Introduction

This library provides a resilient full duplex communication link between a WiFi connected board and a server on the wired LAN. The board may be an ESP8266, ESP32 or other target including the Pyboard D. The design is such that the code can run for indefinite periods. Temporary WiFi or server outages are tolerated without message loss.

The API is simple and consistent between client and server applications, comprising write and readline methods. The ujson library enables various Python objects to be exchanged. Guaranteed message delivery is available.

This project is a collaboration between Peter Hinch and Kevin Köck.

As of July 2020 it has been updated to use (and require) uasyncio V3. See section 3.1.1 for details of consequent API changes.

0. MicroPython IOT application design

IOT (Internet of Things) systems commonly comprise a set of endpoints on a WiFi network. Internet access is provided by an access point (AP) linked to a router. Endpoints run an internet protocol such as MQTT or HTTP and normally run continuously. They may be located in places which are hard to access: reliability is therefore paramount. Security is also a factor for endpoints exposed to the internet.

Under MicroPython the available hardware for endpoints is limited. Testing has been done on the ESP8266, ESP32 and the Pyboard D.

The ESP8266 remains as a readily available inexpensive device which, with care, is capable of long term reliable operation. It does suffer from limited resources, in particular RAM. Achieving resilient operation in the face of WiFi or server outages is not straightforward: see this document. The approach advocated here simplifies writing robust ESP8266 IOT applications by providing a communications channel with inherent resilience.

The usual arrangement for MicroPython internet access is as below.

Running internet protocols on ESP8266 nodes has the following drawbacks:

1. It can be difficult to ensure resilience in the face of outages of WiFi and of the remote endpoint.
2. Running TLS on the ESP8266 is demanding in terms of resources: establishing a connection can take 30s.
3. There are potential security issues for internet-facing nodes.
4. The security issue creates a requirement periodically to install patches to firmware or to libraries. This raises the issue of physical access.
5. Internet applications can be demanding of RAM.

This document proposes an approach where multiple remote nodes communicate with a local server. This runs CPython or MicroPython code and supports the internet protocol required by the application. The server and the remote nodes communicate using a simple protocol based on the exchange of lines of text. The server can run on a Linux box such as a Raspberry Pi; this can run 24/7 at minimal running cost.

Benefits are:

1. Security is handled on a device with an OS. Updates are easily accomplished.
2. The text-based protocol minimises the attack surface presented by nodes.
3. The protocol is resilient in the face of outages of WiFi and of the server: barring errors in the application design, crash-free 24/7 operation is a realistic prospect.
4. The amount of code running on the remote is smaller than that required to run a resilient internet protocol such as this MQTT version.
5. The server side application runs on a relatively powerful machine. Even minimal hardware such as a Raspberry Pi has the horsepower easily to support TLS and to maintain concurrent links to multiple client nodes. Use of threading is feasible.
6. The option to use CPython on the server side enables access to the full suite of Python libraries including internet modules.

The principal drawback is that in addition to application code on the ESP8266 node, application code is also required on the PC to provide the "glue" linking the internet protocol with each of the client nodes. In many applications this code may be minimal.

There are use-cases where conectivity is entirely local, for example logging locally acquired data or using some nodes to control and monitor others. In such cases no internet protocol is required and the server side application merely passes data between nodes and/or logs data to disk.

This architecture can be extended to non-networked clients such as the Pyboard V1.x. This is described and diagrammed here.

1. Contents

This repo comprises code for resilent full-duplex connections between a server application and multiple clients. Each connection is like a simplified socket, but one which persists through outages and offers guaranteed message delivery.

0. MicroPython IOT application design
1. Contents
2. Design
   2.1 Protocol
3. Files and packages
   3.1 Installation
        3.1.1 Existing users
        3.1.2 Firmware and dependency
        3.1.3 Preconditions for demos
   3.2 Usage
        3.2.1 The main demo
        3.2.2 The remote control demo
        3.2.3 Quality of Service demo
        3.2.4 The fast qos demo
        3.2.5 Troubleshooting the demos
4. Client side applications
   4.1 The Client class
   4.1.1 Initial Behaviour
   4.1.2 Watchdog Timer
5. Server side applications
   5.1 The server module
6. Ensuring resilience Guidelines for application design.
7. Quality of service Guaranteeing message delivery.
   7.1 The qos argument
   7.2 The wait argument Concurrent writes of qos messages.
8. Performance
   8.1 Latency and throughput
   8.2 Client RAM utilisation
   8.3 Platform reliability
9. Extension to the Pyboard
10. How it works
    10.1 Interface and client module
    10.2 Server module

2. Design

The code is asynchronous and based on asyncio. Client applications on the remote import client.py which provides the interface to the link. The server side application uses server.py.

Messages are required to be complete lines of text. They typically comprise an arbitrary Python object encoded using JSON. The newline character ('\n') is not allowed within a message but is optional as the final character.

