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From "Tellier Benoit (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MAILBOX-209) A data race is possible with Cassandra modseq generation
Date Wed, 21 Jan 2015 09:05:34 GMT

     [ https://issues.apache.org/jira/browse/MAILBOX-209?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Tellier Benoit updated MAILBOX-209:
-----------------------------------
    Attachment: fix_cassandra_modseq_provider.patch

In this patch, we first get the current modseq value. We increase it only if the value did
not change. If it is the case, we can then return the incremented value. If it is not the
case, we just retry.

Note a tunnable max retry parameter ( to avoid being stuck )

To validate all cassandra unit tests ( mailbox mappers espacially ), it has to behave well
even if the mailbox does not exist. Hence the separated table.

> A data race is possible with Cassandra modseq generation
> --------------------------------------------------------
>
>                 Key: MAILBOX-209
>                 URL: https://issues.apache.org/jira/browse/MAILBOX-209
>             Project: James Mailbox
>          Issue Type: Bug
>            Reporter: Tellier Benoit
>         Attachments: fix_cassandra_modseq_provider.patch
>
>
> The Cassandra mailbox implementation proceed as follow to increase the modseq :
>  - a first call to Cassandra to increase the modseq value for a given mailbox
>  - a second call allow us to retrieve the current value.
> Note that you can not do it in a single call...
> In a distributed environment, imagine that :
>  - for a mailbox the current modseq is n
>  - server A increments modseq. Modseq is n+1
>  - server B increments modseq. Modseq is n+2
>  - server A and B then retrieve the modseq value and have a duplicated modseq : they
for instance both can assign n+2 value to a different message.
> The problem can be solved using conditionnal updates with lightweight transactions.



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