[ZOOKEEPER-2872] Interrupted snapshot sync causes data loss

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.4.10, 3.5.3, 3.6.0
• Fix Version/s: 3.6.0
• Component/s: server
• Labels: pull-request-available
• Assignee: Brian Nixon
• Reporter: Brian Nixon
• Created: 10/Aug/17 18:07
• Updated: 03/Jan/22 04:49
• Resolved: 11/Feb/21 19:19

Description

There is a way for observers to permanently lose data from their local data tree while remaining members of good standing with the ensemble and continuing to serve client traffic when the following chain of events occurs.

1. The observer dies in epoch N from machine failure.
2. The observer comes back up in epoch N+1 and requests a snapshot sync to catch up.
3. The machine powers off before the snapshot is synced to disc and after some txn's have been logged (depending on the OS, this can happen!).
4. The observer comes back a second time and replays its most recent snapshot (epoch <= N) as well as the txn logs (epoch N+1).
5. A diff sync is requested from the leader and the observer broadcasts availability.

In this scenario, any commits from epoch N that the observer did not receive before it died the first time will never be exposed to the observer and no part of the ensemble will complain.

This situation is not unique to observers and can happen to any learner. As a simple fix, fsync-ing the snapshots received from the leader will avoid the case of missing snapshots causing data loss.

Issue Links

• fixes: ZOOKEEPER-2310 Snapshot files must be synced to prevent inconsistency or data loss
• is related to: ZOOKEEPER-4433 Backport ZOOKEEPER-2872 for branch-3.5 (Interrupted snapshot sync causes data loss)
• links to: GitHub Pull Request #333
• links to: GitHub Pull Request #1146