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From Mohit Anchlia <mohitanch...@gmail.com>
Subject Re: How to reliably achieve unique constraints with Cassandra?
Date Fri, 06 Jan 2012 18:38:17 GMT
On Fri, Jan 6, 2012 at 10:03 AM, Drew Kutcharian <drew@venarc.com> 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.

It could be as simple as reading before writing to make sure that
email doesn't exist. But I think you are looking at how to handle 2
concurrent requests for same email? Only way I can think of is:

1) Create new CF say tracker
2) write email and time uuid to CF tracker
3) read from CF tracker
4) if you find a row other than yours then wait and read again from
tracker after few ms
5) read from USER CF
6) write if no rows in USER CF
7) delete from tracker

Please note you might have to modify this logic a little bit, but this
should give you some ideas of how to approach this problem without
locking.

Regarding hijacking accounts, can you elaborate little more?
>
> 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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