[keycloak-dev] UserSessions support for cross-dc

[Marek Posolda]

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