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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7453) Geo-replication in Cassandra
Date Fri, 27 Jun 2014 16:05:24 GMT


Jonathan Ellis commented on CASSANDRA-7453:

Replicas must be uniquely determined by the partition key and *only* the partition key.  Everything
breaks if two different machines compute different replica sets.  Using an additional table
might work, but at best you'll have really terrible performance.  So variants on (1) are the
only sane option.

> Geo-replication in Cassandra
> ----------------------------
>                 Key: CASSANDRA-7453
>                 URL:
>             Project: Cassandra
>          Issue Type: Wish
>            Reporter: Sergio Esteves
>            Priority: Minor
> Currently, a Cassandra cluster spanned across different datacenters replicates all data
to all datacenters when an update is performed. This is a problem for the scalability of Cassandra
as the number of datacenters increases.
> It would be desirable to have some way to make Cassandra aware of the location of data
requests so that it could place replicas close to users and avoid replicating to remote datacenters
that are far away.
> To this end, we thought of implementing a new replication strategy and some possible
solutions to achieve our goals are:
> 1) Using a byte from every row key to identify the location of the primary datacenter
where data should be stored (i.e., where it is likely to be accessed).
> 2) Using an additional CF for every row to specify the origin of the data.
> 3) Replicating only to the 2 closest datacenters from the user (for reliability reasons)
upon a write update. For reads, a user would try to fetch data from the 2 closest datacenters;
if data is not available it would try the other remaining datacenters. If data fails to be
retrieved too many times, it means that the client has moved to other part of the planet,
and thus data should be migrated accordingly. We could have some problems here, like having
the same rows, but with different CFs in different DCs (i.e., if users perform updates to
the same rows from different remote places).
> What would be the best way to do this?
> Thanks.

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