It appears that with some newer version of Tomcat, and specifying a newer servlet spec in web.xml, the following rule applies: "The web.xml resulting from the merge is considered <distributable> only if all its web fragments are marked as as well." See Servlet spec 3.0, section 8.2.3, 5.g.ix

On upgrading to a newer version of Tomcat, we found that Clustering stopped working, and the webapp was being considered NON-distributable. Of the jars we use, ZK.jar is the only one that has a web-fragment.xml, and it is not tagged "distributable".

Our work-around has been to add — metadata-complete="true" — to our XML. If the web-fragment in zk.jar cannot be tagged distributable, then it would be a good idea to add some information about this to the ZK documentation on session clustering / serialization.