Wizard API updated!
Tim Boudreau has released a new version of the Swing Wizard library (version 0.997) that fixes the WizardException bug reported in JavaWorld's recent Open Source Java Project profile. The article's examples have been reworked to test out the new, improved WizardException. Thanks, Tim, for this helpful fix!
Open Source Java Projects: The Wizard API

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J2EE clustering, Part 2

Migrate your application from a single machine to a cluster, the easy way

By Abraham Kang, JavaWorld.com, 08/03/01

Within the J2EE framework, clusters provide an infrastructure for high availability (HA) and scalability. A cluster is a group of application servers that transparently run your J2EE application as if the group were a single entity. However, Web applications behave differently when they are clustered as they must share application objects with other cluster members through serialization. Moreover, you'll have to contend with the extra configuration and setup time.

To avoid major Web application rework and redesign, you should from the very beginning of your development process consider cluster-related programming issues, as well as critical setup and configuration decisions in order to support intelligent load balancing and failover. Finally, you will need to have a management strategy to handle failures.

Read the whole "J2EE clustering" series:



Building on the information in Part 1, I'll impart an applied understanding of clustering. Further, I'll examine clustering-related issues and their possible solutions, as well as the advantages and disadvantages of each choice. I'll also demonstrate programming guidelines for clustering. Finally, I'll show you how to prepare for outages. (Note that, due to licensing constraints, this article will not cover benchmarking.)

Set up your cluster

During cluster setup, you need to make important decisions. First, you have to choose a load balancing method. Second, you must decide how to support server affinity. Finally, you need to determine how you will deploy the server instances among clustered nodes.

Load balancing

You can choose between two generally recognized options for load balancing a cluster: DNS (Domain Name Service) round robin or hardware load balancers.

DNS round robin

DNS is the process by which a logical name (i.e., www.javaworld.com) is converted to an IP address. In DNS round-robin load balancing, a single logical name can return any IP address of the machines in a cluster.

DNS round-robin load balancing's advantages include:

  • Cheap and easy setup
  • Simplicity


Its disadvantages include:

  • No server affinity support. When a user receives an IP address, it is cached on the browser. Once the cache expires, the user makes another request for the IP address associated with a logical name. That second request can return the IP address of any other machine in the cluster, resulting in a lost session.
  • No HA support. Imagine a cluster of n servers. If one of those servers goes down, every nth request to the DNS server will go to the dead server.
  • Changes to the cluster take time to propagate to the rest of the Internet. Many corporations' and ISPs' DNS servers cache DNS lookups from their clients. Even if your DNS list of servers in the cluster could change dynamically, it would take time for the cached entries on other DNS servers to expire. For example, after a downed server is removed from your cluster's DNS list, AOL clients could still attempt to hit the downed server if AOL's DNS servers cached entries to the downed server. As a result, AOL users would not be able connect to your site even if other machines in the cluster were available.
  • No guarantee of equal client distribution across all servers in the cluster. If you don't configure cooperating DNS servers to support DNS load balancing, they could take only the first IP address returned from the initial lookup and use that for their client requests. Imagine a partner corporation with thousands of employees all pinned to a single server in your cluster!

Hardware load balancers



In contrast, a hardware load balancer (like F5's Big IP) solves most of these problems through virtual IP addressing. A load balancer presents to the world a single IP address for the cluster. The load balancer receives each request and rewrites headers to point to other machines in the cluster. If you remove any machine in the cluster, the changes take effect immediately.

Hardware load balancers' advantages include:

  • Server affinity when you're not using SSL
  • HA services (failover, monitoring, and so on)
  • Metrics (active sessions, response time, and so on)
  • Guaranteed equal client distribution across cluster


However, hardware load balancers exhibit disadvantages:

  • High cost -- 0,000 to 0,000, depending on features
  • Complex setup and configuration


Once you have picked your load balancing scheme, you must decide how your cluster will support server affinity.

Server affinity

Server affinity becomes a problem when using SSL without Web server proxies. (Server affinity directs a user to a particular server in the cluster for the duration of her session.) Hardware load balancers rely on cookies or URL readings to determine where requests are directed. If requests are SSL encrypted, hardware load balancers cannot read the header, cookie, or URL information. To solve the problem, you have two choices: Web server proxies or SSL accelerators.

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