Re: Support for N synchronous standby servers - take 2

On Fri, Jan 8, 2016 at 1:53 PM, Kyotaro HORIGUCHI
<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> Hello,
>
> At Mon, 4 Jan 2016 15:29:34 +0900, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote in <CAB7nPqTp5RoHxcp8YxejGMjRjjtLaXCa8=-BEr7ZnBNbPzPdWA(at)mail(dot)gmail(dot)com>
>> > Attached latest v5 patch.
>> > Please review it.
>>
>> Something that I find rather scary with this patch: could it be
>> possible to get actual regression tests now that there is more
>> machinery with PostgresNode.pm? As syncrep code paths get more and
>> more complex, so are debugging and maintenance.
>
> The test on the whole replication system will very likely to be
> too complex and hard to stabilize, and would be
> disproportionately large to other tests.

I don't buy that much. Mind you, there is in this commit fest a patch
introducing a basic regression test suite for recovery using the new
infrastructure that has been committed last month. You may want to
look at it.

> This patch mainly changes the logic to choose the next syncrep
> standbys and calculate the 'synched' LSNs, so performing separate
> module tests for the logics, then perform the test for the
> behavior according to the result of that by, perhaps,
> PostgresNode.pm would remarkably reduce the labor for
> testing.
> Could we have some tapping point for individual testing of the
> logics in appropriate way?

Isn't pg_stat_replication enough for this purpose? What you basically
need to do is set up a master, a set of slaves and then look at the
WAL sender status. Am I getting that wrong?

> In order to do so, the logics should be able to be fed arbitrary
> complete set of parameters, in other words, defining a kind of
> API to use the logics from the core side, even though it is not
> an extension. Then we will *somehow* kick the API with some set
> of parameters in regest.

Well, you will need to craft in the syncrep test suite associated in
this patch a set of routines that allows to set up appropriately
s_s_names and the other parameters that this patch introduces. I does
not sound like a barrier impossible to cross.
--
Michael