[FLINK-2390] Replace iteration timeout with algorithm for detecting termination

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Won't Fix
• Component/s: API / DataStream
• Assignee: Unassigned
• Reporter: Gyula Fora
• Created: 21/Jul/15 20:23
• Updated: 09/Apr/19 08:55
• Resolved: 09/Apr/19 08:55

Description

Currently the user can set a timeout which will shut down the iteration source/sink nodes if no new data is received during that time to allow program termination in iterative streaming jobs.

This method is used due to the non-trivial nature of termination in iterative streaming jobs. While termination is not a main concern in long running streaming jobs, this behaviour makes iterative tests non-deterministic and they often fail on travis due to the timeout. Also setting a timeout can cause jobs to terminate prematurely.

I propose to remove iteration timeouts and replace it with the following algorithm for detecting termination:

-We first identify loop edges in the jobgraph (the channels from the iteration sources to the head operators)
-Once the head operators (the ones with loop input) finish with all their non-loop inputs they broadcast a marker to their outputs.
-Each operator will broadcast a marker once it received a marker from all its non-finished inputs
-Iteration sources are terminated when they receive 2 consecutive markers without receiving any record in-between

The idea behind the algorithm is to find out when no more outputs are generated from the operators inside an iteration after their normal inputs are finished.

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