[infinispan-issues] [JBoss JIRA] (ISPN-7749) A node leaving during rebalance causes extra segment transfers

[Dan Berindei (JIRA)]

Dan Berindei created ISPN-7749:
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             Summary: A node leaving during rebalance causes extra segment transfers
                 Key: ISPN-7749
                 URL: https://issues.jboss.org/browse/ISPN-7749
             Project: Infinispan
          Issue Type: Bug
          Components: Core, State Transfer
    Affects Versions: 9.0.0.Final
            Reporter: Dan Berindei
            Assignee: Dan Berindei
             Fix For: 9.1.0.Final


If a node leaves during a rebalance, the only change is that other nodes will no longer request segments from that node. Otherwise the rebalance proceeds as usual, and at the end a node may delete its copy of a segment even if that leaves less than {{numOwners}} copies in the cluster. 

When the leaver is the joiner, a 2nd rebalance is very likely to return to the initial CH, but only after transferring some segments twice:

{noformat}
Rebalance starts: current_owners(s) = AB, pending_owners(s) = AC
C leaves: current_owners(s) = AB, pending_owners(s) = A
Rebalance finishes: current_owners(s) = A, pending_owners(s) = A
2nd rebalance starts: current_owners(s) = A, pending_owners(s) = AB
{noformat}

Even if the leaver is one of the old owners and the 2 segment transfers are necessary, the cluster stays for too long with less than {{numOwners}} copies of the segment:

{noformat}
Rebalance starts: current_owners(s) = AB, pending_owners(s) = AC
A leaves: current_owners(s) = B, pending_owners(s) = C
C transfers segment s from B
Rebalance finishes: current_owners(s) = C, pending_owners(s) = C
2nd rebalance starts: current_owners(s) = C, pending_owners(s) = DC
{noformat}

We can fix this by making the pending CH a union of the current and pending CH whenever a node leavers, and only removing extra segment copies after the 2nd rebalance.




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