mosquitto-users team mailing list archive

[Jon McDermott <jonmc56@xxxxxxxxx>]

Hi Roger,

Thanks for the help.  Yes - 'crash' was a little melodramatic, I did mean
that the script terminated with a network connection error.

Your suggested fix did the trick.  The publish logic works as advertised,
if I stop the broker service the client queues up the messages until the
broker becomes available again.

Now the subscriber is not working as I expect.  All goes well until I stop
the broker service to simulate a network connection problem.  The client
disconnects as expected, then re-connects once the broker service is
re-started (I can see it reconnect because of some debug print statements
in my on-connect callback method.

The issue is that my subscriber doesn't receive any of the messages that
were queued by the publishing client, and in fact doesn't receive any new
messages that the publishing client publishes after the broker service is
started again.  In other words, after the broker is stopped and restarted,
the subscriber never receives a subsequent message again.

Here's my subscribe code:
mqttclient.connect(HOST, PORT, 60)
mqttclient.subscribe("Jon", 1)
mqttclient.loop_forever()

Apologies if my questions are very rudimentary.

Thanks, Jon


On Tue, Feb 4, 2014 at 1:32 PM, Roger Light <roger@xxxxxxxxxx> wrote:

> Hi Jon,
>
> I presume by "crash" you mean it complains about a broken pipe or
> similar socket error rather than something more dramatic :)
>
> There are a couple of things I can suggest, the first is that you need
> to use one of the loop*() functions. You need to use these to ensure
> that the background network traffic is processed. You have a couple of
> options - you can use loop() which you would call yourself in your
> loop. You can also use loop_start() once after calling connect().
> loop_start() creates a thread that calls loop() for you. The final
> choice which isn't appropriate in your case is to use loop_forever(),
> which is a blocking call and means your loop couldn't work.
>
> The Python client queues up messages that you send with publish() even
> if the client isn't connected.
>
> If you use loop() yourself, you need to check the return value to see
> whether you need to reconnect. If you do, call reconnect().
>
> Using loop_start() takes this out of your hands because it will do the
> reconnection for you, no problem. Likewise loop_forever().
>
> For ease, I'd do:
>
> mqttclient.connect(HOST, PORT, 60)
> mqttclient.loop_start()
> while True:
>     payload = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
>     print('About to publish: {P}'.format(P=payload))
>     mqttclient.publish("Jon", payload, 1, retain=False)
>     # wait for 10 seconds
>     time.sleep(10)
>
> Cheers,
>
> Roger
>
> On Tue, Feb 4, 2014 at 7:34 PM, Jon McDermott <jonmc56@xxxxxxxxx> wrote:
> > Hello,
> >
> > I am trying to understand how to create a Python client that is
> resilient to
> > connection/connectivity problems.
> >
> > I'm using the latest Paho MQTT client on Win8 and Python 2.7.4.  This is
> > just my development platform, my plan is to deploy the code on
> RaspberryPi
> > with a WiFi adapter.
> >
> > When I try this simple publish code:
> >
> > mqttclient.connect(HOST, PORT, 60)
> > while True:
> > payload = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
> > print('About to publish: {P}'.format(P=payload))
> > mqttclient.publish("Jon", payload, 1, retain=False)
> > # wait for 10 seconds
> > time.sleep(10)
> >
> >
> > it publishes to my Mosquitto broker every 10 seconds, and I'm able to
> > subscribe to these messages by topic.
> >
> > If I break the connection, however, or stop the broker service - it
> crashes
> > the client on the next attempt to publish.  (this happens using all 3 QOS
> > options)
> >
> > Is there anything in the Paho MQTT client that can help me queue these
> > published messages until the broker becomes available again?  Any example
> > code available that would show me how to gracefully handle these
> connection
> > errors?
> >
> > Thanks in advance.
> > Jon
> >
> > --
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>