Re: Support for N synchronous standby servers - take 2

On Fri, Jul 10, 2015 at 10:06 PM, Beena Emerson <memissemerson@gmail.com> wrote:
> Hello,
>
> Tue, Jul 7, 2015 at 02:56 AM, Josh Berkus wrote:
>> pro-JSON:
>>
>> * standard syntax which is recognizable to sysadmins and devops.
>> * can use JSON/JSONB functions with ALTER SYSTEM SET to easily make
>> additions/deletions from the synch rep config.
>> * can add group labels (see below)
>
> Adding group labels do have a lot of values but as Amit has pointed out,
> with little modification, they can be included in GUC as well. It will not
> make it any more complex.
>
> 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. Right?
> I am sorry this question is very basic.
>
> The functions could be something like:
> 1. pg_add_synch_set(set_name NAME, quorum INT, is_priority bool, set_members
> VARIADIC)
>
> This will be used to add a sync set. The set_members can be individual
> elements of another set name. The parameter is_priority is used to decide
> whether the set is priority (true) set or quorum (false). This function call
> will  create a folder pg_syncinfo/groups/$NAME and store the json blob?
>
> The root group would be automatically sset by finding the group which is not
> included in other groups? or can be set by another function?
>
> 2. pg_modify_sync_set(set_name NAME, quorum INT, is_priority bool,
> set_members VARIADIC)
>
> This will update the pg_syncinfo/groups/$NAME to store the new values.
>
> 3. pg_drop_synch_set(set_name NAME)
>
> This will update the pg_syncinfo/groups/$NAME folder. Also all the groups
> which included this would be updated?
>
> 4. pg_show_synch_set()
>
> this will display the current sync setting in json format.
>
> Am I missing something?
>
> Is JSON being preferred because it would be ALTER SYSTEM friendly and in a
> format already known to users?
>
> In a real-life scenario, at most how many groups and nesting would be
> expected?
>

I might missing something but, these functions will generate WAL?
If they does, we will face the situation where we need to wait
forever, Fujii-san pointed out.


Regards,

--
Masahiko Sawada