On Sun, Jun 28, 2015 at 5:52 PM, Sawada Masahiko <sawada.mshk@gmail.com> wrote:
> On Fri, Jun 26, 2015 at 2:46 PM, Michael Paquier
> <michael.paquier@gmail.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
>>
>
> I think that you meant "1[elt1,elt2,eltN] <=> elt1,elt2,eltN" in this
> case (for backward compatibility), right?
Yes, [] is where the order of items matter. Thanks for the correction.
Still we could do the opposite, there is nothing decided here.
--
Michael
