David Parker wrote:

>>One of the "high priority items" (fairly urgent, and 
>>considered very important, at least here at Afilias) is the 
>>implementation of Log Shipping, which ought to be an answer to 
>>your problem.
>>
>>The idea of Log Shipping is that you can have a node that, 
>>instead of syncing into a database, generates a series of 
>>files containing the SQL queries to do the updates.  That 
>>series of files may then be distributed to remote sites, and 
>>applied into a database, giving you a replica that is as up to 
>>date as the series of files.
>>
>>One such "log shipping" node can feed as many replicas as you 
>>want it to.  There is no means of feeding anything back to the 
>>origin, but it can certainly scale.
>>
>>So if what you want is to have (say) four reliable boxes at a 
>>central site that talk to one another, each of which could 
>>take over in a pinch, and then have a whole bunch of copies at 
>>different sites, log shipping would allow that to work.
>>
>>It's not implemented yet, but our urgency level on it is 
>>rather high, so there oughta be something at least beta-like 
>>in the not too distant future.
>>    
>>
>
>Thanks! That does sound promising. Is there a spec anywhere that I could
>take a look at? Some questions
>that occur to me initially:
>
>1) Is there still a subscription mechanism, for instance allowing the 
>specification of a "type" of subscription?
>
>  
>
Yes.  Jan and I have both done some "fighting" with the problem and 
independently discovered some of the same bits of trickiness to setting 
up subscriptions.

>2) Does the slon daemon handle the "shipping"?
>
>  
>
It may be a special slon daemon, or log shipping code may be added in; 
not yet sure.

>3) How does a given subscriber node determine it's sync state vis a vis
>the log
>shipper?
>  
>
What appears to be the case (at this point; call it "preliminary 
analysis") is that setting up a subscription involves going to some 
existing subscriber node (which implies you need at least one subscriber 
already in order to start log shipping), and grab a Slony-I-customized 
analogue to a pg_dump which contains the sync status of that subscriber 
at the time the dump starts.

Shipped logs are then "deltas" from that dump.

>Has code for this gone into cvs HEAD for the project, or are you folks
>just working
>on it locally? So many questions....
>  
>
It's not yet at the point of putting something in CVS HEAD...

>If it is in HEAD, I would be interested in taking on some tasks, if you
>need help. Failing that, I would also be eager to help with testing, etc., since this is a feature I
>really need!
>  
>
As soon as it's far enough along for it to make sense to try to 
distribute tasks, I'll see about pestering you.  I'm sure there will be 
room for extra hands.  Log shipping would solve a number of problems 
that we've got, including some that have been persistently thorny, so 
I'm pretty certain that I'm as eager as you are.
-- 
<http://linuxdatabases.info/info/slony.html>