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Date: Fri, 5 Apr 2013 02:10:15 +0000 (UTC)
From: "Keith Turner (JIRA)" <jira@apache.org>
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Subject: [jira] [Created] (ACCUMULO-1243) Multiple assignment may occur if
 tablet server dies during split
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Keith Turner created ACCUMULO-1243:
--------------------------------------

             Summary: Multiple assignment may occur if tablet server dies during split
                 Key: ACCUMULO-1243
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-1243
             Project: Accumulo
          Issue Type: Bug
    Affects Versions: 1.4.0
            Reporter: Keith Turner
            Priority: Critical
             Fix For: 1.5.0


Make the following change to the tablet server code.  The tablet server has to die at this exact point for the bug to occur.

{noformat}
Index: src/main/java/org/apache/accumulo/server/tabletserver/Tablet.java
===================================================================
--- src/main/java/org/apache/accumulo/server/tabletserver/Tablet.java	(revision 1464780)
+++ src/main/java/org/apache/accumulo/server/tabletserver/Tablet.java	(working copy)
@@ -3592,6 +3592,9 @@
       MetadataTable.splitTablet(high, extent.getPrevEndRow(), splitRatio, SecurityConstants.getSystemCredentials(), tabletServer.getLock());
       MetadataTable.addNewTablet(low, lowDirectory, tabletServer.getTabletSession(), lowDatafileSizes, bulkLoadedFiles,
           SecurityConstants.getSystemCredentials(), time, lastFlushID, lastCompactID, tabletServer.getLock());
+      
+      Runtime.getRuntime().halt(2);
+
       MetadataTable.finishSplit(high, highDatafileSizes, highDatafilesToRemove, SecurityConstants.getSystemCredentials(), tabletServer.getLock());
       
       log.log(TLevel.TABLET_HIST, extent + " split " + low + " " + high);
{noformat}

Then create a table and add a split.  

{noformat}
root@test15> createtable foo
root@test15 foo> addsplits -t foo m
{noformat}

If there are multiple tablet servers, then its possible that multiple assignment may occur.  Below is an example of this occurring after tablets were loaded.

{noformat}
root@test15 !METADATA> scan -b 1 -c loc
1;m loc:13d5a86463f4f98 []    127.0.0.1:9998
1;m loc:13d5a86463f4f9f []    127.0.0.1:10000
1< loc:13d5a86463f4f98 []    127.0.0.1:9998
{noformat}

The problem is that the assignment code in the tserver detects an incomplete split and load both children.   However, the master may also assign one of the children.   

I think the assignment code should be modified to fix up the metadata table and only load one tablet. If the new tablet was not created, it should roll back the changes and load the pre split tablets.  If the new tablet was created, then assume the master will assign it and only load the high tablet.  I think these changes would greatly simplify the code also.

I do not think the proposed changes would cause issues with merge, since the chop flag is deleted in the case where this occurs.

Need to ensure that the solution is itself fault tolerant.


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