ReadClusterData Stage Optimisation : ZKCache added in getStat

[ngngwr]

Issues

The PR aims to optimise the ReadClusterData Stage of the rebalance pipeline in Helix. Customers with huge workloads are seeing significant increase in pipeline time. This PR is one of the item to improve on the performance of the Helix pieplines.

Description

getStats() method was not utilising the ZKCache, making direct ZooKeeper calls for every method call. This is causing the performance bottleneck in ReadClusterData Stage where a lot of time is spent on waiting for the ZK response. The changes added logic to utilise ZKCache.

Tests

Verified with the unit tests.

Changes that Break Backward Compatibility (Optional)

NA

Documentation (Optional)

NA

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[LZD-PratyushBhatt]

LGTM, Thanks for this!

Just a note, we should consolidate all such methods as they are mostly 90+% same.