Re: Sync Rep v19 - Mailing list pgsql-hackers

On Thu, 2011-03-03 at 22:27 +0100, Yeb Havinga wrote:
> On 2011-03-03 11:53, Simon Riggs wrote:
> > Latest version of Sync Rep, which includes substantial internal changes
> > and simplifications from previous version. (25-30 changes).
> >
> > Includes all outstanding technical comments, typos and docs. I will
> > continue to work on self review and test myself, though actively
> > encourage others to test and report issues.
> >
> > Interesting changes
> >
> > * docs updated
> >
> > * names listed in synchronous_standby_names are now in priority order
> >
> > * synchronous_standby_names = "*" matches all standby names
> >
> > * pg_stat_replication now shows standby priority - this is an ordinal
> > number so "1" means 1st, "2" means 2nd etc, though 0 means "not a sync
> > standby".
> Some initial remarks:
> 
> 1) this works nice:
> application_name not in synchronous_standby_names -> sync_priority = 0 (OK)
> change synchronous_standby_names to default *, reload conf -> 
> sync_priority = 1 (OK)
> 
> message in log file
> LOG:  00000: standby "walreceiver" is now the synchronous standby with 
> priority 1
> 
> 2) priorities
> I have to get used to mapping the integers to synchronous replication 
> meaning.
> 0 -> asynchronous
> 1 -> the synchronous standby that is waited for
> 2 and higher -> potential syncs
> 
> Could it be hidden from the user? I liked asynchronous / synchronous / 
> potential synchronous

Yes, that sounds good. I will leave it as it is now to gain other
comments since this need not delay commit.

> then the log message could be
> LOG:  00000: standby "walreceiver" is now the synchronous standby

The priority is mentioned in the LOG message, so you can understand what
happens when multiple standbys connect.

e.g.

if you have synchronous_standby_names = 'a, b, c'

and then the standbys connect in the order b, c, a then you will see log
messages

LOG:  standby "b" is now the synchronous standby with priority 2
LOG:  standby "a" is now the synchronous standby with priority 1

It's designed so no matter which order standbys arrive in it is the
highest priority standby that makes it to the front in the end.

> 3) walreceiver is the default application name - could there be problems 
> when a second standby with that name connects (ofcourse the same 
> question holds for two the same nondefault application_names)?

That's documented: in that case which standby is sync is indeterminate.

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services