On Sun, Jun 28, 2015 at 5:52 PM, Sawada Masahiko <sawada.m...@gmail.com> wrote: > On Fri, Jun 26, 2015 at 2:46 PM, Michael Paquier > <michael.paqu...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
