[pgpool-hackers: 4101] 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.

And this is the patch for #2.

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