Tom Lane wrote:
> What I think Simon was actually driving at was query-shipping, which is
> not my idea of "WAL" at all. It has some usefulness, but also a bunch
> of downsides of its very own, mostly centered around reproducibility.
> With the current WAL design I have some faith that the slaves reproduce
> the contents of the master. With any "logical replication" design that
> becomes a faith of the religious kind, because it sure isn't provable.
Actually I think the idea here is to take certain WAL records, translate
them into "portable" constructs, ship them, and let the slave handle the
remaining tasks that need to be done with it. For example you would
only ship heap insert, not index insert; the slave needs to take this
insert and derive the appropriate index operations that the slave needs.
What's needed here is a process that is able to read (or receive from
the backend calling the XLogInsert) WAL records and at the same time
have catalog access for getting type I/O data, converting relation OIDs
into table names if needed, etc.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support