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From "Prakash Khemani (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-3889) NPE in Distributed Log Splitting
Date Mon, 16 May 2011 17:41:47 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3889?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13034139#comment-13034139
] 

Prakash Khemani commented on HBASE-3889:
----------------------------------------

Thanks Lars for finding and providing a fix for this issue. I have a few minor comments on
the patch ...

The following isn't really needed, the earlier check you put in should be good enough.
{code}
+        if (wap == null) {
+          continue;
+        }
{code}


It might be better to catch Throwable in SplitLogWorker.run() and print the Unexpected Error
message there. It might not be a good thing to ignore an unexpected exception in SplitLogWorker.grabTask()
and continue.
{nofrmat}
+++ src/main/java/org/apache/hadoop/hbase/regionserver/SplitLogWorker.java	(working copy)
@@ -297,6 +297,8 @@
           }
           break;
       }
+    } catch (Exception e) {
+      LOG.error("An error occurred.", e);
     } finally {
       if (t > 0) {
         LOG.info("worker " + serverName + " done with task " + path +
{noformat}





> NPE in Distributed Log Splitting
> --------------------------------
>
>                 Key: HBASE-3889
>                 URL: https://issues.apache.org/jira/browse/HBASE-3889
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>    Affects Versions: 0.92.0
>         Environment: Pseudo-distributed on MacOS
>            Reporter: Lars George
>            Assignee: Lars George
>             Fix For: 0.92.0
>
>         Attachments: HBASE-3889.patch
>
>
> There is an issue with the log splitting under the specific condition of edits belonging
to a non existing region (which went away after a split for example). The HLogSplitter fails
to check the condition, which is handled on a lower level, logging manifests it as 
> {noformat}
> 2011-05-16 13:56:10,300 INFO org.apache.hadoop.hbase.regionserver.wal.HLogSplitter: This
region's directory doesn't exist: hdfs://localhost:8020/hbase/usertable/30c4d0a47703214845d0676d0c7b36f0.
It is very likely that it was already split so it's safe to discard those edits.
> {noformat}
> The code returns a null reference which is not check in HLogSplitter.splitLogFileToTemp():
> {code}
> ...
>         WriterAndPath wap = (WriterAndPath)o;
>         if (wap == null) {
>           wap = createWAP(region, entry, rootDir, tmpname, fs, conf);
>           if (wap == null) {
>             logWriters.put(region, BAD_WRITER);
>           } else {
>             logWriters.put(region, wap);
>           }
>         }
>         wap.w.append(entry);
> ...
> {code}
> The createWAP does return "null" when the above message is logged based on the obsolete
region reference in the edit.
> What made this difficult to detect is that the error (and others) are silently ignored
in SplitLogWorker.grabTask(). I added a catch and error logging to see the NPE that was caused
by the above.
> {code}
> ...
>           break;
>       }
>     } catch (Exception e) {
>       LOG.error("An error occurred.", e);
>     } finally {
>       if (t > 0) {
> ...
> {code}
> As a side note, there are other errors/asserts triggered that this try/finally not handles.
For example
> {noformat}
> 2011-05-16 13:58:30,647 WARN org.apache.hadoop.hbase.regionserver.SplitLogWorker: BADVERSION
failed to assert ownership for /hbase/splitlog/hdfs%3A%2F%2Flocalhost%2Fhbase%2F.logs%2F10.0.0.65%2C60020%2C1305406356765%2F10.0.0.65%252C60020%252C1305406356765.1305409968389
> org.apache.zookeeper.KeeperException$BadVersionException: KeeperErrorCode = BadVersion
for /hbase/splitlog/hdfs%3A%2F%2Flocalhost%2Fhbase%2F.logs%2F10.0.0.65%2C60020%2C1305406356765%2F10.0.0.65%252C60020%252C1305406356765.1305409968389
>         at org.apache.zookeeper.KeeperException.create(KeeperException.java:106)
>         at org.apache.zookeeper.KeeperException.create(KeeperException.java:42)
>         at org.apache.zookeeper.ZooKeeper.setData(ZooKeeper.java:1038)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker.ownTask(SplitLogWorker.java:329)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker.access$100(SplitLogWorker.java:68)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker$2.progress(SplitLogWorker.java:265)
>         at org.apache.hadoop.hbase.regionserver.wal.HLogSplitter.splitLogFileToTemp(HLogSplitter.java:432)
>         at org.apache.hadoop.hbase.regionserver.wal.HLogSplitter.splitLogFileToTemp(HLogSplitter.java:354)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker$1.exec(SplitLogWorker.java:113)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker.grabTask(SplitLogWorker.java:260)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker.taskLoop(SplitLogWorker.java:191)
>         at org.apache.hadoop.hbase.regionserver.SplitLogWorker.run(SplitLogWorker.java:164)
>         at java.lang.Thread.run(Thread.java:680)
> {noformat}
> This should probably be handled - or at least documented - in another issue?
> The NPE made the log split end and the SplitLogManager add an endless amount of RESCAN
entries as this never came to an end.

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