Guaranteed message delivery is supported. This is described in section 7. Performance limitations are discussed in section 8.

2.1 Protocol

Client and server applications use readline and write methods to communicate: in the case of an outage of WiFi or the connected endpoint, the method will pause until the outage ends. While the system is tolerant of runtime server and WiFi outages, this does not apply on initialisation. The server must accessible before clients are started.

The link status is determined by periodic exchanges of keepalive messages. This is transparent to the application. If a keepalive is not received within a user specified timeout an outage is declared. On the client the WiFi is disconnected and a reconnection procedure is initiated. On the server the connection is closed and it awaits a new connection.

Each client has a unique ID which is an arbitrary string. In the demo programs this is stored in local.py. The ID enables the server application to determine which physical client is associated with an incoming connection.

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3. Files and packages

This repo has been updated for uasyncio V3. This is incorporated in daily builds of firmware and will be available in release builds later than V1.12. Server code may be run under CPython V3.8 or above. It may be run under MicroPython (Unix build), but at the time of writing this requires this fix to incorporate uasyncio.

Directory iot:

1. client.py / client.mpy Client module. The ESP8266 has insufficient RAM to compile client.py so the precompiled client.mpy should be used. See note below.
2. server.py Server module. (runs under CPython 3.5+ or MicroPython 1.10+). Directory iot/primitives:
3. __init__.py Functions common to Client and Server.
4. switch.py Debounced switch interface. Used by remote demo. Optional directories containing Python packages:
5. iot/examples A simple example. Up to four clients communicate with a single server instance.
6. iot/remote Demo uses the library to enable one client to control another. This may need adapting for your hardware.
7. iot/qos Demonstrates and tests the qos (quality of service) feature, see Quality of service.
8. iot/pb1 Contians packages enabling a Pyboard V1.x to communicate with the server via an ESP8266 connected by I2C. See documentation.

NOTE: The file client.mpy works with daily builds at the time of writing. The bytecode format changes occasionally. If an application throws a bytecode error it is necessary to cross-compile client.py with the associated version of mpy-cross. Or raise an issue and I will post an update.

3.1 Installation

This section describes the installation of the library and the demos. The ESP8266 has limited RAM: there are specific recommendations for installation on that platform.

3.1.1 Existing users

It is recommended to remove the old version and re-install as below.

There have been API changes to accommodate the new uasyncio version: the event loop argument is no longer required or accepted in Client and Server constructors. The directory structure has changed, requiring minor changes to import statements.

3.1.2 Firmware and dependency

On ESP8266, RAM can be saved by building firmware from source, freezing client.py as bytecode. If this is not done, it is necessary to cross compile client.py. The file client.mpy is provided for those unable to do this. If freezing, create an iot directory in your modules directory and copy iot/client.py and the directory iot/primitives and contents there.

Pre-requisites: firmware must be a current daily build or a release build after V1.12. If upgrading, particularly on an ESP8266, it is wise to erase flash prior to installtion. In particular this will ensure the use of littlefs with its associated RAM saving.

This repository is a python package, consequently on the client the directory structure must be retained. The following installs all demos on the target.

On your PC move to a directory of your choice and clone the repository there:

git clone https://github.com/peterhinch/micropython-iot

Installation consists of copying the iot directory and contents to an iot directory on the boot device. On ESP8266 or ESP32 the boot device is/pyboard. On the Pyboard D it will be /flash or /sd depending on whether an SD card is fitted.

Copying may be done using any tool but I recommend rshell. If this is used start in the directory on your PC containing the clone, start rshell and issue (adapting the boot device for your platform):

rsync iot /pyboard/iot

On ESP8266, unless frozen, it is necessary to delete client.py to force the use of client.mpy:

rm /pyboard/iot/client.py

3.1.3 Preconditions for demos

The demo programs store client configuration data in a file local.py. Each demo has its own local.py located in the directory of the demo code. This contains the following constants which should be edited to match local conditions. Remove the use_my_local hack designed for my WiFi privacy.:

MY_ID = '1'  # Client-unique string.
SERVER = '192.168.0.10'  # Server IP address.
SSID = 'use_my_local'  # Insert your WiFi credentials
PW = 'PASSWORD'
PORT = 8123
TIMEOUT = 2000
# The following may be deleted
if SSID == 'use_my_local':
    from iot.examples.my_local import *

The ESP8266 can store WiFi credentials in flash memory. If desired, ESP8266 clients can be initialised to connect to the local network prior to running the demos. In this case the SSID and PW variables may optionally be empty strings (SSID = '').

Note that the server-side examples below specify python3 in the run command. In every case micropython may be substituted to run under the Unix build of MicroPython.

