On Sat, Apr 28, 2012 at 4:43 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
>> Tom Lane wrote:
>>> Simon Riggs writes:
>>>> Translating WAL is a very hard task.
>>> No kidding. I would think it's impossible on its face.
>
>> Well, the xReader approach (Aakash's GSoC project) is to serve as a
>> proxy for a WAL stream going to a hot standby, to interpret each
>> incoming WAL record by "cheating" and querying the HS before passing
>> the WAL along, and then using sync rep to the HS regardless of
>> whether the incoming stream is sync or async. What reliability
>> problem do you see with this approach?
>
> Well, first off, it requires a replica that *is* a physical replica
> (else it won't have the same table OIDs, for instance).
It's even harder than that because the physical replica needs to be
able to be shutdown/restarted. We currently restart WAL from last
checkpoint, but if changes have already been made and committed then
the catalog will be incorrect when we replay the WAL again. So, I
think saying very hard is justified.
We basically have a choice of shipping full metadata with every change
or finding some way to avoid doing that. Investigating the latter
seems like a worthwhile task but there are limits, as you say.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
