[ZOOKEEPER-1856] zookeeper C-client can fail to switch from a dead server in a 3+ server ensemble if the client only has a 2 server list.

• Type: Bug
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Component/s: c client
• Assignee: Michi Mutsuzaki
• Reporter: Dutch T. Meyer
• Created: 02/Jan/14 23:43
• Updated: 15/Jun/23 21:09

Description

If a client has a 2 server list, and is currently connected to the last server in that list, and that server then goes offline, the addrvec_next() call handle_error() will push the client to the start of the list and terminate the connection.

Then, the zoo_cycle_next_server() call in zookeeper_interest will be called in response to the connection failure, and the client will cycle back to the failed server.

In this way, a client who has a list of only 2 servers can get stuck on the one failed server. This would only be an issue in an ensemble larger than 2 of course, because failing 1 out of 2 would lead to quorum loss anyway.

There are other harmonics possible if every other server in the list is failed, but this is simplest to reproduce in a 3 server ensemble where the client only knows about 2 servers, one of which then fails. There are probably some elegant fixes here, but I think the simplest is to add a flag to track whether a server has been accessed before, and if it hasn't, don't call zoo_cycle_next_server() at the top of the zookeeper_interest() function.

Issue Links

• relates to: ZOOKEEPER-2466 Client skips servers when trying to connect