Service-oriented architectures (SOA) provide numerous benefits: reuse of business logic by many clients, location transparency of business logic, simplified unit testing, better scalability through distributed and load-balanced processing, and the composition of new services from existing services. Enterprise JavaBeans are a favorite platform on which to base service-oriented architectures because of their enterprise-class features.
As many new SOA applications are now developed on the J2EE platform, a problem arises: how to maintain 100% availability while deploying maintenance fixes and new versions of the services. Most application server vendors do not recommend hot deployment of applications in production; problems may occur with unloading classes, class loaders, and resources being used by existing deployments. Instead, the vendors recommend restarting the server or cluster of servers after a redeployment; however, the total time to redeploy, test, and restart a cluster of servers can be substantial. This downtime is unacceptable for many production sites due to loss of revenue and customer goodwill, and the mission-critical nature of the services.
A solution to this problem is to provide a mechanism for dynamically switching clients from a cluster of application servers running the old version to another cluster of application servers running the new version. We refer to this as dynamic cluster switching. This can be accomplished by some enhancements to commonly used J2EE patterns in conjunction with JMS-based messaging. The result is that most deployments of new releases can be made without interruption of services to the client.
Why bother with non-stop EJB services? We have all experienced the issues associated with EJB application upgrades and deployments, such as unexpected outages due to limited testing, poor fall-back strategies, and planned downtime for maintenance in the wee hours. For businesses selling commodity goods and services on the Web, downtime directly translates to lost revenue when customers can easily surf to other sites to buy the same product. With non-stop EJB services, you can reduce if not eliminate downtime while seamlessly rolling out new versions of your services. Imagine redeploying and upgrading your EJBs without impacting your clients and their Web sites, Web services, consumers, and business partners. If there are issues with the new software, fallback is low-risk and easy to accomplish. All of this occurs during peak traffic periods when using non-stop EJB services. This article describes how this was accomplished on a large consumer Web site handling over 15,000 concurrent sessions during peak times.
Dynamic Cluster Switching
This solution uses JMS messaging to control a plug-in used by clients of the EJB services. When it's time to release a new version of software, an alternate cluster of servers is deployed with the new code on the same hardware platform as the existing servers. A console or command-line program publishes a "cluster switch" message to the client plug-ins that subscribe to a JMS admin topic. The client plug-ins then start to open connections to the new cluster and allow connections to the old cluster to "die off" as sessions or connections are released. In a short time, all the client plug-ins are seamlessly connected to the new cluster. While this approach sounds simple at a high level, the implementation needs the confluence of many design patterns to be successful in practice.
