Re: Support for N synchronous standby servers - take 2

On Sun, Jan 17, 2016 at 11:09 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Wed, Jan 13, 2016 at 1:54 AM, Alvaro Herrera wrote:
> * Confirm value of pg_stat_replication.sync_state (sync, async or potential)
> * Confirm that the data is synchronously replicated to multiple
> standbys in same cases.
>   * case 1 : The standby which is not listed in s_s_name, is down
>   * case 2 : The standby which is listed in s_s_names but potential
> standby, is down
>   * case 3 : The standby which is considered as sync standby, is down.
> * Standby promotion
>
> In order to confirm that the commit isn't done in case #3 forever
> unless new sync standby is up, I think we need the framework that
> cancels executing query.
> That is, what I'm planning is,
> 1. Set up master server (s_s_name = '2, standby1, standby2)
> 2. Set up two standby servers
> 3. Standby1 is down
> 4. Create some contents on master (But transaction is not committed)
> 5. Cancel the #4 query. (Also confirm that the flush location of only
> standby2 makes progress)

This will need some thinking and is not as easy as it sounds. There is
no way to hold on a connection after executing a query in the current
TAP infrastructure. You are just mentioning case 3, but actually cases
1 and 2 are falling into the same need: if there is a failure we want
to be able to not be stuck in the test forever and have a way to
cancel a query execution at will. TAP uses psql -c to execute any sql
queries, but we would need something that is far lower-level, and that
would be basically using the perl driver for Postgres or an equivalent
here.

Honestly for those tests I just thought that we could get to something
reliable by just looking at how each sync replication setup reflects
in pg_stat_replication as the flow is really getting complicated,
giving to the user a clear representation at SQL level of what is
actually occurring in the server depending on the configuration used
being important here.
-- 
Michael