Re: [paho-dev] non blocking sleep

[Carl Karsten <carl@xxxxxxxxxxxxxxxx>]

On Sun, Oct 6, 2019 at 9:46 AM Frank Pagliughi <fpagliughi@xxxxxxxxxxxxxx> wrote:

You're looking at doing some "event driven programming". The arriving MQTT messages are telling your system what to do, but as you grow the system, there will be other sources of events from sensors, timers, etc. You want to handle these events as quickly as possible and return control so that you're always responsive to new events.

That means you need to do the actual processing of the events outside of callbacks. Typical ways to do this, depending on your language of choice, are with event loop libraries, threads, and timers.

Timers in Python are pretty easy. Just give an amount of time (in seconds) and a callback function, and the function will be called after the specified amount of time.
https://docs.python.org/3.7/library/threading.html#timer-objects

def hello():
    print("hello, world")

t = Timer(30.0, hello)
t.start()  # after 30 seconds, "hello, world" will be printed

Oh wait, that fires that off a thread and can return to paho's event loop, right?

I have no idea what happens in the timer thread, but I trust this will all just work as needed.  I hope.

I was thinking i needed my own event loop, which would need to play nice with paho, which I still only 1/2 understand how it can run in it's own thread if needed.

I think this is the first time I have really had to pay attention to threads.

 

Tracking your internal progress as a state machine is a great way to deal with all of this.

So, basically, in the callback that receives MQTT messages, set a timer to fire some time later. Keep a reference to the timer, "t", in case an event arrives indicating that the action should be canceled before the timer fired. In that case, you call:

t.cancel()

Frank

On 10/5/19 3:10 PM, Carl Karsten wrote:

opps, sorry, I thought this list was specifically for python-paho. 

I'm using Python.

> You basically have to avoid calling sleep and use timers instead.

this sounds funny asking, but What is a timer?

Years ago I used a gui desktop database app designer,  I dragged a "timer widget" onto the form and type in the function it should call every X amount of time.

I understand what that did, I understand i need something like that, but in this context I have no idea how I should implement it. 

I have a few guesses, like check for events in a loop.  maybe add a sleep so the cpu has some cycles to do other things (no sure what other things are needed, pretty sure incoming network traffic will take priority? also pretty sure this is not a problem I should be trying to solve.   I am sure it has been solved  100's of times.

I also guess "event loop" is a thing I need.  Which I also comprehend and could implement one from scratch if I needed to, but I hope I don't need to.

On Sat, Oct 5, 2019 at 7:32 AM Greg Troxel <gdt@xxxxxxxxxx> wrote:

Carl Karsten <carl@xxxxxxxxxxxxxxxx> writes:

> When the door opens, the light should stay on for 60 seconds.
> or if the button is pressed, turn the light off 30 seconds later.
> every door open resets the 60 seconds.
>
> (this is the simplified version that I think is the root of my problem)

This is not an MQTT issue, but I can see it comes up often in things
that want to use MQTT.

> I can't do "on, sleep(60), off" because that will block the on_button
> (right?)

If the main control program calls sleep, it will ignore all other inputs
for that time.  So don't do that :-)

> and when the button is pressed, if the door opens again that should disable
> the "off in 30" thing.

A further complexity is humans asking for lights to be in certain states
and automatic rules at the same time.  In this case I think you want a
virtual light controlled by the human and a virtual light controlled by
the rules and the real light to be the OR of those.

> Is there a tutorial page or something I should be reading?

This is really a fundamental programming issue; you are constructing a
finite state machine with timers.

https://en.wikipedia.org/wiki/Finite-state_machine

There are arguably two approaches to this sort of thing, usually called
threads and async.   It will seems that threads are easier, but really
they just look easier and async is actually easier, when you define that
as "work to get a 100% reliable system".

Frameworks that are set up for your problem include

  https://nodejs.org/en/about/

  http://docs.libuv.org/en/v1.x/

  https://nodemcu.readthedocs.io/en/master/ (but this is Lua on ESP8266,
  probably not where you should run the controller)


but really you sound like you are starting down the path of slowly
reinventing a home automation system, so have a look at

  https://www.home-assistant.io/

Yes, but this system is 'special' otherwise we wouldn't be making it :p

We might be right. not my call.  What I am sure of is someone will make something better some day.  might be me!

Is this what I should be doing?

https://docs.python.org/3/library/asyncio.html

async def main():
    print('Hello ...')
    await asyncio.sleep(1)
    print('... World!')

asyncio.run(main())

--

Carl K

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Carl K