On 5/28/2008 11:59 AM, Simon Riggs wrote:
> On Wed, 2008-05-28 at 11:36 -0400, Christopher Browne wrote:
>> I am looking at the Slony-I CVS HEAD code with a view to eliminating
>> the "abusive" uses of LISTEN/NOTIFY infrastructure, as that will be
>> changing in 8.4.
>> 
>> In slonik.c, the function slonik_failed_node() uses a direct lookup on
>> pg_listener in order to see if there is a "live" slon.
>> 
>> It uses the query:
>>    select listenerpid from pg_catalog.pg_listener 
>>    where relname = '_[Cluster]_Restart';
>> 
>> I don't see another _direct_ way to get at PIDs.  The closest that I
>> can see is to replace this query with something like:
>> 
>>    select nl_backendpid from sl_nodelock 
>>     where nl_nodeid = @NAMESPACE at .getLocalNodeId(''_ at CLUSTERNAME@'') and
>>     exists (select * from pg_stat_activity where procpid = nl_backendpid);
>> 
>> That *does* plug in perfectly nicely. 
>> 
>> Unfortunately, pg_stat_activity pulls the PIDs from the stats
>> collector, so that there is a delay in changes being reported.  (And I
>> have seen situations where the stats collector got "blown," in which
>> case, this wouldn't report anything even *nearly* correct.)
>> 
>> Is there some more direct way to get at PIDs?  A search of
>> pg_attribute and pg_proc for '%pid%' doesn't show up anything.
> 
> Seems like the best way is to encapsulate this. Work out the API you
> would like, then we can take that to pgsql-hackers. Once we agree the
> API we can write a function to do that for 8.3 and below and put the
> function into Postgres backend for 8.4 and later. That way any further
> changes to LISTEN/NOTIFY will be able to change that also, so you are
> future-proofed around the changes.

If all we need is a function that can determine if a backend is alive, 
we do have _Slony_I_killBackend(pid, signame) which at this point does 
support the two signals NULL and TERM. Sending a backend a NULL signal 
just reports if a process with that pid exists or not.


Jan

-- 
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin