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Date: Fri, 25 Aug 2017 19:44:00 +0000 (UTC)
From: "ASF GitHub Bot (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (ZOOKEEPER-2349) Update documentation for
 snapCount
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archived-at: Fri, 25 Aug 2017 19:44:07 -0000


    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2349?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16142090#comment-16142090 ] 

ASF GitHub Bot commented on ZOOKEEPER-2349:
-------------------------------------------

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

    https://github.com/apache/zookeeper/pull/349#discussion_r135338802
  
    --- Diff: src/docs/src/documentation/content/xdocs/zookeeperAdmin.xml ---
    @@ -823,10 +823,10 @@ server.3=zoo3:2888:3888</programlisting>
                   role="bold">zookeeper.snapCount</emphasis>)</para>
     
                   <para>ZooKeeper logs transactions to a transaction
    -              log. After snapCount transactions are written to a log
    -              file a snapshot is started and a new transaction log
    -              file is created. The default snapCount is
    -              100,000.</para>
    +              log. After the count of transactions which are written to a log
    +              file exceed a runtime-set limit in [snapCount/2, snapCount),a snapshot 
    --- End diff --
    
    I think this is an excellent documentation improvement, but we still may be able to phrase this in a clearer way. It may be good to reiterate that ZooKeeper writes to a transaction log between snapshotting here, so the reader understands the purpose of the snapCount.
    
    Perhaps something like:
    
    > ZooKeeper records its transactions using snapshots and a transaction log (think write-ahead log). The number of transactions recorded in the transaction log before a snapshot can be taken (and the transaction log rolled) is determined by snapCount. In order to prevent all of the machines in the quorum from taking a snapshot at the same time, each ZooKeeper server will take a snapshot when the number of transactions in the transaction log reaches a runtime generated random value in the [snapCount/2, snapCount) range.


> Update documentation for snapCount
> ----------------------------------
>
>                 Key: ZOOKEEPER-2349
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2349
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: documentation
>            Reporter: Raghavendra Prabhu
>            Priority: Minor
>         Attachments: ZOOKEEPER-2349.patch
>
>
> The documentation states that 
> {code}
> ZooKeeper logs transactions to a transaction
> log. After snapCount transactions are written to a log
> file a snapshot is started and a new transaction log
> file is created. The default snapCount is
> 100,000.
> {code}
> However, in implementation, snapshotting is done when logCount is somwhere in (snapCount/2, snapCount+1], based on the limit set at runtime:  
> {code}
>         if (logCount > (snapCount / 2 + randRoll)) {
> {code}
> as in 
> https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/SyncRequestProcessor.java#L124


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