On Fri, 24 Jul 2015, David Fetter wrote:

> Folks,
>
> While in the best of all possible worlds, we'd have planned out a
> replication strategy before we get tables whose initial sync via
> "SUBSCRIBE SET" will never finish, we aren't always given that much
> ability to plan that soon.
>
> CLONE is great when you want to light an Nth node for N > 2, but
> that's just adjusting an extant cluster, not creating one in the first
> place.
>
> What stands between the state of the slony code and being able to
> clone an origin node?

My guess is that the hardest part of teaching slony how to do this would be 
to figure out how to get the proper values in sl_setsync such that it knows 
where to start replication from when it starts up as a subscriber.

Specifically something like this scenario is problematic

CLONE PREPARE and/or SYNC 1000 is generated on the origin
T1 commits on the origin
You make  backup copy of the origin with pg_dump or rsync etc..
T2 COMMITS on the origin
SYNC 1001 is generated on the origin

When you restore the backup to the new subscriber the new subscriber will 
need to figure out what rows it needs to replicate from the origin in the 
first sync.  Specifically it needs to somehow figure out that it already has 
the data from T1 but it doesn't have the data from T2 so it really needs 
half of SYNC 1001.

We spoke about this a few years ago in new-york.  I still suspect there is a 
way to solve this, the data in sl_log_1 and sl_log_2 on the new node is 
probably enough information to figure which transactions need to be 
pulled/excluded from the normal SYNC 1000->SYNC 1001 SYNC.




>
> Cheers,
> David.
> -- 
> David Fetter <david at fetter.org> http://fetter.org/
> Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
> Skype: davidfetter      XMPP: david.fetter at gmail.com
>
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