Status

Current state: [ UNDER DISCUSSION ACCEPTED ]

Discussion thread: <link to mailing list DISCUSS thread>

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These fixes only get the pipeline running again but the inflight intermediate stream data and any samza managed state that was reset is consequently lost. Aside from the data loss, this process of a pipeline upgrade is evidently cumbersome. Therefore, it is important to support the ability to drain and stop a pipeline smoothly prior to making an upgrade which involves an incompatible intermediate schema change. 

The work here is scoped to drain only intermediate stream (shuffle) data and system-managed state. The onus of clearing user state like LocalTable, if required, is on the user. System managed state comprises of window aggregation state and timers. The proposed approach will prematurely emit window panes and fire all timers , thereby, draining all state. Continuation of state post drain is beyond the scope of this work.

Proposed Changes

We propose a drain operation for Samza pipelines which halts the pipeline from consuming new data, completes processing of buffered data and then stops the pipeline.

Below is the high level approach with finer details covered in the sections below: 

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