Axon Framework

Axon Framework is a Java toolkit for building systems where every change is stored as an event (event sourcing) and reads are split from writes (CQRS). It gives you the command, event, and query plumbing so you are not inventing it. The core is Apache 2.0 and AxonIQ says it stays free. The important word is core.

Version 5 drew a harder line between free and paid. The Axon Server connector moved out of the open source repo into AxonIQ's commercial modules, along with the Postgres event store, dead-letter queue, distributed messaging, and multi-tenancy. What stays free for storage is a JPA engine on your own database, plus an in-memory one that is not for production. Axon 5 needs JDK 21 and Spring Boot 3, and version 4 reached end of life on June 30, 2026.

Solo and evaluation: the free Individual plan is non-production only. Companies under 20 employees: Team from $150/mo billed yearly. Under 40 employees: Small Business from $1,750/mo. Larger: Enterprise, contact sales. Staying fully free means JPA storage and wiring up your own message distribution.

The catch: Dynamic Consistency Boundaries, the headline feature of version 5, does not work on the free JPA store. It needs Axon Server or a paid extension. Sagas and the DeadlineManager are gone in version 5 too, so moving from 4 to 5 is a rewrite in places, not a version bump.