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https://issues.apache.org/jira/browse/CASSANDRA-1684?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13005536#comment-13005536
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Dave Revell commented on CASSANDRA-1684:
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As jbellis says in the description, atomic batches and entity group locality
are cool and useful. But we should be clear that Cassandra's entity groups
would be a different beast than Megastore's entity groups, and wouldn't have
the same consistency properties unless some un-Cassandra-like changes were made.
In Megastore, transactions can maintain arbitrary consistency constraints among
items in an entity group, since there is a Paxos-agreed total order of
transactions. Cassandra has so far avoided fancy distributed agreement like
this. For example, imagine running (in Cassandra) two different transactions on
two different replicas and imagine what mishmash of the two outcomes you'd get
once timestamp-based conflict resolution happened. In Megastore one of the
transactions would abort. Are we willing to add Paxos?
G-Store's ownership transfer protocol also seems very anti-Cassandra-philosophy
with its concept of single-replica item ownership.
I'd be happy to be corrected on any of this. I think Megastore-like entity
groups are an exciting idea but perhaps make more sense on top of HBase :)
> Entity groups
> -------------
>
> Key: CASSANDRA-1684
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1684
> Project: Cassandra
> Issue Type: New Feature
> Components: Core
> Reporter: Jonathan Ellis
> Assignee: Sylvain Lebresne
> Fix For: 0.8
>
> Original Estimate: 80h
> Remaining Estimate: 80h
>
> Supporting entity groups similar to App Engine's (that is, allow rows to be
> part of a parent "entity group," whose key is used for routing instead of the
> row itself) allows several improvements:
> - batches within an EG can be atomic across multiple rows
> - order-by-value queries within an EG only have to touch a single replica
> even with RandomPartitioner
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