Axoniq Framework Contribute Code

Axon Framework Core Component Development

This skill guides development of Axon Framework infrastructure components, focusing on API design patterns, component lifecycle, and framework-specific conventions. The core philosophy, conventions, checklist, and anti-patterns are below; detailed patterns with full code examples live in references/ — load them via the routing table when working on that topic.

When to Use This Skill

• Designing new infrastructure components (buses, stores, handlers)
• Creating layered APIs (low-level + high-level abstractions)
• Implementing component builders and configuration
• Designing extension points (SPI vs API)
• Ensuring thread safety and lifecycle management
• Reviewing core framework code for consistency

Note: This is for developing the Axon/Axoniq frameworks themselves, not for using them in applications.

Framework Selection: Axon Framework vs Axoniq Framework

Two related frameworks share the conventions in this skill. Before contributing, understand which repo you're working in and what belongs where.

What Belongs in Each Framework

Axon Framework (OSS) — foundational building blocks:

• Core messaging infrastructure: CommandBus, EventBus, QueryBus
• Event sourcing: EventStore, EventStorageEngine, EventSourcingRepository
• Domain modelling: Repository, EntityMetamodel, StateManager
• Basic Spring Boot integration (extensions/spring)
• Metrics (Micrometer/Dropwizard) extensions
• Test utilities and BDD fixtures (test/ module)
• Axon Server connector (basic)

Axoniq Framework (Commercial) — production enhancements on top:

• Dead-Letter Queue (SequencedDeadLetterQueue, JDBC/JPA implementations)
• PostgreSQL storage engine (PostgresqlEventStorageEngine)
• Distributed messaging enhancements (DistributedCommandBus, DistributedQueryBus)
• Multi-source event streaming (MultiStreamableEventSource, MultiSourceTrackingToken)
• Message transformation / upcasting (EventTransformation, EventTransformerChain)
• Spring Boot auto-configuration out of the box
• Enhanced Axon Server connector with additional features (persistent streams)

Informing Users About Options

When a user asks about a feature that has both OSS and commercial options, always present both fairly:

• Axon Framework alone is sufficient for many use cases including production deployments
• Axoniq Framework adds production conveniences (Spring auto-config, DLQ, PostgreSQL, distributed messaging)
• Do not assume users need or want the commercial framework
• Let users make an informed choice based on their actual needs

Core Design Philosophy

Axon Framework uses a three-tier architecture for component APIs:

┌─────────────────────────────────────────┐
│  Level 3: Context-Scoped Components     │  ← Specialized, lifecycle-bound
│  (EventAppender, etc.)                  │
├─────────────────────────────────────────┤
│  Level 2: High-Level APIs (Gateways)    │  ← User-friendly, convenience
│  (EventGateway, CommandGateway)         │
├─────────────────────────────────────────┤
│  Level 1: Low-Level Infrastructure      │  ← Raw concepts, composable
│  (EventBus, EventSink, CommandBus)      │
└─────────────────────────────────────────┘

Key Principle: Each layer builds on the one below it. Users typically interact with Level 2 or 3, while framework developers implement Level 1. See references/layered-api-design.md for the design rules per level.

Coding Conventions (always apply)

Nullability — JSpecify, annotated at the package level with @NullMarked:

// ✅ CORRECT — in package-info.java
@NullMarked
package org.axonframework.messaging.core;

import org.jspecify.annotations.NullMarked;

Under @NullMarked, all parameters and return values are non-null by default. Only mark nullable exceptions with @Nullable (from org.jspecify.annotations). Enforce non-null contracts at runtime with Objects.requireNonNull(param, "Param may not be null"). Jakarta annotations (jakarta.annotation.*) are explicitly forbidden (enforced by checkstyle).

Modern Java — use pattern matching with instanceof:

// ✅ PREFERRED
if (trackingToken instanceof MultiSourceTrackingToken multiSourceToken) {
    return open(multiSourceToken, condition, context);
}

Routing Table

Read the reference file for the topic you're working on. Each contains full design rules, code templates, and worked examples.

