_successful() => _result.ShouldBeSuccessful();
    [Fact] void should_not_have_constraint_violations() => _result.ShouldNotHaveConstraintViolations();
}

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Example: constraint violation (duplicate)

Seed the pre-existing state, then verify the second append is rejected:

public class when_registering_an_author
{
    public class and_the_name_already_exists : Specification
    {
        EventScenario _scenario;
        IAppendResult _result;

        async Task Establish()
        {
            _scenario = new EventScenario();
            await _scenario.Given
                .ForEventSource(AuthorId.New())
                .Events(new AuthorRegistered("Jane Smith"));
        }

        async Task Because() =>
            _result = await _scenario.EventLog.Append(AuthorId.New(), new AuthorRegistered("Jane Smith"));

        [Fact] void should_be_failed() => _result.ShouldBeFailed();
        [Fact] void should_have_a_constraint_violation() => _result.ShouldHaveConstraintViolations();
        [Fact] void should_have_constraint_violation_for_unique_name() =>
            _result.ShouldHaveConstraintViolationFor("UniqueAuthorName");
    }
}

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Example: batch append

public class when_adding_multiple_items_to_cart : Specification
{
    EventScenario _scenario;
    IAppendResult _result;

    void Establish() => _scenario = new EventScenario();

    async Task Because() =>
        _result = await _scenario.EventLog.AppendMany(cartId, [
            new ItemAddedToCart(itemId1),
            new ItemAddedToCart(itemId2),
            new ItemQuantityAdjusted(itemId1, 3)
        ]);

    [Fact] void should_be_successful() => _result.ShouldBeSuccessful();
}

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Checklist

• New EventScenario instance per test method (never shared state)
• Given called before the act phase for all pre-existing state
• Each test asserts a single outcome (one should_ per behavior)
• Constraint violation tests include ShouldHaveConstraintViolationFor("ConstraintName")
• Run dotnet build and dotnet test — fix all failures before completing