[ACCUMULO-1243] Multiple assignment may occur if tablet server dies during split

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 1.4.0
• Fix Version/s: 1.5.0
• Assignee: Keith Turner
• Reporter: Keith Turner
• Created: 05/Apr/13 02:09
• Updated: 23/Apr/13 01:21
• Resolved: 11/Apr/13 19:08

Description

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

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);

Then create a table and add a split.

root@test15> createtable foo
root@test15 foo> addsplits -t foo m

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.

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

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.

Issue Links

• relates to: ACCUMULO-1235 tablet fails to load after split fix