mosquitto-users team mailing list archive

[Roger Light <roger@xxxxxxxxxx>]

Hi Jon,

The point that Jan-Piet alludes to is important here. If you are using
clean_session=False and have a consistent client id, then this should
just work fine. If you have clean_session=True, then the broker will
remove all information regarding your client when you disconnect -
including subscriptions. The client module doesn't keep track of what
topics you are subscribed to, so when it reconnects for you the
subscriptions will not be recreated.

I would suggest putting your call to subscribe() in the on_connect()
callback, then it will be called if the client has to reconnect.

Cheers,

Roger


On Mon, Feb 10, 2014 at 11:35 PM, Jon McDermott <jonmc56@xxxxxxxxx> wrote:
> 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
>> >
>> > --
>> > Mailing list: https://launchpad.net/~mosquitto-users
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>> >
>
>