[ZOOKEEPER-3500] Improving the ZAB UPTODATE semantic to only issue it to learner when there is limited lagging

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: server
• Labels: pull-request-available
• Assignee: Fangmin Lv
• Reporter: Fangmin Lv
• Created: 08/Aug/19 17:12
• Updated: 11/Sep/19 01:08

Description

With large snapshot and high write RPS, when learner is having SNAP syncing with leader, there will be lots of txns need to be replayed between NEWLEADER and UPTODATE packet.
 
Depends how big the snapshot and traffic is, from our benchmark, it may take more than 30s to replay all those txns, which means when we process the UPTODATE packet, it's still 30s lagging behind, with 10K/s txn that's 300K txns lagging.
 
And we start to serve client traffic just after we received UPTODATE packet, which means client will see lots of stale data.
 
The idea here is trying to check and only send UPTODATE packet when there is limited txns lagging behind from leader side. It doesn't change the ZAB protocol, but changed the time when ZK is applying the txns between NEWLEADER and UPTODATE. 
 

Issue Links

• links to: GitHub Pull Request #1047