axon5kotlin-automation-slice

name

axon5kotlin-automation-slice

description

Implement automation slices (Event to Command) using Axon Framework 5, Vertical Slice Architecture, and Event Modeling patterns. An automation is: an Event Handler that reacts to an event by dispatching a command via CommandDispatcher. Automations can be stateless (direct event-to-command mapping) or with a private read model (to look up data needed for command construction). Use when: (1) implementing a new automation / event-to-command reactor in an AF5 project, (2) migrating/porting an automation from Axon Framework 4 (Java or Kotlin) to AF5, (3) user provides a specification, Event Modeling artifact, or natural language description of an event-to-command reaction and asks to implement it, (4) user says "implement", "create", "add", "migrate", "port" an automation, event handler, reactor, or event-to-command flow in an Axon Framework 5 / Vertical Slice Architecture project. Understands AF4 @EventHandler/@ProcessingGroup input as one possible source format.

Axon Framework 5 — Automation Slice

An automation reacts to an event by dispatching a command. In Event Modeling: the orange stripe.

There are two kinds:

• Stateless: Direct event-to-command mapping — no stored state needed
• With read model: Needs a private read model (JPA entity + repository) to look up data required for command construction (e.g., iterate over all entities matching a category)

Relationship to prooph board Event Modeling

This skill implements the automation slice (orange 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 automations is Given (events) → Then (command | hotspot | NOTHING). Events in Given include read-model-building events first, trigger event last. Command in Then is the dispatched command. Hotspot or NOTHING means no command.
• The slice's optional ## Implementation Guidelines — technical requirements beyond the standard pattern

Step 0: Discover Target Project Conventions

Read the target project's context file (e.g., CLAUDE.md, AGENTS.md, .cursorrules) and explore existing slices. Look for:

• File splitting conventions (single .Slice.kt vs separate files per class)
• Visibility modifiers (private on processor/configuration classes)
• Metadata handling — how correlation IDs are attached (see references/kotlin-extensions.md for AxonMetadata helpers)
• Feature flag patterns (Step 4 — optional)
• YAML config files (application.yaml, application-test.yaml)
• Spring Boot test annotation: check if the project defines a meta-annotation for @AxonSpringBootTest

Step 1: Understand the Input

Extract these elements regardless of input format:

Element	What to extract
Trigger event	Which event triggers the automation, and which condition filters it
Target command	Which command to dispatch, with what properties
Mapping logic	How event properties map to command properties
Strategy/calculator	Any injectable strategy for deriving command properties from event data
Metadata	Which metadata keys to propagate from event to command
Read model needed?	Does the automation need to look up data not in the event itself?

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.

Stateless vs With Read Model Decision

Choose with read model when:

• The automation needs data that is NOT in the trigger event (e.g., "find all entities of type X")
• The automation must iterate over a collection of entities to dispatch multiple commands
• The automation combines data from multiple event types (one builds the read model, another triggers commands)

Choose stateless when:

• All data needed for the command is in the trigger event + metadata
• The mapping is direct or uses a pure calculation/strategy

Input: Axon Framework 4 Code

AF4                                  AF5
─────────────────────────────────    ────────────────────────────────────
@ProcessingGroup("name")             (not needed — Spring Boot auto-config)
@DisallowReplay                      (not needed in AF5)
@Component                           @Component
@EventHandler                        @EventHandler (different package)
commandGateway.sendAndWait(cmd, m)   commandDispatcher.send(cmd, metadata)
CommandGateway (constructor-inject)  CommandDispatcher (method parameter)
@MetaDataValue("key")                @MetadataValue("key")
Function<A, B> interface             fun interface Name : (A) -> B

If requirements are unclear, ask the user before proceeding.

Step 2: Ensure Events Exist

Before implementing the automation, verify that all events the processor 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)

sealed interface {Context}Event : DomainEvent {
    @get:EventTag(EventTags.{TAG_CONSTANT})
    val {tagProperty}: {IdType}
}

Concrete Event Classes

@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

Step 3: Implement the Automation

CommandDispatcher vs CommandGateway

Always use CommandDispatcher to dispatch commands from within @EventHandler methods:

• CommandDispatcher is AF5's preferred way to send commands from within message handlers
• It is ProcessingContext-scoped — inject it as a method parameter on the @EventHandler, NOT as a constructor parameter
• CommandGateway is a singleton intended for external callers (REST controllers, etc.)

