[
https://issues.apache.org/jira/browse/BEAM-11216?focusedWorklogId=509744&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-509744
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ASF GitHub Bot logged work on BEAM-11216:
-----------------------------------------
Author: ASF GitHub Bot
Created on: 10/Nov/20 14:54
Start Date: 10/Nov/20 14:54
Worklog Time Spent: 10m
Work Description: scwhittle opened a new pull request #13293:
URL: https://github.com/apache/beam/pull/13293
…ork item retries.
Such retries can occur particularly in autoscaling with Streaming Engine
enabled
when the original commit for the work item is sent to the wrong backend and
is
dropped. Then the retry would have the same cache token but would incorrectly
resume from where the reader last stopped instead of the checkpoint within
the
request.
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Issue Time Tracking
-------------------
Worklog Id: (was: 509744)
Remaining Estimate: 0h
Time Spent: 10m
> StreamingDataflowWorker ReaderCache usage can be incorrect in presence of
> retries
> ---------------------------------------------------------------------------------
>
> Key: BEAM-11216
> URL: https://issues.apache.org/jira/browse/BEAM-11216
> Project: Beam
> Issue Type: Bug
> Components: runner-dataflow
> Reporter: Sam Whittle
> Assignee: Sam Whittle
> Priority: P2
> Time Spent: 10m
> Remaining Estimate: 0h
>
> This is similar to BEAM-7547. The issue and identified there was related to
> the state cache. However for UnboundedSource there is a separate
> ReaderCache. In particular the following sequence could lead to using a
> Reader at the wrong position.
> 1. Work arrives indicating to use reader at state C1
> 2. Work processes and advances reader to C2, commit is prepared with elements
> output from C1 to C2 and reader checkpoint for C2
> 3. Commit of original processing fails (perhaps routed to previous backend
> during autoscaling)
> 4. Work is retried (still indicating to use reader at C1 and still same cache
> token) however the ReaderCache is used, there is a hit, and the existing
> reader (positioned at C2) is used.
> 5. Retry of work processes and advances reader to C3, commit is scheduled
> with elements output from C2 to C3 and reader checkpoint for C3
> 6. Commit succeeds
> At this point there was never a successful commit for the elements between C1
> to C2, though the reader is now advanced past them.
> Possible fixes:
> 1. Use increasing work token as in BEAM-7547 to detect retry and not use the
> ReaderCache entry
> 2. Seek recovered reader from cache to checkpoint or only use if it matches
> the checkpoint. This would probably involve changing the Checkpoint interface
> so 1 is likely preferred.
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