I am not the expert for application server, but my understanding is that WAS Liberty Profile (WLP) implements newer technologies that are not all yet implemented in WAS. WLP is leaner than WAS, but can be expanded with more features.

We are looking into clustering at a customer and so far, the experience is good. My understanding is that clustering is also in productive use within IBM.

You might be aware that we tried to implement clustering in the past with WAS/WAS ND, but were not really successful. The reason was the caching in RTC. The available solutions to do distributed caching in WAS were not efficient/fast enough and cost a fortune. That clustering only helped with up time, but not with scaling and was abandoned because of that.

The key to the new clustering is session affinity using HA proxy and comminication to keep the cache consistent using messaging. This does not require all the complex stuff that WAS and WAS ND provides to deploy many applications automatically in farms and built in distributed caching. At least not for the clustering itself. It also leaves more resources for the application. I am sure clustering can be made available on WAS as well, but there is a need to focus and the decision was to go with the default and bundled application server which has become WLP.

Clustering seems to be pretty stable and actually scales pretty well. As with all new stuff, I expect a learning curve, but I don't think the application server WLP is going to be the issue. So far the experience is very good and the new clustering seems to be a very good mechanism that allows scaling and reliability/availability by using multiple nodes.

There are several customers that are outgrowing what a single server RTC can provide at the moment and clustering is going to be the only way to increase the performance for them and to kee up with the growth. So we have no choice to make it successful this time and I think we will 8).