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From revans2 <...@git.apache.org>
Subject [GitHub] zookeeper pull request #180: ZOOKEEPER-2700 add JMX `takeSnapshot` method an...
Date Tue, 21 Feb 2017 15:13:31 GMT
Github user revans2 commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/180#discussion_r102229805
  
    --- Diff: src/java/main/org/apache/zookeeper/server/ZooKeeperServer.java ---
    @@ -303,15 +305,38 @@ public void loadData() throws IOException, InterruptedException
{
     
         public void takeSnapshot(){
    --- End diff --
    
    We now have a lot of potential paths to take a snapshot.  JMX, SyncRequestProcessor, Admin,
and as part of the sync stage in ZABP.  In the past it was just the SyncRequestProcessor and
ZABP that would trigger snapshots.  We didn't have much concern about collisions, because
SyncRequestProcessor would only run as edits were being sent, and the edits would only be
sent after the ZABP sync phase had completed.  Now we have Admin and JMX possibly doing a
snapshot in the background.  
    
    Now if a snapshot request comes in during the ZABP sync phase after we have clear out
the in memory DB and not yet applied the new snapshot and then we crash before we can write
out the new snapshot we could end up with data corruption.  This should be super super rare
so I don't really know if we care all that much about it, but I think it is something that
we can fix.
    
    I am not an expert on the code, so I am not sure of the cleanest way to fix this, but
it feels like having maybeTakeSnapshot be a part of the SyncRequestProcessor instead of ZooKeeperServer.
 I don't know how simple it is to get to the SyncRequestProcessor from JMX/Admin but if you
can then it means that we can reset the edit count so we are not taking a lot of snapshots
all at once, one right after another.  It also means that we can truly avoid having more then
one snapshot be taken at any time. 


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