Re: SimpleComponents / MQTT

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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