[MAPREDUCE-6237] Multiple mappers with DBInputFormat don't work because of reusing conections

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.5.0, 2.6.0
• Fix Version/s: 2.6.1, 3.0.0-alpha1
• Component/s: mrv2
• Labels: 2.6.1-candidate
• Target Version/s: 2.6.1
• Hadoop Flags: Reviewed
• Assignee: Kannan Rajah
• Reporter: Kannan Rajah
• Created: 31/Jan/15 00:11
• Updated: 30/Aug/16 01:19
• Resolved: 09/Feb/15 18:59

Description

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.