SimpleComponents / MQTT

[slos]

Hello Dmitry,
I'm having fun playing with SimpleComponents MQTT implementation and I have some questions...

I'm on Debian Sid using :
GNAT Studio 23.0w (20220512) hosted on x86_64-pc-linux-gnu
GNAT 11.3.0 targeting x86_64-linux-gnu

GNAT Studio
(c) 2001-2022 AdaCore

Building the test application is fine using :
gprbuild -d -P/home/slos/Ada/SimpleComponents/components_4_62/test_components/components-connections_server-mqtt-test_mqtt.gpr -XObject_Dir=. -Xarch=x86_64 -XTarget_OS=Windows -XDevelopment=Debug -XLegacy=Ada2012 -XAtomic_Access=auto -XTasking=Multiple -XTraced_objects=Off /home/slos/Ada/SimpleComponents/components_4_62/test_components/test_mqtt_client.adb

Running using either "test.mosquitto.org" or local mosquitto gives an exception after some messages received :
Exception raised
raised GNAT.SOCKETS.SERVER.CONNECTION_ERROR : gnat-sockets-server.adb:1145

I don't recall having got exceptions with Gnat Community 2021 on Windows.
Could you please have a look on that behaviour ?

It should be possible to use user name and password but I haven't found how with your implementation.
Could you tell please ?

Of course, one should use TLS in this case. Is it feasible and how ?

Also, how to provide a timestamp ?

Thanks a lot for your kind support.

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

[Dmitry A. Kazakov]

On 2022-06-29 12:15, slos wrote:

> I'm having fun playing with SimpleComponents MQTT implementation and I have some questions...
>
> I'm on Debian Sid using :
> GNAT Studio 23.0w (20220512) hosted on x86_64-pc-linux-gnu
> GNAT 11.3.0 targeting x86_64-linux-gnu
>
> GNAT Studio
> (c) 2001-2022 AdaCore
>
> Building the test application is fine using :
> gprbuild -d -P/home/slos/Ada/SimpleComponents/components_4_62/test_components/components-connections_server-mqtt-test_mqtt.gpr -XObject_Dir=. -Xarch=x86_64 -XTarget_OS=Windows -XDevelopment=Debug -XLegacy=Ada2012 -XAtomic_Access=auto -XTasking=Multiple -XTraced_objects=Off /home/slos/Ada/SimpleComponents/components_4_62/test_components/test_mqtt_client.adb
>
> Running using either "test.mosquitto.org" or local mosquitto gives an exception after some messages received :
> Exception raised
> raised GNAT.SOCKETS.SERVER.CONNECTION_ERROR : gnat-sockets-server.adb:1145
>
> I don't recall having got exceptions with Gnat Community 2021 on Windows.
> 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.

> It should be possible to use user name and password but I haven't found how with your implementation.
> Could you tell please ?

User and password are set after TCP/IP connection established during
handshake (see Send_Connect).

> Of course, one should use TLS in this case. Is it feasible and how ?

Well, MQTT does not require SSL/TLS for user/password during the
handshake. But logically yes, without a secure layer (e.g. provided
GNUTLS or OpenSSL connection handlers) it makes little sense.

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.

(In my opinion MQTT is a toy thing not be used for serious applications,
though I admit, it is hugely popular)

> Also, how to provide a timestamp ?

Hmm, where? MQTT is totally low-level. There are only string topic and
string messages, nothing else. Otherwise? if you want to send a time
stamp in a message use FILETIME or UNIX time epoch and pack 8 bytes UTC
offset to (low or big endian) in the corresponding resolution. MQTT
payload need not to be printable.

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

[slos]

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

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.

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 should be possible to use user name and password but I haven't found how with your implementation.
> > Could you tell please ?
> User and password are set after TCP/IP connection established during
> handshake (see Send_Connect).
> > Of course, one should use TLS in this case. Is it feasible and how ?
> Well, MQTT does not require SSL/TLS for user/password during the
> handshake. But logically yes, without a secure layer (e.g. provided
> GNUTLS or OpenSSL connection handlers) it makes little sense.
>
> 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.

> (In my opinion MQTT is a toy thing not be used for serious applications,
> though I admit, it is hugely popular)

Well, there are many toy things used for serious applications ! ;-)
"Ada for Automation" being one of those... created by an Ada amateur...

> > Also, how to provide a timestamp ?
> Hmm, where? MQTT is totally low-level. There are only string topic and
> string messages, nothing else. Otherwise? if you want to send a time
> stamp in a message use FILETIME or UNIX time epoch and pack 8 bytes UTC
> offset to (low or big endian) in the corresponding resolution. MQTT
> payload need not to be printable.

You are right. I remember having played with nodes in Node-RED where there was something.
But from :
https://github.com/persan/mosquitto-ada
C.Publish (Mid => null, Topic => "test", Payload => "[" & GNAT.Time_Stamp.Current_Time & "] Hej", Qos => QOS_0, Retain => False);
or :
http://mosquitto.org/man/mosquitto_pub-1.html
Publish timestamp and temperature information to a remote host on a non-standard port and QoS 0:
mosquitto_pub -h 192.168.1.1 -p 1885 -t sensors/temperature -m "1266193804 32"
The timestamp is just part of the message string.

Unrelated, you have created your packages as childs of GNAT.Sockets.
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 ?

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

>
> --
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de

[Dmitry A. Kazakov]

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.

[slos]

Hi Dmitry,

Thanks a lot for the provided information. Much appreciated !