Re: Synchronous replication

Fujii Masao wrote:
> On Mon, Aug 2, 2010 at 7:53 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>   
>> Let's not get *the manner of specifying the policy* confused with *the
>> need to update the policy when the master changes*.  It doesn't seem
>> likely you would want the same value for  synchronous_standbys on all
>> your machines.  In the most common configuration, you'd probably have:
>>
>> on A: synchronous_standbys=B
>> on B: synchronous_standbys=A
>>     
>
> Oh, true. But, what if we have another synchronous standby called C?
> We specify the policy as follows?:
>
> on A: synchronous_standbys=B,C
> on B: synchronous_standbys=A,C
> on C: synchronous_standbys=A,B
>
> We would need to change the setting on both A and B when we want to
> change the name of the third standby from C to D, for example. No?
>   
What if the master is named as well in the 'pool of servers that are in 
sync'? In the scenario above this pool would be A,B,C. Working with this 
concept has as benefit that the setting can be copied to all other 
servers as well, and is invariant under any number of failures or 
switchovers. The same could also hold for quorum expressions like A && 
(B || C), if A,B,C are either master or standby.

I initially though that once the definitions could be the same on all 
servers, having them in a system catalog would be a good thing. However 
that'd propably hard to setup, and also in the case of failures during 
change of the parameters it could become very messy.

regards,
Yeb Havinga