Re: Support for N synchronous standby servers - take 2

On Fri, Jun 26, 2015 at 1:46 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Thu, Jun 25, 2015 at 8:32 PM, Simon Riggs wrote:
>> Let's start with a complex, fully described use case then work out how to
>> specify what we want.
>
> Well, one of the most simple cases where quorum commit and this
> feature would be useful for is that, with 2 data centers:
> - on center 1, master A and standby B
> - on center 2, standby C and standby D
> With the current synchronous_standby_names, what we can do now is
> ensuring that one node has acknowledged the commit of master. For
> example synchronous_standby_names = 'B,C,D'. But you know that :)
> What this feature would allow use to do is for example being able to
> ensure that a node on the data center 2 has acknowledged the commit of
> master, meaning that even if data center 1 completely lost for a
> reason or another we have at least one node on center 2 that has lost
> no data at transaction commit.
>
> Now, regarding the way to express that, we need to use a concept of
> node group for each element of synchronous_standby_names. A group
> contains a set of elements, each element being a group or a single
> node. And for each group we need to know three things when a commit
> needs to be acknowledged:
> - Does my group need to acknowledge the commit?
> - If yes, how many elements in my group need to acknowledge it?
> - Does the order of my elements matter?
>
> That's where the micro-language idea makes sense to use. For example,
> we can define a group using separators and like (elt1,...eltN) or
> [elt1,elt2,eltN]. Appending a number in front of a group is essential
> as well for quorum commits. Hence for example, assuming that '()' is
> used for a group whose element order does not matter, if we use that:
> - k(elt1,elt2,eltN) means that we need for the k elements in the set
> to return true (aka commit confirmation).
> - k[elt1,elt2,eltN] means that we need for the first k elements in the
> set to return true.
>
> When k is not defined for a group, k = 1. Using only elements
> separated by commas for the upper group means that we wait for the
> first element in the set (for backward compatibility), hence:
> 1(elt1,elt2,eltN) <=> elt1,elt2,eltN

Nice design.

--
Robert Haas
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