[jira] [Commented] (ZOOKEEPER-2789) Reassign `ZXID` for solving 32bit overflow problem

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16036796#comment-16036796 ] 

ASF GitHub Bot commented on ZOOKEEPER-2789:
-------------------------------------------

Github user maoling commented on the issue:

    https://github.com/apache/zookeeper/pull/262
  
    Hi, @asdf2014 .Thanks for your explanation! But I still have some confusions about the question one:
    look at code like this :
    ```
    int epoch = (int)Long.rotateRight(zxid, 32);// >> 32;
    long count = zxid & 0xffffffffffL;
    ```
    it all depends on that **zxid** can not be altered(no write operation when **zxid** has generated at the first time) in the multithread situation,otherwise epoch and count isn't idempotent.should **zxid** be decorated by **final**?


> Reassign `ZXID` for solving 32bit overflow problem
> --------------------------------------------------
>
>                 Key: ZOOKEEPER-2789
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2789
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: quorum
>    Affects Versions: 3.5.3
>            Reporter: Benedict Jin
>            Assignee: Benedict Jin
>             Fix For: 3.6.0
>
>   Original Estimate: 168h
>  Remaining Estimate: 168h
>
> If it is `1k/s` ops, then as long as $2^32 / (86400 * 1000) \approx 49.7$ days ZXID will exhausted. But, if we reassign the `ZXID` into 16bit for `epoch` and 48bit for `counter`, then the problem will not occur until after  $Math.min(2^16 / 365, 2^48 / (86400 * 1000 * 365)) \approx Math.min(179.6, 8925.5) = 179.6$ years.
> However, i thought the ZXID is `long` type, reading and writing the long type (and `double` type the same) in JVM, is divided into high 32bit and low 32bit part of the operation, and because the `ZXID` variable is not  modified with `volatile` and is not boxed for the corresponding reference type (`Long` / `Double`), so it belongs to [non-atomic operation] (https://docs.oracle.com/javase/specs/jls/se8 /html/jls-17.html#jls-17.7). Thus, if the lower 32 bits of the upper 32 bits are divided into the entire 32 bits of the `long`, there may be a concurrent problem.



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