Shared data in a cluster

I have a web app which is likely at some time to be clustered. I need a couple of things to be shared between instances of the app in the cluster. Any suggestions? Can I do this by storing the data in JNDI? My first thought was having the instances all read and write the same file, but that leaves me open to concurrent update problems. Can I prevent concurrent updates to an object stored in JNDI?

I'm aware that some containers that provide clustering can share session info between instances, but I don't want to do that because there will be too much traffic between the instances.