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https://issues.apache.org/jira/browse/ZOOKEEPER-261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15820065#comment-15820065
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ASF GitHub Bot commented on ZOOKEEPER-261:
------------------------------------------
Github user eribeiro commented on a diff in the pull request:
https://github.com/apache/zookeeper/pull/120#discussion_r95721046
--- Diff:
src/java/main/org/apache/zookeeper/server/persistence/FileTxnSnapLog.java ---
@@ -175,11 +193,20 @@ public long restore(DataTree dt, Map<Long, Integer>
sessions,
"No snapshot found, but there are log entries. " +
"Something is broken!");
}
- /* TODO: (br33d) we should either put a ConcurrentHashMap on
restore()
- * or use Map on save() */
- save(dt, (ConcurrentHashMap<Long, Integer>)sessions);
- /* return a zxid of zero, since we the database is empty */
- return 0;
+
+ if (suspectEmptyDB) {
+ /* return a zxid of -1, since we are possibly missing data
*/
+ LOG.warn("Unexpected empty data tree, setting zxid to -1");
--- End diff --
Are we 100% sure the data tree is empty? Couldn't it be only partially
complete? I mean the machine recorded up to transaction n, but lost
transactions n+1, n+2, n+3, etc?
> Reinitialized servers should not participate in leader election
> ---------------------------------------------------------------
>
> Key: ZOOKEEPER-261
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-261
> Project: ZooKeeper
> Issue Type: Improvement
> Components: leaderElection, quorum
> Reporter: Benjamin Reed
>
> A server that has lost its data should not participate in leader election
> until it has resynced with a leader. Our leader election algorithm and
> NEW_LEADER commit assumes that the followers voting on a leader have not lost
> any of their data. We should have a flag in the data directory saying whether
> or not the data is preserved so that the the flag will be cleared if the data
> is ever cleared.
> Here is the problematic scenario: you have have ensemble of machines A, B,
> and C. C is down. the last transaction seen by C is z. a transaction, z+1, is
> committed on A and B. Now there is a power outage. B's data gets
> reinitialized. when power comes back up, B and C comes up, but A does not. C
> will be elected leader and transaction z+1 is lost. (note, this can happen
> even if all three machines are up and C just responds quickly. in that case C
> would tell A to truncate z+1 from its log.) in theory we haven't violated our
> 2f+1 guarantee, since A is failed and B still hasn't recovered from failure,
> but it would be nice if when we don't have quorum that system stops working
> rather than works incorrectly if we lose quorum.
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