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https://issues.apache.org/jira/browse/MAPREDUCE-6237?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14306716#comment-14306716
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Tsuyoshi OZAWA commented on MAPREDUCE-6237:
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Thank you for clarification. I know you're correct and my suggestion is not
clean fix, but I think this issue is one of critical issues to be fixed for
branch-2 and 2.6.1. To commit this to branch-2 and 2.6.1, incompatible changes
are not accepted since it's public class. My suggestion is:
* Let's fix the bug here with full backward compatibility about public methods
to commit this to branch-2.
* How about fixing the behavior cleanly which targets trunk on separate JIRA?
> DBRecordReader is not thread safe
> ---------------------------------
>
> Key: MAPREDUCE-6237
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6237
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Affects Versions: 2.5.0
> Reporter: Kannan Rajah
> Assignee: Kannan Rajah
> Attachments: mapreduce-6237.patch, mapreduce-6237.patch
>
>
> DBInputFormat.createDBRecorder is reusing JDBC connections across instances
> of DBRecordReader. This is not a good idea. We should be creating separate
> connection. If performance is a concern, then we should be using connection
> pooling instead.
> I looked at DBOutputFormat.getRecordReader. It actually creates a new
> Connection object for each DBRecordReader. So can we just change
> DBInputFormat to create new Connection every time? The connection reuse code
> was added as part of connection leak bug in MAPREDUCE-1443. Any reason for
> caching the connection?
> We observed this issue in a customer setup where they were reading data from
> MySQL using Pig. As per customer, the query is returning two records which
> causes Pig to create two instances of DBRecordReader. These two instances are
> sharing the database connection instance. The first DBRecordReader runs to
> extract the first record from MySQL just fine, but then closes the shared
> connection instance. When the second DBRecordReader runs, it tries to execute
> a query to retrieve the second record on the closed shared connection
> instance, which fail. If we set
> mapred.map.tasks to 1, the query will be successful.
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