Axon Framework – the CQRS framework for Java – version 0.4 released
Last week, I published the 0.4 release of the Axon Framework. Axon helps developers build high performance, scalable and extensible applications using the CQRS pattern. The 0.4 release is a major step towards 1.0, and includes transactional event handling, high-performance caching repositories and easy configuration of event sourcing support. Furthermore, we have also built a demo application that uses Flex to get real-time updates pushed from the server.
Read on to find out more.
About the Axon Framework
Axon Framework helps build scalable, extensible and maintainable applications by supporting developers apply the Command Query Responsibility Segregation (CQRS) architectural pattern. It does so by providing implementations, sometimes complete, sometimes abstract, of the most important building blocks, such as aggregates, repositories and event the bus – the dispatching mechanism for events. Furthermore, Axon provides annotation support, which allows you to build aggregates and event listeners without tying your code to Axon specific logic. This allows you to focus on your business logic, instead of the plumbing, and helps you makes your code easier to test in isolation.
Features available in 0.4
We have made quite a few additions and changes since the 0.3 release. Unfortunately, this also means I had to make some minor API changes. There is now a bigger choice of abstract classes for most of the building blocks. This allows you to choose whether you want to use the logic provided by Axon, or prefer to develop more of your own. Whatever your wish is, there should be a class to help you on your way.
The biggest change since 0.3 is the addition of an Asynchronous Event Bus that supports transactional processing of events. Since it is asynchronous, it means that your command processing doesn’t have to wait for events to be processed. However, this Event Buss can also guarantee the order in which events are delivered to the event handlers. You can choose full concurrent processing (without any guarantees about ordering), full sequential processing (FiFo guarantee) or anything in between. You could, for example, guarantee the sequential processing of Events that come from the same aggregate.
The Event Bus also provides transaction support. That means you can tell the event bus to retry an event after a certain amount of time when processing fails, for example due to an error in the underlying mechanism, such as a database. For performance reasons, you can configure the event handler to process multiple events within a single transaction. Database updates are a lot faster if you process several events within a single transaction before committing it. Within your event handler, you can configure transactions.
Besides Domain Events, Axon now also allows you to explicitly define two other types of events: Application Events and System Events. Application Event can notify your event listeners that something interesting happened in your application. System Events are a special type of application events and can be used to indicate that a part of your application has changed state, for example when the database is no longer available. These System Events could provide interesting operational information about your application.
Another major change since 0.3 is the documentation. We have brought the documentation up to par with the code-base. All features are now extensively described in the reference guide. You can download the manual from the Axon Framework website: www.axonframework.org.
Last, but definitely not least, we have created a demo application that you can easily download and deploy to have a look at how you can use events to your advantage. This demo application is built using a Flex Client and an Axon-based server side component. The Flex Client registers itself as an event listener and will automatically process incoming events, at real time! That means that you can have 2 windows open, change something in one window, and see the change come in immediately on the other window, too. Check our Sample Installation page for more information.
What to expect in upcoming releases
The major components of the CQRS pattern are already available in the 0.4 release of Axon Framework. However, there is still a number of features we want to include in the 1.0 release. Although one of the advantages of CQRS is that it makes an application scalable and extensible, the 1.0 release of Axon Framework focuses on what happens inside a single JVM. Streaming events between JVM’s is not made impossible though, the Spring Integration connectors can be used to publish events to JMS, MQ, mail, HTTP and any other protocol supported by Spring Integration.
Some features to expect before the 1.0 release are rolling snapshots. When aggregates live for a long time, the number of events to read in each time the aggregate is loaded could easily run in the thousands. Reading in a thousand events takes a long time. Instead of reading in a thousand events, you can summarize all these events into a single snapshot event. Axon Framework will provide the plumbing necessary for you to build snapshot events.
Another feature that we’ll be spending some attention on is failure recovery. Unfortunately, applications sometimes behave unexpectedly and sometimes stop entirely. The reason of a crash won’t be the Axon Framework itself, of course. But since we have a reliable source of events in the events store, we can use this to recover from failure. Axon Framework will provide some building blocks that allow you to republish events to event handlers that did not get the chance to process them. This same mechanism will allow you to restore your application state from backups.
The last feature currently on the roadmap is JMX support. Since Axon Framework deals with the dispatching and invocation of event handlers, it has interesting information about an application’s performance and technical state. We are planning to expose this information using JMX MBeans.
If you have any other ideas, you can submit a feature request on www.axonframework.org/issues.
Getting started with Axon
If you’re eager to get started using Axon Framework for your own application, check out the Axon Framework website. There, you can download the binaries, sources documentation and a sample application. If you use Maven, the reference manual will explain how you can configure the dependencies on the Axon binaries.
If the information on the website is not enough to get you started, don’t hesitate to send me a message or post a comment at the bottom of this blog entry.
A special thanks to…
Before I wrap up, I would like to thank Jettro Coenradie for his help on the sample project and the reference documentation. I think the sample turned out into a pretty cool demo environment for some of the advantages that Axon can provide.
Also many thanks to JTeam for the time they allowed me to spend on the project. Without it, I am sure I couldn’t have delivered this release. Thanks!