Sync Rep v17

Well, good news all round.

v17 implements what I believe to be the final set of features for sync
rep. This one I'm actually fairly happy with. It can be enjoyed best at
DEBUG3.

The patch is very lite touch on a few areas of code, plus a chunk of
specific code, all on master-side. Pretty straight really. I'm sure
problems will be found, its not long since I completed this; thanks to
Daniel Farina for your help with patch assembly.

Which is just as well, because the other news is that I'm off on holiday
for a few days, which is most inconvenient. I won't be committing this
for at least a week and absent from the list. OTOH, I think its ready
for a final review and commit, so I'm OK if you commit or OK if you
leave it for me.

That's not the end of it. I can see a few things we could/should do in
this release, but this includes all the must-do things. Docs could do
with a little love also. So I expect work for me when I return.

Config Summary
==============

Most parameters are set on the primary. Set

primary: synchronous_standby_names = 'node1, node2, node3'

which means that whichever of those standbys connect first will
become the main synchronous standby. Servers arriving later will
be potential standbys (standby standbys doesn't sound right...).
synchronous_standby_names can change at reload.

Currently, the standby_name is the application_name parameter
set in the primary_conninfo.

When we set this for a client, or in postgresql.conf

primary: synchronous_replication = on

then we will wait at commit until the synchronous standby has
reached the WAL location of our commit point.

If the current synchronous standby dies then one of the other standbys
will take over. (I think it would be a great idea to make the
list a priority order, but I haven't had time to code that).

If none of the standbys are available, then we don't wait at all
if allow_standalone_primary is set.
allow_standalone_primary can change at reload.

Whatever happens, if you set sync_replication_timeout_client
then backends will give up waiting if some WALsender doesn't
wake them quickly enough.

You can generally leave these parameters at their default settings

primary: sync_replication_timeout_client = 120s
primary: allow_standalone_primary = on
standby: wal_receiver_status_interval = 10s

--
Simon Riggs http://www.2ndQuadrant.com/books/
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