[KUDU-1345] HybridClock can run backwards when clocks are far out of sync

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: 0.7.0
• Fix Version/s: 0.7.1, 0.8.0
• Target Version/s: 0.8.0
• Code Review: http://gerrit.cloudera.org:8080/#/c/2266/
• Assignee: Todd Lipcon
• Reporter: Todd Lipcon
• Created: 20/Feb/16 02:51
• Updated: 03/Mar/16 00:00
• Resolved: 23/Feb/16 04:23

Description

Nick Wolf reported an error he hit on his cluster where transactions were assigned out-of-order timestamps. After looking at his WALs for a while, I realized we have this bug:

• the clock gets an update from another node a second in the future
• we generate a lot of timestamps locally, which causes the clock to increment its logical portion
• the logical portion exceeds the number of bits allocated to it (12 bits iirc). The way we return timestamps, this is OK – we simply add the logical and physical portions, so it "wraps" into the physical portion, causing it to increment forward a few microseconds as we do thousands of writes.
• however, our host clock eventually reaches a higher value than the last physical portion. This causes us to reset our logical clock portion back to 1, which may actually rewind it back in time.