axoniq-app-dev

Axon Framework 5 — Application Developer Guide

This skill covers everything needed to build an application with Axon Framework 5 (AF5) and its optional commercial extension, Axoniq Framework. When working on a specific topic, use the Read tool to load the relevant guide file from the subdirectories listed in the routing table below.

Target version: this skill targets the Axon Framework 5.1.x line (latest stable 5.1.1). API names and package locations follow the 5.1 layout. Version-specific feature availability is called out inline in the guides (for example: replay/reset from 5.1; event upcasting and handler timeouts are scheduled for 5.2). When advising a user, confirm their version in the build file if a feature's availability is borderline.

Investigating an API not covered here: the published Javadoc at https://apidocs.axoniq.io/ is the reference for any class, method, or package this skill's guides don't document. The docs are versioned by minor line: for a version x.y.z, the URL contains the x.y segment — e.g. https://apidocs.axoniq.io/5.1/ for the 5.1.x line. Match the segment to the user's version (check the build file). It covers both the open-source org.axonframework packages and the commercial io.axoniq.framework packages — browse by package, e.g. https://apidocs.axoniq.io/5.1/org/axonframework/.../package-summary.html.

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Framework choices

Axon Framework 5 (org.axonframework, Apache 2.0) is fully open source and provides the complete core feature set: command handling, DCB decision models, event sourcing, event handling, query handling, interceptors, dead letter queue (in-memory, JDBC, JPA), Spring Boot integration, and testing utilities. It is the default choice.

Axoniq Framework (io.axoniq.framework, commercial license) is an optional extension layer that adds production infrastructure features on top of AF5:

• PostgreSQL event store — a production-grade EventStorageEngine backed by PostgreSQL with optimised DCB tag indexing
• Distributed messaging — DistributedCommandBus and DistributedQueryBus for spreading load across multiple application instances
• Multi-source event streaming — MultiStreamableEventSource for consuming events from multiple independent event stores simultaneously

Axoniq Framework is free for non-production use; production deployments require a paid subscription. Users who prefer to stay on open-source only can cover all core use cases with AF5 alone — Axoniq Framework features should only be suggested when the user asks for them or their use case clearly requires them.

Detecting which frameworks the user has

Before suggesting Axoniq Framework features, check the project's build file (pom.xml, build.gradle, or build.gradle.kts) for the groupId prefixes:

GroupId prefix	Framework	Notes
org.axonframework	Axon Framework 5 (open source)	Always present in AF5 projects
io.axoniq.framework	Axoniq Framework (commercial)	Only present if user explicitly added it
io.axoniq (other)	Other Axoniq commercial products	e.g. Axon Server connector, Inspector

If the build file contains only org.axonframework dependencies: the user is on Axon Framework only. Give AF5-only advice. Do not suggest io.axoniq.framework modules unless the user asks.

If the build file contains io.axoniq.framework dependencies: Axoniq Framework is available. It is safe to reference its APIs (PostgresqlEventStorageEngine, DistributedCommandBus, MultiStreamableEventSource, etc.).

When no build file is visible and the question is ambiguous, default to AF5 open-source advice and note that Axoniq Framework provides commercial alternatives where relevant.

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DCB Philosophy — the primary stateful pattern

The primary model for stateful command handling in AF5 is DCB (Dynamic Consistency Boundaries), not aggregates. Aggregates are a specialization of DCB, not the default.

Core idea: for each command, identify the minimum state needed to make the decision, source only the events relevant to that decision using tag-based criteria, then append the resulting events with a condition that detects concurrent conflicts on that same boundary.

Why DCB over aggregates: an aggregate loads everything ever recorded for an identity. A DCB decision model loads only the events that are relevant to the specific decision being made — nothing more. This is more precise, more performant, and avoids the overhead of identity-scoped loading when only a subset of that history is needed.

The workflow:

1. Tag events with @EventTag so they can be found later by meaningful keys (e.g., "course", "student")
2. Define a minimal decision state record that holds only what the command needs to decide
3. In the command handler, open an EventStoreTransaction, source the relevant events using SourcingCondition + EventCriteria, fold them into the decision state, apply business rules, then append the result
4. The transaction automatically enforces that no conflicting event was written concurrently — if one was, AppendEventsTransactionRejectedException is thrown and the command should be retried

Key AF5 primitives: EventStore/EventStoreTransaction, SourcingCondition, AppendCondition, ConsistencyMarker, EventCriteria/Tag/@EventTag, AppendEventsTransactionRejectedException

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Routing table

When working on a topic, read the corresponding guide file. Guides are grouped into domain folders that mirror the AF5 reference guide. Each guide contains full API reference, code examples, and patterns.