// CORRECT: CommandDispatcher as method parameter
@EventHandler
fun react(event: {TriggerEvent}, commandDispatcher: CommandDispatcher) {
    commandDispatcher.send(command, metadata)
}

// WRONG: CommandGateway as constructor parameter
class MyProcessor(private val commandGateway: CommandGateway) {
    @EventHandler
    fun react(event: {TriggerEvent}) {
        commandGateway.send(command, metadata) // Don't do this
    }
}

Error Propagation with CompletableFuture

When the automation dispatches commands, return a CompletableFuture from the @EventHandler method so AF5 awaits command completion. If a command fails, the event handler fails too, and the event processor will retry.

• commandDispatcher.send(command, metadata) returns a CommandResult
• CommandResult.resultMessage returns CompletableFuture
• For multiple commands: use CompletableFuture.allOf() to await all

AF5's MessageStreamResolverUtils handles CompletableFuture return types via MessageStream.fromFuture().

---

Stateless Automation

An automation slice has up to 3 files (adapt to project conventions on splitting):

File 1: Strategy Interface (if needed)

A fun interface for injectable logic deriving command properties from event data:

fun interface {StrategyName} : ({InputType}) -> {OutputType}

Skip if the mapping from event to command is trivial/direct.

File 2: Configuration (if strategy exists)

@ConditionalOnProperty(...)  // if using feature flags
@Configuration
private class {AutomationName}Configuration {

    @Bean
    fun {strategyName}(): {StrategyName} =
        {StrategyName} { input -> /* default implementation */ }
}

File 3: Processor

@ConditionalOnProperty(...)  // if using feature flags
@Component
private class {AutomationName}Processor(
    private val strategy: {StrategyName}  // if strategy exists
) {

    @EventHandler
    fun react(
        event: {TriggerEvent},
        @MetadataValue("tenantId") tenantId: String,  // use the project's correlation key name
        commandDispatcher: CommandDispatcher
    ) {
        if ({condition}) {
            val command = {TargetCommand}(...)
            val metadata = AxonMetadata.with("tenantId", tenantId)
            commandDispatcher.send(command, metadata)
        }
    }
}

---

Automation with Read Model

When the automation needs to look up data, create a dedicated read model within the same slice. Slices are independent — never reuse another slice's read model.

Everything goes in a single .Slice.kt file:

JPA Entity (Read Model)

@Entity
@Table(
    name = "{context}_automation_{readmodel_name}",
    indexes = [Index(name = "idx_{context}_{readmodel_name}_{columns}", columnList = "tenantId, categoryId")]
)
internal data class {ReadModelName}(
    val tenantId: String,
    @Id
    val primaryId: String,
    val filterField: String
)

Key rules:

• Composite index on the columns used in WHERE clause — filter at DB level, not client-side
• Table name prefixed with context to avoid collisions
• internal visibility — only the processor in this slice should access it

Repository

@ConditionalOnProperty(...)  // if using feature flags
@Repository
private interface {ReadModelName}Repository : JpaRepository<{ReadModelName}, String> {
    fun findAllByTenantIdAndFilterField(tenantId: String, filterField: String): List<{ReadModelName}>
}

Key rules:

• DB-level filtering — use Spring Data derived queries that filter on all relevant columns, not findAll() + client-side filter
• private visibility
• Same @ConditionalOnProperty as the processor (if using feature flags)

Processor

@ConditionalOnProperty(...)  // if using feature flags
@Component
@SequencingPolicy(type = MetadataSequencingPolicy::class, parameters = ["tenantId"])
private class {AutomationName}Processor(
    private val repository: {ReadModelName}Repository
) {

    @EventHandler
    fun react(
        event: {TriggerEvent},
        @MetadataValue("tenantId") tenantId: String,
        commandDispatcher: CommandDispatcher
    ): CompletableFuture<Void> {
        val futures = repository.findAllByTenantIdAndFilterField(tenantId, event.filterValue)
            .map { entity -> dispatchCommand(entity, event, tenantId, commandDispatcher) }
        return CompletableFuture.allOf(*futures.toTypedArray())
    }

    private fun dispatchCommand(
        entity: {ReadModelName},
        event: {TriggerEvent},
        tenantId: String,
        commandDispatcher: CommandDispatcher
    ): CompletableFuture<out Any?> {
        val command = {TargetCommand}(
            // map entity fields + event data to command properties
        )
        val metadata = AxonMetadata.with("tenantId", tenantId)
        return commandDispatcher.send(command, metadata).resultMessage
    }

    @EventHandler
    fun on(event: {BuildingEvent},  tenantId: ) {
        repository.save(
            {ReadModelName}(
                tenantId = tenantId,
                primaryId = event.entityId.raw,
                filterField = event.filterValue.raw
            )
        )
    }
}

Key rules:

• @SequencingPolicy(MetadataSequencingPolicy, "tenantId") — ensures events for the same tenant/correlation unit are processed sequentially, preventing race conditions on the read model
• CommandDispatcher as method parameter — ProcessingContext-scoped, not constructor-injected
• CompletableFuture<Void> return — CompletableFuture.allOf() awaits all dispatched commands; if any fails, the event handler fails and the processor retries
• commandDispatcher.send(command, metadata).resultMessage — returns CompletableFuture for the command result
• Two @EventHandler methods in one class: one builds the read model, the other reacts by dispatching commands
• Repository is constructor-injected (Spring bean), CommandDispatcher is method-injected (ProcessingContext)

Step 4: 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 for the full @ConditionalOnProperty example (processor, repository, application.yaml, additional-spring-configuration-metadata.json) and alternatives.