3.2 Usage

3.2.1 The main demo

This illustrates up to four clients communicating with the server. The demo expects the clients to have ID's in the range 1 to 4: if using multiple clients edit each one's local.py accordingly.

On the server navigate to the parent directory of iot and run:

python3 -m iot.examples.s_app_cp

or

micropython -m iot.examples.s_app_cp

On each client run:

import iot.examples.c_app

3.2.2 The remote control demo

This shows one ESP8266 controlling another. The transmitter should have a pushbutton between GPIO 0 and gnd, both should have an LED on GPIO 2.

On the server navigate to the parent directory of iot and run:

python3 -m iot.remote.s_comms_cp

or

micropython -m iot.remote.s_comms_cp

On the esp8266 run (on transmitter and receiver respectively):

import iot.remote.c_comms_tx
import iot.remote.c_comms_rx

3.2.3 Quality of Service demo

This test program verifies that each message (in each direction) is received exactly once. On the server navigate to the parent directory of iot and run:

python3 -m iot.qos.s_qos_cp

or

micropython -m iot.qos.s_qos_cp

On the client, after editing /pyboard/qos/local.py, run:

import iot.qos.c_qos

3.2.4 The fast qos demo

This tests the option of concurrent qos writes. This is an advanced feature discussed in section 7.1. To run the demo, on the server navigate to the parent directory of iot and run:

python3 -m iot.qos.s_qos_fast

or

micropython -m iot.qos.s_qos_fast

On the client, after editing /pyboard/qos/local.py, run:

import iot.qos.c_qos_fast

3.2.5 Troubleshooting the demos

If local.py specifies an SSID, on startup the demo programs will pause indefinitely if unable to connect to the WiFi. If SSID is an empty string the assumption is an ESP8266 with stored credentials; if this fails to connect an OSError will be thrown. An OSError will also be thrown if initial connectivity with the server cannot be established.

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4. Client side applications

A client-side application instantiates a Client and launches a coroutine which awaits it. After the pause the Client has connected to the server and communication can begin. This is done using Client.write and Client.readline methods.

Every client ha a unique ID (MY_ID) typically stored in local.py. The ID comprises a string subject to the same constraint as messages:

Messages comprise a single line of text; if the line is not terminated with a newline ('\n') the client library will append it. Newlines are only allowed as the last character. Blank lines will be ignored.

A basic client-side application has this form:

import uasyncio as asyncio
import ujson
from iot import client
import local  # or however you configure your project

class App:
    def __init__(self, verbose):
        self.cl = client.Client(local.MY_ID, local.SERVER,
                                local.PORT, local.SSID, local.PW,
                                local.TIMEOUT, conn_cb=self.state, 
                                verbose=verbose)
        asyncio.create_task(self.start())

    async def start(self):
        await self.cl  # Wait until client has connected to server
        asyncio.create_task(self.reader())
        await self.writer()  # Wait forever

    def state(self, state):  # Callback for change in connection status
        print("Connection state:", state)

    async def reader(self):
        while True:
            line = await self.cl.readline()  # Wait until data received
            data = ujson.loads(line)
            print('Got', data, 'from server app')

    async def writer(self):
        data = [0, 0]
        count = 0
        while True:
            data[0] = count
            count += 1
            print('Sent', data, 'to server app\n')
            await self.cl.write(ujson.dumps(data))
            await asyncio.sleep(5)

    def close(self):
        self.cl.close()

app = None
async def main():
    global app  # For closure by finally clause
    app = App(True)
    await app.start()  # Wait forever

try:
    asyncio.run(main())
finally:
    app.close()  # Ensure proper shutdown e.g. on ctrl-C
    asyncio.new_event_loop()

If an outage of server or WiFi occurs, the write and readline methods will pause until connectivity has been restored. The server side API is similar.

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4.1 The Client class

The constructor has a substantial number of configuration options but in many cases defaults may be accepted for all but the first five.

Constructor args:

1. my_id The client id.
2. server The server IP-Adress to connect to.
3. port=8123 The port the server listens on.
4. ssid='' WiFi SSID. May be blank for ESP82666 with credentials in flash.
5. pw='' WiFi password.
6. timeout=2000 Connection timeout in ms. If a connection is unresponsive for longer than this period an outage is assumed.
7. conn_cb=None Callback or coroutine that is called whenever the connection changes.
8. conn_cb_args=None Arguments that will be passed to the connected_cb callback. The callback will get these args preceeded by a bool indicating the new connection state.
9. verbose=False Provides optional debug output.
10. led=None If a Pin instance is passed it will be toggled each time a keepalive message is received. Can provide a heartbeat LED if connectivity is present. On Pyboard D a Pin or LED instance may be passed.
11. wdog=False If True a watchdog timer is created with a timeout of 20s. This will reboot the board if it crashes - the assumption is that the application will be restarted via main.py.

