[jboss-jira] [JBoss JIRA] (JGRP-2470) JBDC_PING can face a split-brain issue when restarting a coordinator node

[Radoslav Husar (Jira)]

    [ https://issues.redhat.com/browse/JGRP-2470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14118598#comment-14118598 ] 

Radoslav Husar commented on JGRP-2470:
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bq. What's the rationale for reverting JGRP-2199? If the old coord removes all information, the new one will re-insert it (actually multiple times, if configured)...

That's the problem – these are not synchronized in any way and thus it's racey. As Masafumi explained, the problem is that if these are ordered in the database in a way 1. the new coordinator reinserted the data and 2. then the stopping coordinators clear table query {{clearTable(String clustername)}} the restarted member won't discovery anything and start a singleton cluster.

bq. It seems that a periodic findMembers() triggered by MERGE3 can heal the singleton clusters situation. The findMembers() can insert their own node information when the table is empty. But this happens after the interval calculated by Math.max(min_interval, Util.random(max_interval) + max_interval/2) where min_interval is 10000 and max_interval is 30000 in JBoss EAP by default. And, the actual merge happens after this. So, it could take a long time to heal the singleton clusters situation.

We don't even need to go into details on merging partitions, discovery can never lead by design into this as this causes partitions and data loss.


> JBDC_PING can face a split-brain issue when restarting a coordinator node
> -------------------------------------------------------------------------
>
>                 Key: JGRP-2470
>                 URL: https://issues.redhat.com/browse/JGRP-2470
>             Project: JGroups
>          Issue Type: Bug
>    Affects Versions: 4.1.9, 4.0.22
>            Reporter: Masafumi Miura
>            Assignee: Radoslav Husar
>            Priority: Major
>             Fix For: 4.2.5
>
>
> After [the change|https://github.com/belaban/JGroups/commit/215cdb6] for JGRP-2199, JDBC_PING deletes all entries from the table during the shutdown of the coordinator node. 
> This behavior has a possibility to cause a split-brain when restarting a coordinator node. Because, as all entries are lost in the following scenario, the restarting node can not find any information about existing nodes from the table and does not form a cluster.
> 0. node1 and node2 form a cluster. The node1 is a coordinator.
> 1. Trigger a restart of the node1
> 2. The node1 removes their node information from the table
> 3. The node2 becomes a new coordinator
> 4. The node2 updates their node information in the table
> 5. The node1 clears all entries from the table
> 6. The node1 starts again
> 7. The node1 does not join the existing cluster because there's no node information in the table
> Note: If step 5 happens before step 4, the split-brain issue does not happen. However, as step 4 and step 5 happen on different nodes, these steps can happen in parallel. So, the order is undefined. So, for example, if the shutdown of node1 takes a long time, there's a high possibility to face this issue.



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