[keycloak-dev] UserSessions support for cross-dc

I've sent PR https://github.com/keycloak/keycloak/pull/4357 for subject. It adds cross-dc support for userSessions, so that if you write userSession "abc" in DC1, you will be able to read it in DC2 and viceversa. Among cross-dc, it also provides the solution for lost updates (write skew) issues where 2 threads on different cluster nodes (or in different data-centers) updates same userSession. They both read the userSession in same state and then both update it, but 2nd update will overwrite the 1st one, which was committed first. I've used the pattern based on tracking changes (events) and infinispan atomic-replace operation described in the earlier mail: http://lists.jboss.org/pipermail/keycloak-dev/2017-May/009347.html So there was some refactoring of InfinispanUserSessionProvider to support the event-based approach. One difference from the previous proposal was, that events are not sent between data-centers but instead userSession entities are directly written to remoteCache itself - however the writes are still protected to avoid write skew issues. The reason is, that with multiple datacenters, it can happen that datacenters lost the network connection between each other (split brain). Infinispan has some ways to restore from this state and sync the entities after network connection is fixed. With the entities directly in the cache, this should be easier to achieve then the case when the remoteCache is used just as an event bus to send "changes" among datacenters. There is still lots of work for the cross-dc support, but hopefully it's another step forward :) Marek