Axon5kotlin Read Slice

Axon Framework 5 — Read Slice

Relationship to prooph board Event Modeling

This skill implements the read slice (green stripe) from a prooph board Event Modeling board. It consumes:

• The slice's ## Scenarios (GWTs) section (written with the slice-scenarios skill) — GWT format for read slices

is Given (events) → Then (information). Events in Given tell you which events the projector must handle. The information element in Then describes the expected query result shape and values.

• The slice's optional ## Implementation Guidelines — technical requirements that extend the standard pattern

Step 0: Discover Target Project Conventions

Before writing any code, read the target project's context file (e.g., CLAUDE.md, AGENTS.md, .cursorrules) and explore at least one existing read slice. Look for:

• Feature flag pattern (Step 2 — Feature Flags)
• Assertion library (AssertJ, AssertK, etc.)
• Test naming conventions (backtick-quoted method names)
• Metadata handling — how correlation IDs are attached to events (see

[references/kotlin-extensions.md](references/kotlin-extensions.md) for AxonMetadata helpers)

• Existing read slice files and tests as patterns
• REST API test pattern — whether RestAssured, MockMvc, or another tool is used
• Spring configuration metadata file location

Also identify the established convention for:

• REST API exposure (Step 4 — optional)
• Feature flags (Step 2 — optional)

Step 1: Ensure Events Exist

Before implementing the read slice, verify that all events the projector will handle exist in the codebase. If they don't, create them first.

Event Hierarchy

Recommended hierarchy (check what the target project already uses):

DomainEvent                        ← root marker (project-defined)
  └─ {Context}Event                ← sealed interface per bounded context ({context}/events/)
       └─ {ConcreteEvent}          ← data class ({context}/events/)

Context Event Interface (if it doesn't exist)

// File: {context}/events/{Context}Event.kt
sealed interface {Context}Event : DomainEvent {
    @get:EventTag(EventTags.{TAG_CONSTANT})
    val {tagProperty}: {IdType}
}

Concrete Event Classes

// File: {context}/events/{EventName}.kt
@Event(namespace = "{Context}", name = "{EventName}", version = "1.0.0")
data class {EventName}(
    override val {tagProperty}: {IdType},
    val property1: ValueType1
) : {Context}Event

Key rules:

• @Event(namespace, name, version) — import from org.axonframework.messaging.eventhandling.annotation.Event
• namespace = bounded context name, name = class name, version = "1.0.0" for new events
• Use value object types for properties
• When an event participates in a Dynamic Consistency Boundary (DCB), add extra @EventTag on cross-stream properties

Step 2: Implement the Read Slice

If the Event Modeling artifact includes slice details with ## Scenarios (GWTs), use them to derive test cases.

If the slice details contain ## Implementation Guidelines, follow them.

Query Annotation

Every query data class must have @Query(namespace, name, version):

@Query(namespace = "{Context}", name = "{QueryName}", version = "1.0.0")
data class GetOrders(val tenantId: TenantId, ...) {
    data class Result(val items: List) {
        data class OrderSummary(...)
    }
}

• @Query — import from org.axonframework.messaging.queryhandling.annotation.Query
• namespace = bounded context name, name = query class name, version = "1.0.0" for new queries

A read slice file contains all layers in a single file. Do NOT add section comments (Domain/Application/Presentation) for read slices — those are only for write slices.

Slice File Structure

// Query DTO + Result DTO
@Query(namespace = "{Context}", name = "GetOrders", version = "1.0.0")
data class GetOrders(val tenantId: TenantId, ...) {
    data class Result(val items: List)
}

// JPA Entity (read model) — internal to projection
@Entity @Table(name = "...", indexes = [...])
data class OrderReadModel(...)

// Repository — private
@ConditionalOnProperty(...)  // if using feature flags
@Repository
private interface OrderReadModelRepository : JpaRepository { ... }

// Projector — private
@ConditionalOnProperty(...)  // if using feature flags
@Component
@SequencingPolicy(type = MetadataSequencingPolicy::class, parameters = ["correlationId"])
private class OrderReadModelProjector(...) { ... }

// Query Handler — private
@ConditionalOnProperty(...)  // if using feature flags
@Component
private class OrderReadModelQueryHandler(...) { ... }

// REST Controller — internal (only if REST API chosen in Step 4)
@ConditionalOnProperty(...)  // if using feature flags
@RestController
internal class OrdersRestApi(...) { ... }

Result DTO Rules

• If the read model matches the desired query result 1:1, expose the JPA entity directly in the Result.
• If the read model contains fields the caller already knows from the query (e.g., tenantId, entityId), create a

separate result data class nested inside the query that strips those redundant fields. The query handler maps from the JPA entity to the result DTO.

Idiomatic Kotlin in Projectors

Use findByIdOrNull (from org.springframework.data.repository.findByIdOrNull) with scope functions:

// Upsert pattern
val updated = repository.findByIdOrNull(id)
    ?.let { it.copy(quantity = it.quantity + event.quantity.raw) }
    ?: OrderReadModel(...)
repository.save(updated)

// Delete-or-update pattern
repository.findByIdOrNull(id)?.let { existing ->
    if (shouldDelete) repository.deleteById(id) else repository.save(existing.copy(...))
}

JPA Index

Add @Table(indexes = [...]) for columns used in repository query methods:

@Table(
    name = "ordering_read_getorders",
    indexes = [Index(name = "idx_orders_tenant_order", columnList = "tenantId, orderId")]
)

Step 3: Feature Flags (Optional)

Check the target project's convention first — scan existing slices for @ConditionalOnProperty, @Profile, or custom feature-flag integrations.

