Re: max_standby_delay considered harmful

On May 6, 2010, at 12:48 , Simon Riggs wrote:
> On Thu, 2010-05-06 at 11:36 +0200, Florian Pflug wrote:
>> If there was an additional SQL-callable function that returned the backends the recovery process is currently waiting for, plus one that reported that last timestamp seen in the WAL, than all those different cancellation policies could be implemented as daemons that monitor recovery and kill backends as needed, no?
>>
>> That would allow people to experiment with different cancellation policies, and maybe shed some light on what the useful policies are in practice.
>
> It would be easier to implement a conflict resolution plugin that is
> called when a conflict occurs, allowing users to have a customisable
> mechanism. Again, I have no objection to that proposal.

True, providing a plugin API would be even better, since no SQL callable API would have to be devised, and possible algorithms wouldn't be constrained by such an API's limitations.

The existing max_standby_delay logic could be moved to such a plugin, living in contrib. Since it was already established (I believe) that the existing max_standby_delay logic is sufficiently fragile to require significant knowledge on the user's side about potential pitfalls, asking those users to install the plugin from contrib shouldn't be too much to ask for.

This way, users who really need something more sophisticated than recovery wins always or standby wins always are given the tools they need *if* they're willing to put in the extra effort. For those who don't, offering max_standby_delay probably does more harm than good anyway, so nothing is lost by not offering it in the first place.

best regards,
Florian Pflug