Step 5: Implement Tests

Spring Boot integration test with AxonTestFixture Kotlin DSL.

Critical: Enable BOTH the automation AND its target write slice in the feature flag settings.

If the automation uses a strategy, override it with a deterministic @TestConfiguration bean.

The AxonTestFixture Kotlin DSL must be copied into the project's test sources. See references/axon-test-fixture-kotlin-dsl.md.

For AxonMetadata — use the typealias from references/kotlin-extensions.md.

AF5 Imports

import org.axonframework.messaging.commandhandling.gateway.CommandDispatcher
import org.axonframework.messaging.core.annotation.MetadataValue
import org.axonframework.messaging.eventhandling.annotation.EventHandler
import org.axonframework.messaging.eventhandling.annotation.SequencingPolicy
import org.axonframework.messaging.eventhandling.sequencing.MetadataSequencingPolicy

AxonTestFixture API for Automations

Assert exact command dispatched (stateless automation — single command):

fixture.Scenario {
    Given {
        event({TriggerEvent}(...), metadata)
    } Then {
        await({ it.commands(expectedCommand) })
    }
}

Assert multiple commands in any order (automation with read model — multiple commands):

fixture.Scenario {
    Given {
        event({BuildingEvent}(...), metadata)  // builds read model
        event({TriggerEvent}(...), metadata)   // triggers commands
    } Then {
        await({
            it.commandsSatisfy { commands ->
                val relevantPayloads = commands.map { cmd -> cmd.payload() }
                    .filterIsInstance<{TargetCommand}>()
                    .filter { cmd -> cmd.entityId in testEntityIds }

                assertThat(relevantPayloads).containsExactlyInAnyOrder(
                    {TargetCommand}(...),
                    {TargetCommand}(...)
                )
            }
        })
    }
}

Assert no commands (condition not met):

fixture.Scenario {
    Given {
        event({TriggerEvent}(... /* condition not met */), metadata)
    } Then {
        await({ it.noCommands() })
    }
}

Testing Automations with Read Model — Important Notes

1. RecordingCommandBus accumulates commands across test methods in the same Spring context. It is NOT reset between tests when using Given { } Then { } (only the When { } phase resets it).

2. Isolate tests using entity ID filtering: Generate unique IDs per test method, then filter assertions to only check commands relevant to the current test's IDs:

   val testEntityIds = setOf(entity1, entity2)
   // ... in assertion:
   .filter { cmd -> cmd.entityId in testEntityIds }
   

3. Use commandsSatisfy + containsExactlyInAnyOrder: JPA/DB returns results in unpredictable order, so commands may be dispatched in any order. Don't use commands(cmd1, cmd2) which asserts strict ordering.

4. Put all events in a single Given block: For temporal ordering tests, interleave building events and trigger events in a single Given block and assert all expected commands at once.

Test Cases to Cover

Stateless automations:

1. Happy path: Event matching condition → expected command dispatched
2. Condition not met: Event not matching condition → no commands dispatched

Automations with read model:

1. Happy path: Build read model entries + trigger event → commands for matching entries only
2. Non-matching entries: Build entries of different types + trigger for one type → only matching type gets commands
3. Temporal ordering: Build some entries, trigger, build more entries, trigger again → each trigger only affects entries that existed at that point

Mapping Event Model GWT Scenarios to Tests

GWT Element	Test Code
:::element event in Given	Given { event(EventClass(...), metadata) }
Multiple events in Given	Multiple event(...) calls — read-model-building events first, trigger event last
:::element command in Then	Then { await({ it.commands(expectedCommand) }) }
:::element hotspot in Then	Then { await({ it.noCommands() }) } — exception/failure
NOTHING in Then	Then { await({ it.noCommands() }) } — no reaction

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

References

• Stateless Automation Test Example — Complete working test with deterministic strategy override
• Automation with Read Model Test Example — Complete working test with private read model and multi-command assertions
• Feature Flag Patterns — @ConditionalOnProperty and alternatives
• Kotlin Extensions — AxonMetadata typealias and helper functions
• AxonTestFixture Kotlin DSL — Given-When-Then DSL source to copy into the project