Re: Proposal: "Causal reads" mode for load balancing reads without stale data

On Tue, Mar 29, 2016 at 1:56 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Mon, Mar 28, 2016 at 8:54 PM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com> wrote:
>> I have been also thinking a lot about this patch, and the fact that
>> the WAL receiver latch is being used within the internals of
>> libpqwalreceiver has been bugging me a lot, because this makes the
>> wait phase happening within the libpqwalreceiver depend on something
>> that only the WAL receiver had a only control on up to now (among the
>> things thought: having a second latch for libpqwalreceiver, having an
>> event interface for libpqwalreceiver, switch libpq_receive into being
>> asynchronous...).
>
> Yeah, it bugs me too. Do you prefer this?
>
> int walrcv_receive(char **buffer, int *wait_fd);
>
> Return value -1 means end-of-copy as before, return value 0 means "no
> data available now, please call me again when *wait_fd is ready to
> read". Then walreceiver.c can look after the WaitLatchOrSocket call
> and deal with socket readiness, postmaster death, timeout and latch,
> and libpqwalreceiver.c doesn't know anything about all that stuff
> anymore, but it is now part of the interface that it must expose a
> file descriptor for readiness testing when it doesn't have data
> available.
>
> Please find attached a new patch series which does it that way.

Oops, there is a bug in the primary disconnection case when len == 1
and it breaks out of the loop and wait_fd is invalid. I'll follow up
on that tomorrow, but I'm interested to hear your thoughts (and anyone
else's!) on that interface change and general approach.

--
Thomas Munro
http://www.enterprisedb.com