[HBASE-27402] Clone Scan in ClientScanner to avoid errors with Scan re-used

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.6.0
• Labels: patch-available
• Assignee: Bryan Beaudreault
• Reporter: Bryan Beaudreault
• Created: 01/Oct/22 13:31
• Updated: 15/Jan/24 07:09
• Resolved: 13/Jan/24 20:14

Description

This has come up before in https://issues.apache.org/jira/browse/HBASE-1774 and https://issues.apache.org/jira/browse/HBASE-4891. The major pushback was around ScanMetrics, which relied on sharing a mutable Scan object.

Since https://issues.apache.org/jira/browse/HBASE-17584, ScanMetrics are available on ResultScanner and the method on Scan was deprecated (removed in master).

I think this issue became pretty urgent in https://issues.apache.org/jira/browse/HBASE-17167, when we started passing mvcc into the Scan object. If a user unknowingly reuses the Scan object, this can seem like data loss since the Scan will return none of the expected data.  We recently hit this in our upgrade from hbase client 1.2 to 2.4.6, where use-cases that had worked in 1.2 suddenly started returning no results in 2.4.6. It's very hard to debug.

I suggest that we now add the clone in master branch. For branch-2, I think we could put it behind a config param to preserve backwards compatibility of Scan.getScanMetrics. If the config param is enabled, scan cloning occurs and Scan.getScanMetrics will be inaccurate. Personally I think this is far better scenario, because data result accuracy is more important than metrics. But we can leave it to the user to decide, and provide a release note.

Issue Links

• links to: GitHub Pull Request #5614