Re: Support for N synchronous standby servers - take 2 - Mailing list pgsql-hackers

On 4 February 2016 at 14:34, Fujii Masao <masao.fujii@gmail.com> wrote:
> On Wed, Feb 3, 2016 at 11:00 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Tue, Feb 2, 2016 at 8:48 PM, Fujii Masao <masao.fujii@gmail.com> wrote:
>>> So you disagree with only third version that I proposed, i.e.,
>>> adding some hooks for sync replication? If yes and you're OK
>>> with the first and second versions, ISTM that we almost reached
>>> consensus on the direction of multiple sync replication feature.
>>> The first version can cover "one local and one remote sync standbys" case,
>>> and the second can cover "one local and at least one from several remote
>>> standbys" case. I'm thinking to focus on the first version now,
>>> and then we can work on the second to support the quorum commit
>>
>> Well, I think the only hard part of the third problem is deciding on
>> what syntax to use.  It seems like a waste of time to me to go to a
>> bunch of trouble to implement #1 and #2 using one syntax and then have
>> to invent a whole new syntax for #3.  Seriously, this isn't that hard:
>> it's not a technical problem.  It's just that we've got a bunch of
>> people who can't agree on what syntax to use.  IMO, you should just
>> pick something.  You're presumably the committer for this patch, and I
>> think you should just decide which of the 47,123 things proposed so
>> far is best and insist on that.  I trust that you will make a good
>> decision even if it's different than the decision that I would have
>> made.
>
> If we use one syntax for every cases, possible approaches that we can choose
> are mini-language, json, etc. Since my previous proposal covers only very
> simple cases, extra syntax needs to be supported for more complicated cases.
> My plan was to add the hooks so that the developers can choose their own
> syntax. But which might confuse users.
>
> Now I'm thinking that mini-language is better choice. A json has some good
> points, but its big problem is that the setting value is likely to be very long.
> For example, when the master needs to wait for one local standby and
> at least one from three remote standbys in London data center, the setting
> value (synchronous_standby_names) would be
>
>   s_s_names = '{"priority":2, "nodes":["local1", {"quorum":1,
> "nodes":["london1", "london2", "london3"]}]}'
>
> OTOH, the value with mini-language is simple and not so long as follows.
>
>   s_s_names = '2[local1, 1(london1, london2, london3)]'
>
> This is why I'm now thinking that mini-language is better. But it's not easy
> to completely implement mini-language. There seems to be many problems
> that we need to resolve. For example, please imagine the case where
> the master needs to wait for at least one from two standbys "tokyo1", "tokyo2"
> in Tokyo data center. If Tokyo data center fails, the master needs to
> wait for at least one from two standbys "london1", "london2" in London
> data center, instead. This case can be configured as follows in mini-language.
>
>   s_s_names = '1[1(tokyo1, tokyo2), 1(london1, london2)]'
>
> One problem here is; what pg_stat_replication.sync_state value should be
> shown for each standbys? Which standby should be marked as sync? potential?
> any other value like quorum? The current design of pg_stat_replication
> doesn't fit complicated sync replication cases, so maybe we need to separate
> it into several views. It's almost impossible to complete those problems.
>
> My current plan for 9.6 is to support the minimal subset of mini-language;
> simple syntax of "<number>[name, ...]". "<number>" specifies the number of
> sync standbys that the master needs to wait for. "[name, ...]" specifies
> the priorities of the listed standbys. This first version supports neither
> quorum commit nor nested sync replication configuration like
> "<number>[name, <number>[name, ...]]". It just supports very simple
> "1-level" configuration.

Whatever the solution, I'm really don't like the idea of changing the
definition of s_s_names based on the value of another GUC, mainly
because it seems hacky, but also because the name of the GUC stops
making sense.

Thom