Event versioning (or why I sometimes modify the event store)

Requirements and applications evolve and change, leading to refactoring. For example, user registration now requires a first and last name, what was once an error may no longer be considered one, etc. In an event sourced application, that poses a few problems. In this post I’ll discuss a few strategies and my views on event versioning.

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Distributed command handling with Axon, JGroups and Docker

In a recent project I used Axon Framework together with JGroups, to create a clustered, or distributed command bus.

In that project we had some concurrency issues. One of those issues was that two events were applied on a single aggregate with an identical sequence number. The JGroupsConnector in Axon uses a consistent hashing algorithm to route commands. This ensures that commands with the same routing key will be sent to the same member, regardless of the sending member of that message. Within the project we used the aggregate identifier as the routing key, ensuring that commands for a single aggregate are processed in a single JVM, thus preventing duplicate sequence numbers.

To demonstrate such a setup, I’ve created a simple demo application, based on the latest version of Axon Framework (3.0-M3). Using Docker Compose, the application is launched twice (in two containers). The containers should then form a JGroups cluster, and handle a number of commands. Go check it out on GitHub!