[SOLR-7660] Avoid redundant 'exists' calls made to ZK while fetching cluster state updates

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 5.3, 6.0
• Component/s: SolrCloud
• Labels: difficulty-easy impact-medium
• Assignee: Shalin Shekhar Mangar
• Reporter: Shalin Shekhar Mangar
• Created: 10/Jun/15 15:56
• Updated: 09/May/16 18:43
• Resolved: 11/Jun/15 06:12

Description

As noted in https://issues.apache.org/jira/browse/SOLR-7636?focusedCommentId=14578332&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14578332

we have CollectionsHandler -> ZK getChildren (/live_nodes) + ZK read (/clusterstate.json) + ZK getChildren + N * ZK exists (for getIndividualColls) + W * ZK exists + W * getData (for watched collections) + (N-W) * ZK exists + (N-W) * ZK getData (for non-watched collections).

Some of the ZK exists calls can be avoided without changing the outcome.