Re: How is PG replication typically used to create a High Availability (HA) config ?

Hi David,

> On 11. Aug, 2020, at 17:12, David Gauthier <davegauthierpg@gmail.com> wrote:
>
> Hi:
>
> Our IT dept has created what they call a High Availability DB for our PG DB (9.6.7 on linux).  If the primary fails, they promise to promote the backup to be the new primary but leave it at that.  But from the perspective of the app, I'm left with....
> 1) detecting an SQL error is a DB connectivity problem.
> 2) Attempt to reconnect.  If fails, try connecting to the backup server (assume it's the new primary ?)
> 3) If that works, then test to see if it is indeed a primary (try a write statement) ?
> 4) Somehow remember that the backup server is the one to connect to as the primary for future connections.
> 5) When the original primary server is fixed and brought back on-line, and they promote it to being the new primary, then I guess my write statements will fail because I'm still connecting to the backup.  So go figure that out and reset a pointer of sorts to the new primary ?
>
> Seems like a lot of work for an app and that it should be more seamless.  But maybe I'm wrong.
>
> I was hoping for something like a server alias that the IT team toggles as needed between the servers so that I always know what to connect to.  The best solution would be something completely seamless... my app isn't even aware that the primary went down, the backup was promoted, and that I'm actually running on a different server. Is something like that possible ?
>
> Thanks for any replies !

If you're a developer, you can take a look at:
https://www.postgresql.org/docs/current/libpq.html

If your application uses libpq, then you can have it connect to the primary database, no matter on what host the primary and the replica currently is.

Look at: https://jdbc.postgresql.org/documentation/head/connect.html

You can especially do something like:

jdbc:postgresql://host-a:5432,host-b:5433/postgres?targetServerType=master

You can always download the current driver from:
https://jdbc.postgresql.org

Does that help?

Cheers,
Paul