On 12 April 2016 at 08:36, Michael Paquier <michael.paqu...@gmail.com>

> > Also, the entire point here is that extensions DON'T
> > get to have custom apply routines, because we have no way for
> > replay to know about such apply routines.  (It surely cannot look
> > in the catalogs for them, but there's no other suitable infrastructure
> > either.)  In turn, that means there's little value in having any custom
> > data associated with the WAL records.
> Well, yes, the startup process has no knowledge of that. That's why it
> would need to go through a hook, but the startup process has no
> knowledge of routines loaded via _PG_init perhaps?

The only way we really have at the moment is shared_preload_libraries.

This got discussed much earlier, possibly on a previous iteration of the
generic xlog discussions rather than this specific thread. IIRC the gist
was that we need a way to:

- Flag the need for a redo routine so that replay stops if that redo
routine isn't available
- Find out *which* redo routine the generic log message needs during redo
- Get a pointer to that routine to actually call it

... and also possibly allow the admin to force skip of redo calls for a
given extension (but not others) in case of a major problem.

If you rely on shared_preload_libraries, then "oops, I forgot to put
myextension in shared_preload_libraries on the downstream" becomes a Bad
Thing. There's no way to go back and redo the work you've passed over. You
have to recreate the standby. Worse, it's silently wrong. The server has no
idea it was meant to do anything and no way to tell the user it couldn't do
what it was meant to do.

We can't register redo routines in the catalogs because we don't have
access to the syscaches etc during redo (right?).

So how do we look at a generic log record, say "ok, the upstream that wrote
this says it needs to invoke registered generic xlog hook 42, which is
<func> from <extension> at <ptr>" ?

Personally I'd be willing to accept the limitations of the s_p_l and hook
approach to the point where I think we should add the hook and accept the
caveats of its use ... but I understand the problems with it. I understand
not having custom redo routine support in this iteration of the patch. It's
somewhat orthogonal job to this patch anyway - while handy for specific
relation generic xlog, custom redo routines make most sense when combined
with generic xlog that isn't necessarily associated with a specific
relation. This patch doesn't support that.

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

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