[SOLR-6261] Run ZK watch event callbacks in parallel to the event thread

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 4.9
• Fix Version/s: 4.10, 6.0
• Component/s: SolrCloud
• Assignee: Mark Miller
• Reporter: Ramkumar Aiyengar
• Created: 20/Jul/14 20:17
• Updated: 09/May/16 18:52
• Resolved: 11/Aug/14 21:23

Description

Currently checking for leadership (due to the leader's ephemeral node going away) happens in ZK's event thread. If there are many cores and all of them are due leadership, then they would have to serially go through the two-way sync and leadership takeover.

For tens of cores, this could mean 30-40s without leadership before the last in the list even gets to start the leadership process. If the leadership process happens in a separate thread, then the cores could all take over in parallel.

Issue Links

• relates to: SOLR-6336 DistributedQueue (and it's use in OCP) leaks ZK Watches