Re: Synchronizing slots from primary to standby

On Mon, Nov 29, 2021 at 9:40 AM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Mon, Nov 29, 2021 at 1:48 AM SATYANARAYANA NARLAPURAM
> <satyanarlapuram(at)gmail(dot)com> wrote:
> >
> >> 3) Instead of the subscriber pulling the slot info, why can't the
> >> publisher (via the walsender or a new bg worker maybe?) push the
> >> latest slot info? I'm not sure we want to add more functionality to
> >> the walsender, if yes, isn't it going to be much simpler?
> >
> > Standby pulling the information or at least making a first attempt to connect to the primary is a better design as primary doesn't need to spend its cycles repeatedly connecting to an unreachable standby. In fact, primary wouldn't even need to know the followers, for example followers / log shipping standbys
>
> My idea was to let the existing walsender from the primary/publisher
> to send the slot info (both logical and physical replication slots) to
> the standby/subscriber, probably by piggybacking the slot info with
> the WAL currently it sends. Having said that, I don't know the
> feasibility of it. Anyways, I'm not in favour of having a new bg
> worker to just ship the slot info. The standby/subscriber, while
> making connection to primary/publisher, can choose to get the
> replication slot info.

I think it is possible that the standby is restoring the WAL directly
from the archive location and there might not be any wal sender at
time. So I think the idea of standby pulling the WAL looks better to
me.

> As I said upthread, the problem I see with standby/subscriber pulling
> the info is that: how frequently the standby/subscriber is going to
> sync the slot info from primary/publisher? How can it ensure that the
> latest information exists say when the subscriber is down/crashed
> before it picks up the latest slot information?

Yeah that is a good question that how frequently the subscriber should
fetch the slot information, I think that should be configurable
values. And the time delay is more, the chances of losing the latest
slot is more.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com