Thanks for the information. It was very useful
josh

On Dec 14, 2007 4:18 PM, Christopher Browne <cbbrowne at ca.afilias.info>
wrote:

> "Josh Harrison" <joshques at gmail.com> writes:
> > I have a question regarding slony's switchover and failover option
> > The document says
> > "At the time of this writing switchover to another server requires the
> application to reconnect to the database. So in order to avoid any
> complications, we simply
> > shut down the web server. Users who use pg_pool for the applications
> database connections merely have to shut down the pool."
>
> If I describe this issue differently (and maybe that means changing
> the docs!), maybe it'll make it clearer...
>
> Any time you do switchover from one server to another, say, from "DB1"
> to "DB2", this requires that the applications that were connecting to
> DB1 drop those connections, and connect to DB2, instead.
>
> How you will accomplish this is hugely dependent on how you set your
> environment up.
>
> Options might include:
>
>  - Stop the application, change "DB1" to "DB2" in a config file, and
>   restart the application.
>
>  - Use a DNS CNAME so that your application points to "DBSERVER;" that
>   way, you initially have DNS set up so that DBSERVER is a CNAME
>   pointing at DB1, and you later change it to be a CNAME pointing at
>   DB2.
>
>   You may need for your application to be restarted in order for it
>   to know to reopen DB connections.
>
>  - If you're using pg_pool, or something similar, then the
>   reconfiguration involves reconfiguring pg_pool, rather than your DNS
>   server.
>
>   But for the most part, the behaviour is logically similar to the
>   DNS-based example above.
>
> > 1 So does this mean that during the switchover, the web server
> > should be shut down and users will not be able to access the
> > database at that time?
>
> Ideally, yes.  Cutting off access pretty forcibly tends to be a good
> idea during a switchover; that way, there aren't any lingering locks
> kicking around possibly preventing the switchover from completing
> quickly.
>
> > 2. Is there any time frame for this switchover ie., how long it
> > takes ? what does this depend on?
>
> The reason for it to take > 1s is that there are processes connected
> to either the former master node or the soon-to-be-master node,
> holding locks on tables that the switchover process needs to modify.
>
> > 3. Is there any other way to keep the web server up running during
> > this switch over operation?
>
> If you're using some mechanism like CNAME/pg_pool so that it's always
> pointing at "something current," and the application knows enough to
> re-open connections any time it finds they fail, that may "play OK."
> --
> output =3D ("cbbrowne" "@" "linuxfinances.info")
> http://cbbrowne.com/info/wp.html
> Your latest program has been judged UNTASTEFUL by the T daemon;
> and automatically deleted.
>
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