Re: [xmlblaster-devel] Reconnected xml-rpc callbacks result in duplicate messages

[Michele <laghi at swissinfo.org>]

Hi Brad,

Brad Clements wrote:
> I've been googling for an answer to this problem, and I'm wondering what's the best 
> way to handle this.
>
> I'm using Python and pyBlaster.py to subscribe an xml-rpc server for callbacks.
>
> My xml-rpc server keeps track of pings from the server, and if it doesn't get any for a 
> while it will re-login and re-subscribe to one key.
>
> Also, I'm running my server under DaemonTools, so if it dies, daemontools will restart 
> it.
>
> It seems that if my server gets restarted, xml-blaster sees it as a new login (but with the 
> same callback information).  It then sends two messages where I expect only one.
>
> And, if I restart my server again, then I'll get 3 messages and so on.
>
> I know this is a hard problem for xmlblaster to deal with, especially since I can 
> subscribe on behalf of another callback listener, so how's xmlblaster supposed to know 
> what to do? 
>
> xmlblaster thinks my server is still logged in (because the client doesn't get the chance 
> to logout in this particular case), and the client subscribes to the same key every time.
>
> So, there's two things I need to do:
>
> 1. stop multiple duplicate messages 
>
> I've been going through the reference guide, and finding a complete list of QoS 
> parameters is hard.
>
> I know I whined about this last year (sorry), but it sure would be nice to have a 
> complete QoS list on one page in the docs.
>
> And, having Docs not in a frame would make it easier to bookmark.
>
> Anyway, I found
>
> qos.subscribe.multiSubscribe
if you subscribe multiple times, only one is considered.
> and
>
> qos.subscribe.duplicateUpdates

if you publish several identical messages on the same topic you only 
will get the first update of them (not really what you want here).

> Which of these is better to use in my situation? I only ever want one copy of any single 
> message. I think multiSubscribe=false is the better choice, but neither option mentions 
> the other one and under what conditions I'd pick one over the other.
>
>
> 2. Cleanup the old logins to avoid resource exhaustion 
don't need to do that (see below)

> my server logs in, subscribes and makes no further requests on the client connection. 
> So, it's sessionTimeout = 0
>
> I login with the same userid every time. Is there any way I can say "hey, only one login 
> allowed at a time and each new login will logout the previous instance and clear it's 
> callback". ?
Yes there is:
maxSessions=1 in the conntectQos, however I believe you would do better 
here to use persistent subscriptions and avoid to re-subscribe, see

http://www.xmlblaster.org/xmlBlaster/doc/requirements/engine.persistence.subscription.html

and don't forget to use positive sessionId, that way you make sure to 
regain the same session and not a new one.

> Ah, looks like <session timeout="0" clearSessions="true" />  is what I want here.
>
> Yes?
Regards
Michele