Domain	Topic	Guide file	Framework
getting-started	Maven dependencies / project setup	getting-started/dependencies.md	AF5 + Axoniq Framework
foundations	Message anatomy, processing context, correlation	foundations/messages-and-processing-context.md	AF5 (open source)
foundations	MessageStream API (entries, factories, reduce/consume, transform)	foundations/message-streams.md	AF5 (open source)
foundations	Message annotations reference	foundations/annotations.md	AF5 (open source)
foundations	Supported handler parameters reference	foundations/supported-parameters.md	AF5 (open source)
foundations	Exception handling & handler timeouts	foundations/exception-handling.md	AF5 (open source)
foundations	Interceptors	foundations/interceptors.md	AF5 (open source)
foundations	Customizing handlers (parameter resolvers, enhancers, meta-annotations)	foundations/handler-customization.md	AF5 (open source)
foundations	Identifier generation	foundations/identifiers.md	AF5 (open source)
commands	Stateless command handling	commands/stateless.md	AF5 (open source)
commands	Stateful command handling (DCB)	commands/decision-models-dcb.md	AF5 (open source)
commands	Event-sourced entities (creator, hierarchies, polymorphism)	commands/entities.md	AF5 (open source)
commands	Dispatching commands & command bus	commands/dispatching.md	AF5 (open source)
events	Event handling and projections	events/handling-projections.md	AF5 (open source)
events	Event processors (subscribing / pooled streaming)	events/processors.md	AF5 (open source)
events	Publishing events	events/publishing.md	AF5 (open source)
events	Event versioning & upcasting	events/versioning-upcasting.md	AF5 (open source)
queries	Query handling	queries/query-handling.md	AF5 (open source)
event-store	Event store API reference (sourcing / append conditions)	event-store/primitives.md	AF5 (open source)
event-store	Event store internals (EventStore / EventStorageEngine)	event-store/internals.md	AF5 (open source)
event-store	Conversion & serialization	event-store/conversion-serialization.md	AF5 (open source)
configuration	Application configuration (plain Java)	configuration/plain-java.md	AF5 + Axoniq Framework options
configuration	Spring Boot configuration	configuration/spring-boot.md	AF5 Spring extension
testing	Testing — basics	testing/basics.md	AF5 (open source)
testing	Testing — advanced (time, integration, Spring)	testing/advanced.md	AF5 (open source)
testing	Testing — matchers & field filters	testing/matchers.md	AF5 (open source)
production-infra	Distributed messaging	production-infra/distributed-messaging.md	Axoniq Framework (commercial)
production-infra	Multi-source event streaming	production-infra/multi-source-streaming.md	Axoniq Framework (commercial)

To load a guide: use the Read tool with the path relative to this file, e.g.:

Read: skills/axoniq-app-dev/commands/decision-models-dcb.md

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Quick orientation

Getting started

• Start with getting-started/dependencies.md when setting up a new project or adding a module — it covers all Maven/Gradle coordinates and versions.

Commands

• Start with commands/stateless.md for any command handler that does not need to read past state.
• Start with commands/decision-models-dcb.md for any command handler that must validate against prior events. This is the more common case.
• Use commands/entities.md when modelling identity-scoped state with @EventSourcedEntity / @InjectEntity, entity hierarchies, or polymorphism (the identity-scoped specialization of DCB).
• Use commands/dispatching.md for the CommandGateway/CommandBus API, routing keys, and command results.

Events

• Use events/handling-projections.md when building projections, reactions, replay control, sequencing, or a dead letter queue.
• Use events/processors.md for the deep processor reference — subscribing vs pooled streaming, tracking tokens, segments, multi-node, reset.
• Use events/publishing.md when publishing events via EventAppender (inside handlers) or EventGateway (outside).
• Use events/versioning-upcasting.md when evolving event schemas (revisions; note upcasting is forward-looking — see the guide's caveat).

Queries

• Use queries/query-handling.md when serving read requests or pushing live updates via subscription queries.

Foundations (messaging internals & cross-cutting)

• Use foundations/messages-and-processing-context.md for message anatomy, ProcessingContext/unit-of-work lifecycle, and correlation.
• Use foundations/message-streams.md for the MessageStream API — the Entry wrapper, factory methods, reduce/imperative consumption, and transforms. Read it whenever you source events, write a low-level handler/interceptor, or consume a MessageStream<?>.
• Use foundations/annotations.md as a complete reference for any annotation's attributes (@Command, @Event, @CommandHandler, @EventHandler, @InjectEntity, @EventSourcedEntity, @EventTag, etc.).
• Use foundations/supported-parameters.md to see what can be injected into a handler method.
• Use foundations/exception-handling.md for @ExceptionHandler, execution-exception wrapping, and handler timeouts.
• Use foundations/interceptors.md when adding cross-cutting behaviour (validation, logging, retry, correlation propagation).
• Use foundations/handler-customization.md for custom ParameterResolvers, HandlerEnhancerDefinitions, and meta-annotations.
• Use foundations/identifiers.md when customizing message/identifier generation.

Event store

• Use event-store/primitives.md alongside the DCB guide for exact sourcing/append condition APIs.
• Use event-store/internals.md for the EventStore/EventStorageEngine layer and storage engines.
• Use event-store/conversion-serialization.md for the Converter layer, Jackson/Avro/CBOR, and content types.

Configuration

• Use configuration/plain-java.md when wiring up a plain Java application. Includes event store options (in-memory, PostgreSQL) and dead letter queue configuration.
• Use configuration/spring-boot.md for a Spring Boot application — auto-detection, @EventSourced, @Namespace, EventProcessorDefinition, application.yml processor properties, and Axon Server connection properties (axon.axonserver.servers/context/token).

Testing

• Use testing/basics.md for the AxonTestFixture given-when-then basics.
• Use testing/advanced.md for time control, integration tests, and Spring Boot test setup.
• Use testing/matchers.md for the matcher API and field filters.

Production infrastructure (Axoniq Framework, commercial)

• Use production-infra/distributed-messaging.md when distributing command or query load across multiple application nodes.
• Use production-infra/multi-source-streaming.md when consuming events from multiple independent event stores simultaneously.