[IMPALA-3019] next_nonlocal_backend_entry_ is reset on empty Statestore messages

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.0
• Fix Version/s: Impala 2.6.0, impala 2.5.1
• Component/s: Backend
• Target Version: Impala 2.6.0, impala 2.5.1
• Assignee: Lars Volker
• Reporter: Lars Volker
• Created: 17/Feb/16 22:45
• Updated: 03/Jun/16 22:26
• Resolved: 13/May/16 22:41

Description

Currently processing updates from the Statestore roughly looks like this in the scheduler:

• if !is_delta: clear current information
• Process all new entries
• Remove all old entries
• Invalidate next_nonlocal_backend_entry_

However the invalidation of the next_nonlocal_backend_entry_ pointer happens unconditionally, whether there where changes to the current backend_map_ or not - including when an empty message was sent by the Statestore. This breaks most of the round-robin functionality in GetBackend(). As a fix, we should only reset next_nonlocal_backend_entry_ if there were changes to the backend map.