[FLINK-3174] Add merging WindowAssigner

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.1.0
• Component/s: API / DataStream
• Assignee: Aljoscha Krettek
• Reporter: Aljoscha Krettek
• Created: 15/Dec/15 15:04
• Updated: 28/Feb/19 11:14
• Resolved: 05/Apr/16 15:21

Description

We should add the possibility for WindowAssigners to merge windows. This will enable Session windowing support, similar to how Google Cloud Dataflow supports.

For session windows, each element would initially be assigned to its own window. When triggering we check the windows and see if any can be merged. This way, elements with overlapping session windows merge into one session.