[slantedview]

General things: when your session fails to migrate, when log compaction can't keep up with writes, when logs fill up your disk (ZK used to require external cron jobs to prune logs), when watcher notifications arrive late, when the leader's GC exceeds its heartbeat timeout, and so on.

Lots of things can and do go wrong with Zookeeper. I suspect it depends on the use case, but building a Zookeeper dependency into any system is potentially asking a lot of users/operators.

[growse]

Well, like many things, if you deploy it on sensible hardware and put some thought into how its operated, zookeeper isn't particularly problematic. It's certainly not a 'nightmare'.

[sciurus]

As an operator I do feel that pain but the alternative of every piece of software that needs distributed coordination reimplementing it, likely poorly, it isn't great either.

[curun1r]

There are (IMHO, superior) alternatives like Etcd and Consul that could be used.

Ideally, these kinds systems should be built such that the distributed coordination piece is pluggable and the implementation can be chosen based on deployment concerns.