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Hive >> mail # user >> Files does not exist error: concurrency control on hive queries...


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RE: Files does not exist error: concurrency control on hive queries...
Another option is to deal with this using versioning. Some ideas on this are at

https://issues.apache.org/jira/browse/HIVE-718

Ashish
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From: Eva Tse [[EMAIL PROTECTED]]
Sent: Wednesday, September 09, 2009 10:45 PM
To: [EMAIL PROTECTED]
Subject: Re: Files does not exist error: concurrency control on hive queries...

Zookeeper sounds like a decent alternative, though it would add a new dependency for deployment.
Maybe we could open a jira for it first to track this issue?
Thanks,
Eva.
On 9/9/09 2:49 PM, "Prasad Chakka" <[EMAIL PROTECTED]> wrote:

Yeah, metastore db is logical place to do locking but there have to be periodic cleanups (when clients die without releasing locks) etc which is hacky so less preferrable. Another option is to point a ZooKeeper cluster to Hive and ask Hive to use it for locking. So those who are not concerned about concurrency control, don’t have to install ZooKeeper but other can. ZooKeeper provides leases so there won’t be any problem of hanging locks and it will be easier for admins to clean it up.

I suppose it depends on whoever wants to take this task up :)

Prasad
________________________________
From: Eva Tse <[EMAIL PROTECTED]>
Reply-To: <[EMAIL PROTECTED]>
Date: Wed, 9 Sep 2009 14:32:20 -0700
To: <[EMAIL PROTECTED]>
Subject: Re: Files does not exist error: concurrency control on hive queries...
Regardless of whether the user uses a HiveServer, looks like the logical place to do locking or concurrency control would be at the metastore DB. This is actually one big advantage of Hive. The r/w lock or access control can be achieved by a DB row with lock count for each partition, etc. This might be over-simplfying it, but the metastore DB seems to be the ideal candidate.  Thoughts?
On 9/9/09 12:52 PM, "Prasad Chakka" <[EMAIL PROTECTED]> wrote:

I thought your script runs the two job sequentially. If these two queries are run in parallel then the error can be expected since Hive doesn’t try to acquire locks before reading or writing. I don’t think there are any plans to support this kind of locking (this can only be done if all queries go through HiveServer otherwise lot of orphaned locks will bring the system to halt). I think you should do some kind of locking (possibly with HDFS files) to prevent queries being executed simultaneously.

Any other ideas?

Prasad
________________________________
From: Eva Tse <[EMAIL PROTECTED]>
Reply-To: <[EMAIL PROTECTED]>
Date: Wed, 9 Sep 2009 12:36:11 -0700
To: <[EMAIL PROTECTED]>, Dhruba Borthakur <[EMAIL PROTECTED]>
Subject: Re: Files does not exist error: concurrency control on hive queries...

Hi Prasad,

Are you implying the expected behavior for these queries should be run sequentially by hive because one is r/w and one is read-only ?

For clarifications, these two queries are running concurrently in two separate jobs as below.

Query 1 is run within a job that does the following essentially:
For every hour:
   - parse log files to generate completed sessions information
   - load completed sessions into 48 partitions (for the prior 48 hours)
   - merge small files using ‘insert overwrite ... select from’ on every other 8 partitions. Essentially, we would issue 6 separate queries to merge 6 partitions at the same time, not sequentially. (We do this to minimize time required.) And this is query 1.

Query 2 is run within another job that does select on 24 partitions (aka daily summary) for the previous day. This job just run this query in a loop for testing purposes.

The error comes from query 2 saying ‘file not found’ for a file that we are merging in query 1 at that point in time.

We need to rerun the test to be able to catch the failure at that time to see if the file was there at that instance. In the previous run, the merge query succeeded, so I would imagine the file not there after the merge. And, am not sure if that file was still there at that instance when the failure happens.

Thanks for the help!
Eva.

On 9/9/09 10:29 AM, "Prasad Chakka" <[EMAIL PROTECTED]> wrote:

The first query will not return unless it copied the files to the dest directory and this operation is atomic (FileSystem.rename() guarantees that). Since second query is not executed until the first query returns, this problem may be due to a bug in HDFS (highly unlikely) or an issue with HDFS configuration or related to EC3.

The second query knows the file name ‘sessionsFacts_P20090909T021823L20090908T09-r-00006’ so Hive client does was able to successfully call getFileStatus() on it but the mapper (of second query) is not able to do the same thing. So either this file has been deleted after the Hive client accessed it but before the mapper access it or the machine on which the mapper is being executed can’t see this file. Can you manually check whether the file exists at all after the job fails?

Prasad
________________________________
From: Eva Tse <[EMAIL PROTECTED]>
Reply-To: <[EMAIL PROTECTED]>
Date: Wed, 9 Sep 2009 10:19:24 -0700
To: <[EMAIL PROTECTED]>
Subject: Re: Files does not exist error: concurrency control on hive queries...
Prasad,
We believe the problem is that one of the query is doing an ‘insert overwrite ... select from’ which actually is deleting and merging the small files. The other query somehow couldn’t find those files that it thought it has seen before and failed. So, it looks like a concurrency issue.

Yongqiang,
Could you elaborate a bit on why you say this is not a bug?

Thanks,
Eva.
On 9/9/09 9:55 AM, "Prasad Chakka" <[EMAIL PROTECTED]> wrote:

If a certain input file/dir does not exist then the job can’t be submitted. Since only a few reducers are failing, the problem could be something else.
Eva, Does the same job succeed on a second try? Ie. Is the file/dir available eventually? What is the replication factor?

Prasad
________________________________
From: Yongqiang He <hey
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB