Chris Travers wrote:
> Hi all;
>
> I have been looking into how to ensure that synchronous replication, etc.
> could best be implimented. To date, I see only two options: incorporate
> the replication code into the database backend or have a separate "proxy"
> which handles the replication.
There are many problems with a "proxy" solution. One is that you really
don't know if a statement does modify the database or not. A SELECT for
example can call a user defined function somewhere and that can do
whatever the programmer likes it to do. So you would have to "replicate"
all that too. Granted, you can exclude this type of database usage from
your supported list.
Next you don't have control over sequence allocation. Every application
that uses sequence allocated ID's is in danger, because they are not
blocking, you cannot force the order of assignments and they don't roll
back either.
And you get into deadlock problems if you don't guarantee that your
proxy uses the same order to access all databases. And you cannot
guarantee that if your proxy tries to do it parallel. So it has to do
the queries against all databases one by one, that doesn't scale well.
The last thing (I mention for now) is that I cannot imagine any way that
such proxy code allows for a new member to join without stopping the
whole application, creating an identical copy of one member
(dump+restore) and continue. So it is impossible to build 24*7 support
that way.
No, separate proxy code doesn't strike me as the superior solution.
Jan
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