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From ShawnWalker <>
Subject [GitHub] accumulo pull request #121: ACCUMULO-4353: Stabilize tablet assignment durin...
Date Wed, 29 Jun 2016 16:16:19 GMT
Github user ShawnWalker commented on a diff in the pull request:
    --- Diff: server/base/src/main/java/org/apache/accumulo/server/master/state/
    @@ -93,6 +95,9 @@ public TServerInstance getServer() {
       public TabletState getState(Set<TServerInstance> liveServers) {
    +    if (suspend != null) {
    --- End diff --
    Offline discussion with @keith-turner turned up a race condition here:
    1. Tablet is assigned to tserver
    2. `TabletGroupWatcher` reads tablet's location state, sees a `TabletLocationState` in
which `current` is null, `future` is set, `suspend` is null)
    3. Tserver begins hosting tablet (`current` is set, `future` is null, `suspend` is null)
    4. Tserver dies, lock is lost
    5. `TabletGroupWatcher` still thinks `current` is set, `future` is null, `suspend` is
null, and also thinks tserver is dead, so state ASSIGNED_TO_DEAD_SERVER.  `TabletGroupWatcher`
suspends tablet.
    7. Now `current` is set, `future` is null, `suspend` is set (state SUSPENDED).  
    8. Tserver comes back.
    9. On next scan, `TabletGroupWatcher` assigns the tablet back to tserver, setting `future`
(and clearing `suspend`).  Now `TabletLocationState` is in the illegal situation where both
`current` is `future` are simultaneously set.
    While this race seems a bit unlikely, it is a possible concern.  To address this concern,
I believe that if either `current` or `future` is set, we should favor returning state ASSIGNED_TO_DEAD_SERVER
over state SUSPENDED.  That way, on the first pass, `TabletGroupWatcher` suspends the tablet,
clearing `future` (which is a no-op, since `future` is already clear).  On the second pass,
`TabletGroupWatcher` suspends the tablet again, this time clearing `current`.

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