[pgpool-hackers: 4100] Re: What to do with ParamterStatus?

[Tatsuo Ishii]

> Currently Pgpool-II only processes ParameterStatus messages at the
> connection starting up phase. In the phase all ParameterStatus
> messages from the primary node are forwarded to frontend. If value for
> a ParameterStatus is differ among backends, only the value of primary
> node is forwarded and the value of standby nodes are ignored.  In this
> case it is logged that the message lenth (thus values) are differ
> (except is_hot_standby, which is always differ among primary and
> standby). So far so good.
> 
> Question is, in the case ParameterStatus message arrives after
> establishing the session. This could happen in certain cases. For
> example, postgresql.conf is changed and reloaded. The other case is,
> SET command is rolled back. Currently Pgpool-II ignores
> ParameterStatus messages in these cases.
> 
> I think this is not good because frontend never knows the value of
> parameters have been changed. So what should we do?
> 
> 1) Every time ParameterStatus message is received, forward it to
> frontend. Even the value for standby servers are forwarded. This may
> result in receiving multiple ParameterStatus for frontend if user
> changes postgresql.conf and reloads it on each server.
> 
> 2) Process ParameterStatus only when the message is received from the
> primary server.
> 
> 3) Other.
> 
> I think #2 is reasonable because it is consistent with the process
> done at the starting up.

After thinking a little bit harder, it seems #1 is more reasonable
than #2. The event (ParameterStatus is generated) is mostly happening
when admin changes a parameter and reloads config file.  This will be
done at each server of the cluster one by one. If pgpool only picks up
the ParameterStatus at primary server, the admin might be confused
when changing/reloading config file at standby servers. So I guess #1
is less confusing than #2 for admins.

Best reagards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp