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https://issues.apache.org/jira/browse/CRUNCH-515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14963256#comment-14963256
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Gabriel Reid commented on CRUNCH-515:
-------------------------------------
Implementing finalize() sounds interesting to do cleanup sounds interesting,
especially if it's accompanied with an error log entry to warn the
user/developer that they forgot to call cleanup. I realize that in general
nobody will see that error message, but if just one person heeds its warning
then it's already worth it as far as I'm concerned.
It sounds pretty clear that neither shutdown hooks nor finalize will completely
solve the issue, so I think that up to a certain extent it is indeed up to the
user to get this right.
I'm fine with either finalize or shutdown hooks as an attempt to help out the
user, but I do think that we should at least try to do something that makes the
issue visible to the user/developer.
> Decrease probability of collision on Crunch temp directories
> ------------------------------------------------------------
>
> Key: CRUNCH-515
> URL: https://issues.apache.org/jira/browse/CRUNCH-515
> Project: Crunch
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.8.4, 0.11.0
> Reporter: Ben Roling
> Assignee: Josh Wills
> Attachments: CRUNCH-515-1.patch
>
>
> I've heard reports of failures of Crunch pipelines at our organization due to
> collision on temp directories.
> Take the following stack trace from an old internal email thread I dug up as
> an example:
> {noformat}
> 2015-04-02 04:45:49,208 INFO
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob:
> org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory
> /tmp/crunch-686245394/p2/output already exists
> at
> org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:132)
> at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:1013)
> at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:974)
> at java.security.AccessController.doPrivileged(Native Method)
> at javax.security.auth.Subject.doAs(Subject.java:394)
> at
> org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1438)
> at
> org.apache.hadoop.mapred.JobClient.submitJobInternal(JobClient.java:974)
> at org.apache.hadoop.mapreduce.Job.submit(Job.java:582)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchControlledJob.submit(CrunchControlledJob.java:340)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.startReadyJobs(CrunchJobControl.java:277)
> at
> org.apache.crunch.hadoop.mapreduce.lib.jobcontrol.CrunchJobControl.pollJobStatusAndStartNewOnes(CrunchJobControl.java:316)
> at
> org.apache.crunch.impl.mr.exec.MRExecutor.monitorLoop(MRExecutor.java:113)
> at
> org.apache.crunch.impl.mr.exec.MRExecutor.access$000(MRExecutor.java:55)
> at org.apache.crunch.impl.mr.exec.MRExecutor$1.run(MRExecutor.java:84)
> at java.lang.Thread.run(Thread.java:682)
> {noformat}
> What we found in this case is the pre-existing directory was rather old. It
> hung around because we're doing a poor job of cleaning old garbage out of our
> HDFS /tmp directory. We intend to set up a job to delete stuff older than a
> couple of weeks or so out of /tmp but I think the chances of a collision will
> still be high enough that failures like this might still happen on occasion.
> The temp directory Crunch chooses is a random 31-bit value:
> https://github.com/apache/crunch/blob/apache-crunch-0.11.0/crunch-core/src/main/java/org/apache/crunch/impl/dist/DistributedPipeline.java#L326
> I say 31 bit value because it comes from a 32-bit random integer but only
> includes positive values, thereby excluding 1 bit.
> The following blog post shows some probabilities for 32-bit hash collisions,
> which are essentially the same problem:
> http://preshing.com/20110504/hash-collision-probabilities/
> Since we're dealing with 31 bits instead of 32 the probabilities will be
> higher than expressed there for 32 bits. Even with 32 bits the probability
> of collision is 1 in 100 with just 9292 values.
> I have not done any thorough investigation to understand why, but in our
> production environment we have a lot of Crunch jobs and we are leaving
> 200-300 stray Crunch temp directories per day. Depending on how aggressive
> we get with a scheduled job to clean old stuff out of temp we could still
> have a realistic chance of hitting a collision.
> My proposal is to change the random integer component of the temp path to a
> UUID or something similar to make it drastically more unlikely that a
> collision will ever occur regardless of whether or not "/tmp" is ever cleaned
> up.
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