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From "Ben Roling (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CRUNCH-515) Decrease probability of collision on Crunch temp directories
Date Wed, 29 Apr 2015 17:30:06 GMT
Ben Roling created CRUNCH-515:
---------------------------------

             Summary: 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: 0.11.0, 0.8.4
            Reporter: Ben Roling
            Assignee: Josh Wills


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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