Simon Riggs <simon(at)2ndQuadrant(dot)com> wrote:
> On Wed, 2011-01-19 at 19:05 -0600, Kevin Grittner wrote:
>
>> The idea is that whenever we see a valid snapshot which would
>> yield a truly serializable view of the data for a READ ONLY
>> transaction, we add a WAL record with that snapshot information.
>
> You haven't explained why this approach is the way forwards. What
> other options have been ruled out, and why. The above approach
> doesn't sound particularly viable to me.
Why not? We already generate appropriate snapshots for this in SSI,
so is the problem in getting the appropriate information into the
WAL stream or in having a request for a snapshot within a
serializable transaction while running in hot standby the problem?
> It's not clear to me what the reason is that this doesn't just
> work on HS already. If you started there it might help.
Because the standby would need to bombard the server with a stream
of predicate lock information, we would need to allow transactions
on the master to be canceled do in part to activity on the standby,
and I don't even know how you would begin to track read/write
conflicts between transactions on two different clusters.
If any of that didn't make sense, it would probably be more
efficient for everyone involved if those interested browsed the
Overview section of the Wiki page than to have me duplicate its
contents in a post.
http://wiki.postgresql.org/wiki/Serializable
-Kevin