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From "Benedict Jin (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-2789) Reassign `ZXID` for solving 32bit overflow problem
Date Tue, 23 May 2017 01:45:04 GMT
Benedict Jin created ZOOKEEPER-2789:
---------------------------------------

             Summary: 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
             Fix For: 3.6.0


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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