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From Tatu Saloranta <tsalora...@gmail.com>
Subject Re: Auto Increament
Date Thu, 25 Mar 2010 17:37:39 GMT
On Thu, Mar 25, 2010 at 9:20 AM, Benjamin Black <b@b3k.us> wrote:
> Cassandra is not being used to generate the Twitter identifiers.
> Twitter, like most places using Cassandra, has more than one database
> system in production.
>
> UUIDs are not at risk of conflicts with billions of rows.

Exactly: UUIDs were _designed_ not to. Biggest theoretical limit would
be that with time+location based variant, you can  generate "only" 10
million uuids per second due to timer resolution. And that's per
physical address (ethernet/mac), which you can add more of. 128-bit
space for random uuids (an alternative) can be shown to be more than
adequate, although it does obviously assume good random number
generator.

As to question on how sequences are created: no company only uses
Cassandra. Considering that rate of user account creation is rather
low (in grand scheme of things), it can be done using any number of
ways. Simplest: use an SQL database. :-)
(and that'd be my guess as well -- right tool for the job etc)

Nonetheless, it may just be an implementation detail; that is, whether
it's a contiguous monotonically increasing sequence may not be an
important invariant. Just needs to be unique.

-+ Tatu +-

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