Re: Exposing the Xact commit order to the user

Jan Wieck wrote:

> Since the actual row level change information and other event data is 
> found inside of regular tables, identified by TXID and sequence number, 
> I am pretty sure I want that data in a server-side query. What you are 
> proposing is to read the xid's and timestamps with an external process, 
> that now forcibly needs to reside on the DB server itself (neither 
> Londiste nor Slony have that requirement as of today), then bring it 
> back into the DB at least inside the WHERE clause of a query.


It depends on how you approach the problem.

If you had a process that could scan WAL files (or a platform/version 
independent representation of these WAL files) you could run that 
process on any server (the origin server, a replica, or some third 
server with the software installed).  Where you run it involves making 
trade-offs on the costs of storing transferring and processing the files 
and would ideally be configurable.

You could then have a process that transfers all of the data logged by 
the triggers to the replicas as soon as it is committed.  Basically 
saying 'copy any rows in sl_log from the origin to the replica that we 
haven't already sent to that replica'

You could then move the work of figuring out the commit order onto the 
replica where you would combine the output of the WAL scanning process 
with the transaction data that has been copied to the replica.


> 
> 
> Jan
> 


-- 
Steve Singer
Afilias Canada
Data Services Developer
416-673-1142