Re: Support for N synchronous standby servers - take 2

On Mon, Jul 13, 2015 at 10:34 PM, Fujii Masao wrote:
> On Fri, Jul 10, 2015 at 10:06 PM, Beena Emerson wrote:
>> On Tue, Jul 7, 2015 at 2:19 PM, Michael Paquier wrote:
>>
>>> Something like pg_syncinfo/ coupled with a LW lock, we already do
>>> something similar for replication slots with pg_replslot/.
>>
>> I was trying to figure out how the JSON metadata can be used.
>> It would have to be set using a given set of functions.
>
> So we can use only such a set of functions to configure synch rep?
> I don't like that idea. Because it prevents us from configuring that
> while the server is not running.

If you store a json blob in a set of files of PGDATA you could update
them manually there as well. That's perhaps re-inventing the wheel
with what is available with GUCs though.

>> Is JSON being preferred because it would be ALTER SYSTEM friendly and in a
>> format already known to users?
>
> At least currently ALTER SYSTEM cannot accept the JSON data
> (e.g., the return value of JSON function like json_build_object())
> as the setting value. So I'm not sure how friendly ALTER SYSTEM
> and JSON format really. If you want to argue that, probably you
> need to improve ALTER SYSTEM so that JSON can be specified.
>
>> In a real-life scenario, at most how many groups and nesting would be
>> expected?
>
> I don't think that many groups and nestings are common.

Yeah, in most common configurations people are not going to have more
than 3 groups with only one level of nodes.
--
Michael