8000 Update README.rstmicropython/umqtt.robust/README.rst: Easing document readability. by Naidile-P-N · Pull Request #1009 · micropython/micropython-lib · GitHub
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38 changes: 19 additions & 19 deletions micropython/umqtt.robust/README.rst
Original file line number Diff line number Diff line change
Expand Up @@ -16,15 +16,15 @@ there's no power (mains outage or battery ran out). As you may imagine,
umqtt.robust won't help you with your flat battery. Most computing
systems are now networked, and communication is another weak link.
This is especially true for wireless communications. If two of your
systems can't connect reliably communicate via WiFi, umqtt.robust
systems can't connect or reliably communicate over WiFi, umqtt.robust
can't magically resolve that (but it may help with intermittent
WiFi issues).

What umqtt.robust tries to do is very simple - if while trying to
perform some operation, it detects that connection to MQTT breaks,
it tries t 10000 o reconnect to it. That's good direction towards "robustness",
but the problem that there is no single definition of what "robust"
is. Let's consider following usecase:
perform some operation, it detects that the connection to MQTT breaks,
it tries to reconnect. That's a good direction towards "robustness",
but the problem is that there no single definition of what "robust"
is. Let's consider the following usecases:

1. A temperature reading gets transmitted once a minute. Then the
best option in case of a transmission error might be not doing
Expand All @@ -35,11 +35,11 @@ battery-powered, any connection retries will just drain battery and
make device "less robust" (it will run out of juice sooner and more
unexpectedly, which may be a criteria for "robustness").

2. If there's a button, which communicates its press event, then
perhaps it's really worth to retry to deliver this event (a user
2. If there's a button which communicates it's press event, then
perhaps it's really worth it to retry delivering this event (a user
expects something to happen when they press the button, right?).
But if a button is battery-power, unconstrained retries won't do
much good still. Consider mains power outage for several hours,
But if a button is on battery-power, unconstrained retries still won't do
much good. Consider mains power outage for several hours,
MQTT server down all this time, and battery-powered button trying
to re-publish event every second. It will likely drain battery
during this time, which is very non-robust. Perhaps, if a press
Expand All @@ -48,10 +48,10 @@ on what press does, the above may be good for a button turning
on lights, but not for something else!)

3. Finally, let's consider security sensors, like a window broken
sensor. That's the hardest case. Apparently, those events are
sensor. That's the hardest case. Those events are
important enough to be delivered no matter what. But if done with
short, dumb retries, it will only lead to quick battery drain. So,
a robust device would retry, but in smart manner, to let battery
a robust device would retry, but in a smart manner to let the battery
run for as long as possible, to maximize the chance of the message
being delivered.

Expand All @@ -61,19 +61,19 @@ a) There's no single definition of what "robust" is. It depends on
a particular application.
b) Robustness is a complex measure, it doesn't depend on one single
feature, but rather many different features working together.
Consider for example that to make button from the case 2 above
Consider for example that to make the button from the case 2 above
work better, it would help to add a visual feedback, so a user
knew what happens.
knows what happened.

As you may imagine, umqtt.robust doesn't, and can't, cover all possible
"robustness" scenarios, nor it alone can make your MQTT application
"robust". Rather, it's a barebones example of how to reconnect to an
MQTT server in case of a connection error. As such, it's just one
of many steps required to make your app robust, and majority of those
of the many steps required to make your app robust, and majority of those
steps lie on *your application* side. With that in mind, any realistic
application would subclass umqtt.robust.MQTTClient class and override
application would inherit umqtt.robust.MQTTClient class and override
delay() and reconnect() methods to suit particular usage scenario. It
may even happen that umqtt.robust won't even suit your needs, and you
may even be possible that umqtt.robust doesn't suit your needs, and you
will need to implement your "robust" handling from scratch.


Expand All @@ -82,18 +82,18 @@ Persistent and non-persistent MQTT servers

Consider an example: you subscribed to some MQTT topics, then connection
went down. If we talk "robust", then once you reconnect, you want any
messages which arrived when the connection was down, to be still delivered
messages which arrived when the connection was down, to still be delivered
to you. That requires retainment and persistency enabled on MQTT server.
As umqtt.robust tries to achieve as much "robustness" as possible, it
makes a requirement that the MQTT server it communicates to has persistency
enabled. This include persistent sessions, meaning that any client
subscriptions are retained across disconnect, and if you subscribed once,
subscriptions are retained on disconnectivity, and if you subscribed once,
you no longer need to resubscribe again on next connection(s). This makes
it more robust, minimizing amount of traffic to transfer on each connection
(the more you transfer, the higher probability of error), and also saves
battery power.

However, not all broker offer true, persistent MQTT support:
However, not all brokers offer true, persistent MQTT support:

* If you use self-hosted broker, you may need to configure it for
persistency. E.g., a popular open-source broker Mosquitto requires
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