If no clear convention exists, ask the user: > How should slice-level feature flags be managed? > - @ConditionalOnProperty (Spring Boot default) > - Custom flag library (FF4J, Unleash, LaunchDarkly, etc.) > - No feature flags — ship all slices unconditionally

See [references/feature-flag-patterns.md](references/feature-flag-patterns.md) for the full @ConditionalOnProperty example and alternatives.

Step 4: REST API Exposure (Optional)

Check the target project's convention first — does it expose read models via REST (@RestController presence)?

If no convention is established, ask the user: > Does this read slice need a REST API endpoint? > - Yes — add a @RestController and a REST API test (see references/rest-api-patterns.md) > - No — projection + query handler only; callers use QueryGateway directly

Step 5: Design Test Cases

Cover these scenarios (adapt to the specific slice):

1. Empty state: No events published → query returns empty result
2. Single entity: One creation event → query returns single item
3. Multiple entities: Multiple creation events → query returns all items
4. State updates: Creation event + update event → query returns updated state
5. Aggregation: Same entity updated multiple times → values accumulated correctly
6. Deletion: Entity added then fully removed → disappears from query result
7. Isolation: Multiple entities exist → query returns only matching ones (by ID, tenant, etc.)
8. Lifecycle: Full sequence of events reflecting real usage

Mapping Event Model GWT Scenarios to Tests

When the slice details contain ## Scenarios (GWTs), map each scenario to a test method:

| GWT Element | Test Code | |---|---| | NOTHING in Given | When { nothing() } Then { expect { ... empty result ... } } (synchronous) | | :::element event in Given | Given { event(EventClass(...), metadata) } | | Multiple :::element event in Given | Multiple event(...) calls in Given { } block | | :::element information in Then | Then { awaitAndExpect { cfg -> assertThat(queryResult.field).isEqualTo(value) } } |

Properties in :::element blocks are rule-relevant only — fill remaining constructor params with test fixture values.

Step 6: Implement the Spring Slice Test

Use @AxonSpringBootTest (org.axonframework.extension.springboot.test.AxonSpringBootTest) with AxonTestFixture injected. Check if the project defines a meta-annotation that wraps @AxonSpringBootTest with shared config (@ActiveProfiles, @Import for Testcontainers, etc.); if one exists, use it.

The AxonTestFixture Kotlin DSL (Given { } Then { awaitAndExpect { } }) must be copied into the project's test sources. See [references/axon-test-fixture-kotlin-dsl.md](references/axon-test-fixture-kotlin-dsl.md).

For AxonMetadata — use the typealias from [references/kotlin-extensions.md](references/kotlin-extensions.md).

Test Class Structure

@TestPropertySource(properties = ["slices.{context}.read.{feature}.enabled=true"])
@AxonSpringBootTest
internal class {Feature}SpringSliceTest @Autowired constructor(
    private val fixture: AxonTestFixture
) {
    private val tenantId = TenantId.random()
    private val metadata = AxonMetadata.with("tenantId", tenantId.raw)

    @Test
    fun `given no events, when query, then empty`() {
        fixture.When { nothing() } Then {
            expect { cfg ->
                val result = query{Feature}(cfg)
                assertThat(result.items).isEmpty()
            }
        }
    }

    @Test
    fun `given event, then result contains item`() {
        fixture.Given {
            event(SomeEvent(...), metadata)
        } Then {
            awaitAndExpect { cfg ->
                val result = query{Feature}(cfg)
                assertThat(result.items).containsExactlyInAnyOrder(
                    ExpectedResult(...)
                )
            }
        }
    }

    private fun query{Feature}(cfg: Configuration): {Query}.Result =
        cfg.getComponent(QueryGateway::class.java)
            .query({Query}(tenantId), {Query}.Result::class.java)
            .orTimeout(1, TimeUnit.SECONDS)
            .join()
}

Key Rules

• Metadata is required: If the projector uses @MetadataValue("correlationId"), every event must be published

with metadata containing that key: .event(payload, metadata)

• awaitAndExpect for events, expect for empty state: Use Given { } Then { awaitAndExpect { } } when

events were given (async processing). Use When { nothing() } Then { expect { } } when no events.

• Query via Configuration: Access QueryGateway through cfg.getComponent(QueryGateway::class.java) inside

the expect or awaitAndExpect block.

• Timeout on query: Always add .orTimeout(1, TimeUnit.SECONDS) before .join().
• Assert with full objects: Use containsExactlyInAnyOrder(ResultDto(...)) with explicitly constructed result

instances rather than field-by-field assertions.

• Explicit expected values: Define expected values as explicit properties rather than deriving them from domain

objects. This makes tests more readable and catches serialization issues.

• Constructor injection: Inject AxonTestFixture via constructor, not field injection.

Step 7: REST API Test (Optional, only if Step 4 chosen REST)

See [references/rest-api-patterns.md](references/rest-api-patterns.md) for a full RestAssured + @WebMvcTest example with mocked QueryGateway.

References

• [Read Slice Test Example](references/read-slice-test-example.md) — Complete working example
• [REST API Patterns](references/rest-api-patterns.md) — RestAssured REST controller and test examples
• [Feature Flag Patterns](references/feature-flag-patterns.md) — @ConditionalOnProperty and alternatives
• [Kotlin Extensions](references/kotlin-extensions.md) — AxonMetadata typealias and helper functions
• [AxonTestFixture Kotlin DSL](references/axon-test-fixture-kotlin-dsl.md) — Given-When-Then DSL source to copy into the project

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: proophboard
• Source: proophboard/skills
• License: MIT
• Homepage: https://skills.prooph-board.com/

Install and usage instructions live in the source repository linked above.