There was a little change to the licensing terms and conditions that was sneaked in with the release of WAS v220.127.116.11 fixpak.
We're used clustering being available in WAS Network Deployment. This provides the Deployment Manager, which provides a central point of administration to manage a distributed environment and clustered application servers. The clustering of application servers allows for the load balancing and failover of HTTP, EJB, Transactional and Messaging workloads.
There is no centralised administration (although the v7 features admin agent and job manager provide a neat option here), there is no centralised deployment of applications, and so on.
<EDIT Oct 14th 2013>The following licensing restrictions are removed as of WAS v8.5.5. It is now permitted for an unlimited number of profiles to be load balanced across WAS base, Express and Liberty core as stated in the announcement letter</EDIT>
There are restrictions on how many application servers can be routed to from the web server plugin depending on which version you are using.
(I provide links to the official license, but it's not easy reading!)
For WAS (base), the licensee is authorized to configure the Web server plug-ins for up to
- 25 Profiles into one Web server plug-in file for the HTTP server.
Within a single Web server plug-in, any simple load balancing and failover must be limited to routing requests to a maximum of 5 profiles and is limited to HTTP/HTTPS requests only.
(this means that whilst you can load balance to a number of applications across up to 25 profiles, you can only route to a single application that is "clustered" on a maximum of 5 application servers - a cluster maximum size of 5.
- 2 Profiles into one Web server plug-in file for the HTTP server - a "cluster" maximum size of 2
The key steps are to configure session affinity and then to merge the plugin configuration files from the various application server profiles. WebSphere provides the pluginMerge tool to do that for you