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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (GEARPUMP-32) Minimum clock of source Tasks maybe inaccurate
Date Mon, 22 Aug 2016 03:05:21 GMT

    [ https://issues.apache.org/jira/browse/GEARPUMP-32?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15429986#comment-15429986
] 

ASF GitHub Bot commented on GEARPUMP-32:
----------------------------------------

Github user huafengw commented on a diff in the pull request:

    https://github.com/apache/incubator-gearpump/pull/67#discussion_r75613138
  
    --- Diff: examples/streaming/state/src/main/scala/org/apache/gearpump/streaming/examples/state/processor/NumberGeneratorProcessor.scala
---
    @@ -31,14 +32,14 @@ class NumberGeneratorProcessor(taskContext: TaskContext, conf: UserConfig)
       private var num = 0L
       override def onStart(startTime: Instant): Unit = {
         num = startTime.toEpochMilli
    -    self ! Message("start")
    +    self ! Watermark(Instant.ofEpochMilli(num))
    --- End diff --
    
    self ! Watermark(startTime)


> Minimum clock of source Tasks maybe inaccurate
> ----------------------------------------------
>
>                 Key: GEARPUMP-32
>                 URL: https://issues.apache.org/jira/browse/GEARPUMP-32
>             Project: Apache Gearpump
>          Issue Type: Bug
>          Components: streaming
>    Affects Versions: 0.8.0
>            Reporter: Manu Zhang
>            Assignee: Manu Zhang
>             Fix For: 0.8.1
>
>
> Moved from [https://github.com/gearpump/gearpump/issues/1835] and reported by [Zhu Yueqian|https://github.com/yueqianzhu]
> {quote}
> Source tasks have not any upstreamClocks. So, startClock is the minimum of pending clocks
when recover happen.
> eg below:
> source task1: timeStamp:15,not ACK, minClockValue maybe is 15(<= 15).
> source task2: timeStamp:10,ACKed, minClockValue maybe is Long.MaxValue
> when recover happen,startClock maybe is 15. where is the data between 10 to 15 at task2?
> {quote}
> More context on this issue:
> In Gearpump, we maintain a global minimum clock tracked from a message's timestamp across
all tasks. It means messages with timestamp before this clock have all been processed. An
application will restart from this value on failure, and thus at-least-once message delivery
could be guaranteed. 
> The global minimum clock is the lower bound of all the Tasks' minimum clocks. 
> For a task, the minimum clock is the lower of 
> # upstream minimum clock
> # a. the minimum timestamp of unacked messages
>    b. Long.MaxValue if all messages have been acked.
>  
> Note that 2.b allows the global minimum clock to progress and it is almost safe since
the clock is also bounded by the upstream minimum clock. I said "almost safe" because a source
task has no upstream but we assume the upstream minimum clock is Long.MaxValue. Thus, the
scenario described by Zhu Yueqian could happen and breaks at-least-once guarantee. 



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