Re: Logical decoding on standby

On 19 January 2016 at 23:30, Дмитрий Сарафанников <dimon99901(at)mail(dot)ru>
wrote:

When you plan to add logical decoding on standby?
>

> it is useful to have separate standby server for logical replication that
> will not break the master if you make a mistake in plugin.
>

Indeed. It might PANIC it and force a restart, if that's what you mean. I
guess in an extreme case you could clobber shmem silently, which would be
bad. (I wonder if we could make shared_buffers mmap'ed read-only for
walsender backends?). So it certainly seems like it'd be nice to have.
There's also the advantage that you could shift load off the master by
having physical master=>replica replication on a fast wide link, then do
logical decoding from there to send over the wire to WAN sites etc.

Unfortunately it's not particularly simple and nobody seems to have time to
implement it. As Álvaro pointed out, sometimes you have to do the work if
you want the change to happen. Or find someone with the existing skills and
convince them to want to do it, but most of those people are already very,
very busy.

As part of the failover slots work Simon noted that:

"To prevent needed rows from being removed we need we would need to enhance
hot_standby_feedback so it sends both xmin and catalog_xmin to the master."

... which means a protocol change in the walsender protocol. So you're
looking at that plus the other comment given above, that

"We need to be able to correctly and quickly identify the timeline a LSN
belongs to"

.... which is new internal infrastructure and new state in the replica that
must be maintained. Probably persistently.

In other words ... this isn't especially simple to do. It requires at least
two non-trivial core changes. Nobody who has the skills to do it reasonably
quickly also has the time to do it at all or has other higher priority
things to do first. So it's not likely to happen soon unless you or someone
like you steps up and has a go at it.

If you do decide to attempt to implement it and you're willing to read a
fair bit of code and documentation to do so you'll find people here pretty
helpful with suggestions, ideas, and help if you get stuck. But only if you
put in the work yourself.

(On a side note, failover slots probably won't be usable from a standby
either. They have to write to WAL and you can't write to WAL from a
standby. So if slot support is ever added on a standby it'll probably be
ephemeral slots only.)

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services