Until Hadoop 0.20, the JobClient was injecting the property 'group.name' on the JobConf submitted to the JobTracker.
Since Hadoop 0.21, due to security related changes, this is not done anymore.
This breaks backwards compatibility for jobs/components that expect the 'group.name' to be automatically set at submission time.
An example of a component being affected by this change is the FairScheduler where it is common to use the group.name as pool name. Different from other properties, a special characteristic of the group.name is that its value cannot be tampered by a user.
For security reasons this should not be done (as it was done before) in the JobClient side. Instead, it should be done in the JobTracker when the JobConf is received.