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From Jeremiah Jordan <>
Subject Re: How to reliably achieve unique constraints with Cassandra?
Date Fri, 06 Jan 2012 21:02:22 GMT
Since a Zookeeper cluster is a quorum based system similar to Cassandra, 
it only goes down when n/2 nodes go down.  And the same way you have to 
stop writing to Cassandra if N/2 nodes are down (if using QUoRUM), your 
App will have to wait for the Zookeeper cluster to come online again 
before it can proceed.

On 01/06/2012 12:03 PM, Drew Kutcharian wrote:
> Hi Everyone,
> What's the best way to reliably have unique constraints like functionality with Cassandra?
I have the following (which I think should be very common) use case.
> User CF
> Row Key: user email
> Columns: userId: UUID, etc...
> UserAttribute1 CF:
> Row Key: userId (which is the uuid that's mapped to user email)
> Columns: ...
> UserAttribute2 CF:
> Row Key: userId (which is the uuid that's mapped to user email)
> Columns: ...
> The issue is we need to guarantee that no two people register with the same email address.
In addition, without locking, potentially a malicious user can "hijack" another user's account
by registering using the user's email address.
> I know that this can be done using a lock manager such as ZooKeeper or HazelCast, but
the issue with using either of them is that if ZooKeeper or HazelCast is down, then you can't
be sure about the reliability of the lock. So this potentially, in the very rare instance
where the lock manager is down and two users are registering with the same email, can cause
major issues.
> In addition, I know this can be done with other tools such as Redis (use Redis for this
use case, and Cassandra for everything else), but I'm interested in hearing if anyone has
solved this issue using Cassandra only.
> Thanks,
> Drew

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