The demands on software projects increase rapidly as time progresses. Companies no longer accept a brochure-like homepage to promote their business; they want their (web)applications to evolve together with their business. That means that not only projects and code bases become more complex, it also means that functionality is constantly added, changed and (unfortunately not enough) removed. It can be frustrating to find out that a seemingly easy-to-implement feature can require development teams to take apart an entire application. Furthermore, today's webapplications target an audience of potentially billions of people, making scalability an indisputable requirement.

Although there are many applications and frameworks around that deal with scalability issues, such as GigaSpaces and Terracotta, they share one fundamental flaw. These stacks try to solve the scalability issues while letting developers develop applications using the layered architecture they are used to. In some cases, they even prevent or severely limit the use of a real domain model, forcing all domain logic into services. Although that is faster to start building an application, eventually this approach will cause complexity to increase and development to slow down.

Greg Young, initiator of the Command Query Responsiblity Segregation (CQRS) pattern addressed these issues by drastically changing the way applications are architected. Instead of separating logic into separate layers, logic is separated based on whether it is changing an application's state or querying it. That means that executing commands (actions that potentially change an application's state) are executed by different components than those that query for the application's state. The most important reason for this separation is the fact that there are different technical and non-technical requirements for each of them. When commands are executed, the query components are (a)synchronously updated using events. This mechanism of updates through events, is what makes this architecture is extensible, scalable and ultimately more maintainable.

	Note
	A full explanation of CQRS is not within the scope of this document. If you would like to have more background information about CQRS, visit the Axon Framework website: www.axonframework.org. It contains links to background information.

Since CQRS is so fundamentally different than the layered-architecture which dominates the software landscape nowadays, it is quite hard to grasp. It is not uncommon for developers to walk into a few traps while trying to find their way around this architecture. That's why Axon Framework was conceived: to help developers implement CQRS applications while focussing on the business logic.

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 of the most important building blocks, such as aggregates, repositories and event buses (the dispatching mechanism for events). Furthermore, Axon provides annotation support, which allows you to build aggregates and event listeners withouth tying your code to Axon specific logic. This allows you to focus on your business logic, instead of the plumbing, and helps you to make your code easier to test in isolation.

Axon does not, in any way, try to hide the CQRS architecture or any of its components from developers. Therefore, depending on team size, it is still advisable to have one or more developers with a thorough understanding of CQRS on each team. However, Axon does help when it comes to guaranteeing delivering events to the right event listeners and processing them concurrently and in the correct order. These multi-threading concerns are typically hard to deal with, leading to hard-to-trace bugs and sometimes complete application failure. When you have a tight deadline, you probably don't even want to care about these concerns. Axon's code is thoroughly tested to prevent these types of bugs.

Most of the concerns Axon addresses are located within a single JVM. However, for an application to be scalable, a single JVM is not enough. Therefore, Axon provides the axon-intregration module, which allows events to be sent to a Spring Integration channel. From there, you can use Spring Integration to dispatch events to application components on different machines. In the near future, Axon will provide more ways to dispatch commands and events between JVM's and physical machines.

Will each application benefit from Axon? Unfortunately not. Simple CRUD (Create, Read, Update, Delete) applications which are not expected to scale will probably not benefit from CQRS or Axon. Fortunately, there is a wide variety of applications that does benefit from Axon.

Applications that will most likely benefit from CQRS and Axon are those that show one or more of the following characteristics:

You can download the Axon Framework from our downloads page: axonframework.org/download.

This page offers a number of downloads. Typically, you would want to use the latest stable release. However, if you're eager to get started using the latest and greatest features, you could consider using the snapshot releases instead. The downloads page contains a number of assemblies for you to download. Some of them only provide the Axon library itself, while others also provide the libraries that Axon depends on. There is also a "full" zip file, which contains Axon, its dependencies, the sources and the documentation, all in a single download.

If you really want to stay on the bleeding edge of development, you can also checkout the sources from the subversion repository: http://axonframework.googlecode.com/svn/trunk/.

If you use maven as your build tool, you need to configure the correct dependencies for your project. Add the following code in your dependencies section:

<dependency>
    <groupId>org.axonframework</groupId>
    <artifactId>axon-core</artifactId>
    <version>1.2</version>
</dependency>

Most of the features provided by the Axon Framework are optional and require additional dependencies. We have chosen not to add these dependencies by default, as they would potentially clutter your project with artifacts you don't need. This section discusses these dependencies and describes in what scenarios you need them.

<dependency>
    <groupId>org.axonframework</groupId>
    <artifactId>axon-integration</artifactId>
    <version>1.2</version>
</dependency>
<dependency>
    <groupId>org.springframework.integration</groupId>
    <artifactId>spring-integration-core</artifactId>
    <version><!-- Add version here --></version>
</dependency>

Axon Framework doesn't impose many requirements on the infrastructure. It has been built and tested against Java 6, making that more or less the only requirement.

Since Axon doesn't create any connections or threads by itself, it is safe to run on an Application Server. Axon abstracts all asynchronous behavior by using Executors, meaning that you can easily pass a container managed Thread Pool, for example. If you don't use an Application Server (e.g. Tomcat, Jetty or a stand-alone app), you can use the Executors class or the Spring Framework to create and configure Thread Pools.