Re: recovery_connections cannot start (was Re: master in standby mode croaks)

On Fri, Apr 23, 2010 at 1:04 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> One way we could fix this is use 2 bits rather than 1 for
> XLogStandbyInfoMode.  One bit could indicate that either
> archive_mode=on or max_wal_senders>0, and the second bit could
> indicate that recovery_connections=on.  If the second bit is unset, we
> could emit the existing complaint:
>
> recovery connections cannot start because the recovery_connections
> parameter is disabled on the WAL source server
>
> If the other bit is unset, then we could instead complain:
>
> recovery connections cannot start because archive_mode=off and
> max_wal_senders=0 on the WAL source server
>
> If we don't want to use two bits there, it's hard to really describe
> all the possibilities in a reasonable number of characters.  The only
> thing I can think of is to print a message and a hint:
>
> recovery_connections cannot start due to incorrect settings on the WAL
> source server
> HINT: make sure recovery_connections=on and either archive_mode=on or
> max_wal_senders>0
>
> I haven't checked whether the hint would be displayed in the log on
> the standby, but presumably we could make that be the case if it's not
> already.
>
> I think the first way is better because it gives the user more
> specific information about what they need to fix.  Thinking about how
> each case might happen, since the default for recovery_connections is
> 'on', it seems that recovery_connections=off will likely only be an
> issue if the user has explicitly turned it off.  The other case, where
> archive_mode=off and max_wal_senders=0, will likely only occur if
> someone takes a snapshot of the master without first setting up
> archiving or SR.  Both of these will probably happen relatively
> rarely, but since we're burning a whole byte for XLogStandbyInfoMode
> (plus 3 more bytes of padding?), it seems like we might as well snag
> one more bit for clarity.
>
> Thoughts?

I like the second choice since it's simpler and enough for me.
But I have no objection to the first.

When we encounter the error, we would need to not only change
those parameter values but also take a fresh base backup and
restart the standby using it. The description of this required
procedure needs to be in the document or error message, I think.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center