We have just done an upgrade of ZK to 3.3.0. Previous to this, ZK has been up for about a year with no problems.
On two nodes, we killed the previous instance and started the 3.3.0 instance. The first node was a follower and the second a leader. All went according to plan and no clients seemed to notice anything. The stat command showed connections moving around as expected and all other indicators were normal. When we did the third node, we saw this in the log: 2010-04-20 14:07:49,010 - FATAL [QuorumPeer:/0.0.0.0:2181:follo...@71] - Leader epoch 18 is less than our epoch 19 The third node refused all connections. We brought down the third node, wiped away its snapshot, restarted and it joined without complaint. Note that the third node was originally a follower and had never been a leader during the upgrade process. Does anybody know why this happened? We are fully upgraded and there was no interruption to normal service, but this seems strange.