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https://issues.apache.org/jira/browse/CASSANDRA-1039?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Roland Hänel updated CASSANDRA-1039:
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Attachment: cassandra-1039-patch.txt
patch to introduce tie breaker for Column.comparePriority
> Make read repair deterministic to enable unique ID creation function in
> Cassandra
> ---------------------------------------------------------------------------------
>
> Key: CASSANDRA-1039
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1039
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.6.1
> Reporter: Roland Hänel
> Priority: Minor
> Attachments: cassandra-1039-patch.txt
>
>
> When deciding to initiate read repair on a certain column, in the special
> case that there are different column values with the same timestamp (this
> situation can occur if values were 'simultaneously' inserted by several
> clients), the behavior should be deterministic (tie-breaker). This prevents
> ongoing back-and-forward read repair when one node repairs towards value X,
> and another one repairs towards value Y.
> I propose to deploy the following rule: if we are in doubt whether to repair
> a column with timestamp T (because two values X and Y are present within the
> cluster, both at timestamp T), then we always repair towards X if
> some_func(X)<some_func(Y). some_func() could be md5() or just plain
> byte-by-byte comparison (maybe the latter is better because it will never
> result in equality).
> The major use case for this improvement is to enable Cassandra as a means to
> generate cluste-wide unique identifiers. This could be implemented like this:
> - client wants to grab <ID>, and inserts a column with key <ID>, value
> <some_random_stuff>, timestamp <ID>
> - client reads the just inserted column with consistency level ALL or QUORUM
> - if the read value (<some_random_stuff>) matches the value originally
> inserted by the client, the <ID> is unique for this client
> In this algorithm, collisions are prevented by the fact that if to clients
> try to aquire the same <ID> at the same time, the proposed tie-breaking will
> take care that exactly one client wins after the first read repair.
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