Re: Re: 24 x 7 x 365

There is a problem with this approach
that may only become apparent at high
concurrency.

Since you are operating with two-phase
commits, you may come up against the case where "writers block readers".

Your client issues a commit to both servers. Each server get the PREPARE message,
and when both have responded, each gets
the COMMIT message.

Between the PREPARE and COMMIT,
any blocks updated in the transaction
cease to be available to ANY query
that started after the PREPARE arrived.

For the (hopefully) brief interval between the prepare and commit, neither database knows whether the transaction as a whole has prepared or committed, so any process that wants to see the current version of the data has to wait until there is a known current version.

In a high-concurrency system, a problem
that used to be "buffer busy waits" on updates only can turn into enqueue waits on updates and queries.

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Saturday, December 13, 2003 8:19 PM

Yep, I also think so. I'm currently developing a small prototype for this kind of transparent proxy, which I'll post here when it's stable...

Tanel.

> Tanel,
>
> I think this is a good solution, provided the application can handle
> two phased commit protocol across both the databases, else there
> could be orphan records on one or both these databases.
>

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