ZooKeeper 3.5.5 release is the latest stable one. It contains many new features (including SSL related improvements which are very important for production use; see the release notes). Yet there should be no backward incompatible changes on the API, so the applications using ZooKeeper clients should be built against the new zookeeper without any problem and the new ZooKeeper client should work with the older (3.4) servers without any issue, at least until someone is start to use new functionality.
The aim of this ticket is not to change the ZooKeeper version used by Hadoop YARN yet, but to enable people to rebuild and test Hadoop with the new ZooKeeper version.
Currently the Hadoop build (with ZooKeeper 3.5.5) fails because of a YARN test case: TestSecureRegistry.testLowlevelZKSaslLogin(). This test case seems to use low-level ZooKeeper internal code, which changed in the new ZooKeeper version. Although I am not sure what was the original reasoning of the inclusion of this test in the YARN code, I propose to remove it, and if there is still any missing test case in ZooKeeper, then let's issue a ZooKeeper ticket to test this scenario there.