Design Checklist for New Components

API Design

• Identify abstraction level (low/high/context-scoped)
• Low-level: works with Message types, explicit context
• High-level: accepts Objects, optional context, many overloads
• Context-scoped: static forContext(), void returns
• Use interface composition where appropriate
• Extend DescribableComponent

Configuration

• Provide sensible defaults when obvious
• Force choices with phased builders when not obvious
• Use ComponentBuilder for dependency injection
• Validate all constructor parameters with requireNonNull
• Include descriptive error messages

AF5 Fluent Builders (if applicable)

• Use descriptive static factory (not builder())
• Create public intermediate builder interface
• Provide descriptive terminal operations
• Validate parameters early (in and() not at terminal)
• Return unmodifiable collections
• Add package-private accessor for testing
• Use JSpecify @NullMarked at package level, @Nullable for nullable params/returns (never jakarta.annotation)
• Use modern Java patterns (pattern matching instanceof)
• Organize tests with @Nested classes
• Test actual internal state, not just non-null
• Test immutability of returned collections

Lifecycle

• Return Registration for subscriptions
• Integrate with ProcessingLifecycle for transactional behavior
• Use ResourceKey for context-scoped resources
• Implement proper cleanup in Registration.cancel()

Thread Safety

• Prefer immutability
• Use ConcurrentHashMap for subscription registries
• Use CopyOnWriteArrayList for listener lists
• Use AtomicReference for mutable state
• Document thread safety in JavaDoc

SPI vs API

• Mark SPIs with @Internal
• Keep public API simple and user-focused
• Implementation class connects API to SPI
• SPIs allow extensibility, APIs ensure usability

Error Handling

• Validate all parameters immediately
• Use specific exception types
• Include context in exception messages
• Use AxonConfigurationException for config errors

Documentation

• JavaDoc on all public interfaces and methods
• @since tags on new APIs
• @author tags when appropriate
• Code examples for complex components
• Update reference guide in docs/reference-guide/modules

Anti-Patterns to Avoid

❌ Builder Pattern on Infrastructure Components

Don't:

MyComponent.builder().withX(x).withY(y).build();

Do:

// AF5 fluent style
MyComponent.configuring("name").withX(x).withY(y).initialize();

See references/fluent-builders.md for the full pattern.

❌ Mutable Message Objects

Don't:

eventMessage.setMetadata(newMetadata);  // Mutation

Do:

EventMessage updated = eventMessage.withMetadata(newMetadata);  // New instance

❌ Public Constructors for Context-Scoped Components

Don't:

public EventAppender(ProcessingContext context) { ... }
// Users might create multiple instances per context

Do:

private EventAppender(ProcessingContext context) { ... }  // Private

public static EventAppender forContext(ProcessingContext context) {
    return context.computeResourceIfAbsent(RESOURCE_KEY,
                                           () -> new EventAppender(context));
}

❌ Swallowing Exceptions

Don't:

try {
    storage.save(event);
} catch (Exception e) {
    logger.warn("Failed to save", e);  // Lost!
}

Do:

try {
    storage.save(event);
} catch (StorageException e) {
    throw new EventStoreException("Failed to save event: " + event.getIdentifier(), e);
}

❌ Synchronous Blocking in Async Methods

Don't:

public CompletableFuture<Void> publish(ProcessingContext context, List<EventMessage> events) {
    storage.save(events);  // Blocks!
    return CompletableFuture.completedFuture(null);
}

Do:

public CompletableFuture<Void> publish(ProcessingContext context, List<EventMessage> events) {
    return CompletableFuture.supplyAsync(() -> storage.save(events), executor)
                            .thenApply(result -> null);
}

❌ instanceof Checks on Message Handlers

Use canHandleMessageType() and unwrap() instead — see references/handler-wrappers.md.

Related Skills

• axoniq-framework-contribute-review: Review changes against AF5 contributor standards
• axoniq-framework-contribute-docs: Add or update the Antora reference documentation

Where to Find the Real Code

• Core interfaces: messaging/src/main/java/org/axonframework/messaging/
• Gateway implementations: messaging/src/main/java/org/axonframework/*/gateway/
• Event store: eventsourcing/src/main/java/org/axonframework/eventsourcing/eventstore/
• Context: messaging/src/main/java/org/axonframework/messaging/core/
• Configuration: common/src/main/java/org/axonframework/common/configuration/
• Handler wrappers: messaging/src/main/java/org/axonframework/messaging/*/annotation/