Re: SimpleComponents / MQTT

[slos <new.stephane.los@gmail.com>]

Le vendredi 1 juillet 2022 à 13:05:43 UTC+2, Dmitry A. Kazakov a écrit :
> On 2022-07-01 10:51, slos wrote: 
> > Le mercredi 29 juin 2022 à 17:00:20 UTC+2, Dmitry A. Kazakov a écrit : 
> >>> Could you please have a look on that behaviour ? 
> >> It means that the other side (mosquitto) dropped the connection. 
> >> 
> >> One possible case is when ping timeout (Keep_Alive) is set but the 
> >> client was silent for the period of time. You should send ping before 
> >> the timeout expires or reconnect. 
> > Hello Dmitry, 
> > Thanks for your answer. 
> > I had set the "Keep_Alive" parameter : 
> > Send_Connect (Client, "TestMQTTclient", Keep_Alive => 60.0); 
> > 
> > If it is not set, one gets : 
> > MQTT client test 1 started 
> > MQTT client connected to test.mosquitto.org 
> > Connect rejected identifier rejected
> AFAIK it is a bug in mosquitto that it does not accept zero ping interval.
> > Indeed, I get exceptions because of this line : 
> > GNAT.Exception_Traces.Trace_On (GNAT.Exception_Traces.Every_Raise); 
> > 
> > I have played with mosquitto on localhost and wanted to see what would happen if the broker was shut down. 
> > Problem is that I could not get an information or exception in this case.
> It is unrelated to MQTT, just general TCP/IP behavior: 
> 
> 1. If one side (broker) shuts the socket down, the other side's (the 
> client) read ends with 0 payload. 
> 
> 2. If it crashes, the socket is closed and nothing happens until some 
> OS-specific timeout (might be seconds) and then the socket get shut down 
> and see above. 
> 
> The second case could be improved by setting KEEP_ALIVE on the socket. 
> Not to confuse with MQTT's ping, which is rather a useless thing.
> > So I had to add a check of the ping response in order to know if the broker is still there but maybe you have a better idea.
> It is reverse. Ping is a sort of watchdog on the server side to drop off 
> sluggish clients. 
> 
> If you want to speed up recognition that the broker had crashed use 
> TCP/IP KEEP_ALIVE option. Differently to ping it requires nothing from 
> you, being implemented transparently by the TCP/IP stack.
> >> With SSL/TLS you could probably map users to certificates/keys and drop 
> >> user/password, but for the server side it is easier to have them in 
> >> order to maintain user access rights. 
> > I use Gnoga as well and there are explanations on how to use SSL/TLS in Gnoga's documentation. 
> > I wondered if such explanations where available to use with MQTT as well.
> There is nothing specific to MQTT. All protocol implementations in 
> Simple Components (MQTT, HTTP, SMTP etc) use a connection server object. 
> If you take one like GNAT.Sockets.Server.Secure (GNUTLS) or 
> GNAT.Sockets.Server.OpenSSL (OpenSSL) with MQTT client/server and that 
> will give you MQTTS.
> > Unrelated, you have created your packages as childs of GNAT.Sockets.
> GNAT.Sockets.Server needs access to the private part of GNAT.Sockets. 
> That is the reason.
> > Although it seems not a problem for using them, the applications can be built, that seems to be a problem for code navigation with GNAT Studio, even in latest version. 
> > Is that a limitation of the tool or something I'm doing wrong ?
> It never worked well, if you mean "go to declaration" stuff. I know that 
> AdaCore is changing the cross-reference stuff. Maybe it will get better 
> at some point. (before getting worse first! (:-))
> > By the way, thanks to your MQTT implementation, this demo application is sending SIEMENS S7 PLC data to the mosquitto broker : 
> > https://gitlab.com/ada-for-automation/ada-for-automation/-/tree/master/demo/142%20a4a_k0_S7
> Nice. 
> 
> P.S. In the proprietary code we rather use iTOT for SPS communication 
> which is far more efficient. Unfortunately it cannot be included into 
> the Simple Components.
> -- 
> Regards, 
> Dmitry A. Kazakov 
> http://www.dmitry-kazakov.de
Hi Dmitry,

Thanks a lot for the provided information. Much appreciated !

Best Regards,
Stéphane
https://www.ada4automation.org/