Methods (asynchronous):

1. readline No args. Pauses until data received. Returns a line.
2. write Args: buf, qos=True, wait=True. buf holds a line of text.
   If qos is set, the system guarantees delivery. If it is clear messages may (rarely) be lost in the event of an outage.
   The wait arg determines the behaviour when multiple concurrent writes are launched with qos set. See Quality of service.

The following asynchronous methods are described in Initial Behaviour below. In most cases they can be ignored.

3. bad_wifi
4. bad_server

Methods (synchronous):

1. status Returns True if connectivity is present. May also be read using function call syntax (via __call__).
2. close Closes the socket. Should be called in the event of an exception such as a ctrl-c interrupt. Also cancels the WDT in the case of a software WDT.

Bound variable:

1. connects The number of times the Client instance has connected to WiFi. This is maintained for information only and provides some feedback on the reliability of the WiFi radio link.

The Client class is awaitable. If

await client_instance

is issued, the coroutine will pause until connectivity is (re)established.

Applications which always await the write method do not need to check or await the client status: write will pause until it can complete. If write is launched using create_task it is essential to check status otherwise during an outage unlimited numbers of coroutines will be created.

The client buffers up to 20 incoming messages. To avoid excessive queue growth applications should have a single coroutine which spends most of its time awaiting incoming data.

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4.1.1 Initial Behaviour

When an application instantiates a Client it attemps to connect to WiFi and then to the server. Initial connection is handled by the following Client asynchronous bound methods (which may be modified by subclassing):

1. bad_wifi No args.
2. bad_server No args. Awaited if server refuses an initial connection.

Note that, once a server link has been initially established, these methods will not be called: reconnection after outages of WiFi or server are automatic.

The bad_wifi coro attempts to connect using the WiFi credentials passed to the constructor. This will pause until a connection has been achieved. The bad_server coro raises an OSError. Behaviour of either of these may be modified by subclassing.

Platforms other than ESP8266 launch bad_wifi unconditionally on startup. In the case of an ESP8266 which has WiFi credentials stored in flash it will first attempt to connect using that data, only launching bad_wifi if this fails in a timeout period. This is to minimise flash wear.

4.1.2 Watchdog Timer

This option provides a last-ditch protection mechanism to keep a client running in the event of a crash. The ESP8266 can (rarely) crash, usually as a result of external electrical disturbance. The WDT detects that the Client code is no longer running and issues a hard reset. Note that this implies a loss of program state. It also assumes that main.py contains a line of code which will restart the application.

Debugging code with a WDT can be difficult because bugs or software interrupts will trigger unexpected resets. It is recommended not to enable this option until the code is stable.

On the ESP8266 the WDT uses a sofware timer: it can be cancelled which simplifies debugging. See examples/c_app.py for the use of the close method in a finally clause.

The WDT on the Pyboard D is a hardware implementation: it cannot be cancelled. It may be necessary to use safe boot to bypass main.py to access the code.

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5. Server side applications

A typical example has an App class with one instance per physical client device. This enables instances to share data via class variables. Each instance launches a coroutine which acquires a Connection instance for its individual client (specified by its client_id). This process will pause until the client has connected with the server. Communication is then done using the readline and write methods of the Connection instance.

Messages comprise a single line of text; if the line is not terminated with a newline (\n) the server library will append it. Newlines are only allowed as the last character. Blank lines will be ignored.

A basic server-side application has this form:

import asyncio
import json
from iot import server
import local  # or however you want to configure your project

class App:
    def __init__(self, client_id):
        self.client_id = client_id  # This instance talks to this client
        self.conn = None  # Will be Connection instance
        self.data = [0, 0, 0]  # Exchange a 3-list with remote
        asyncio.create_task(self.start())

    async def start(self):
        # await connection from the specific EP8266 client
        self.conn = await server.client_conn(self.client_id)
        asyncio.create_task(self.reader())
        asyncio.create_task(self.writer())

    async def reader(self):
        while True:
            # Next line will pause for client to send a message. In event of an
            # outage it will pause for its duration.
            line = await self.conn.readline()
            self.data = json.loads(line)
            print('Got', self.data, 'from remote', self.client_id)

    async def writer(self):
        count = 0
        while True:
            self.data[0] = count
            count += 1
            print('Sent', self.data, 'to remote', self.client_id, '\n')
            await self.conn.write(json.dumps(self.data))  # May pause in event of outage
            await asyncio.sleep(5)

async def main():
    clients = {1, 2, 3, 4}
    apps = [App(n) for n in clients]  # Accept 4 clients with ID's 1-4
    await server.run(clients, True, local.PORT, local.TIMEOUT)  # Verbose

def run():
    try:
        asyncio.run(main())
    except KeyboardInterrupt:  # Delete this if you want a traceback
        print('Interrupted')
    finally:
        server.Connection.close_all()
        asyncio.new_event_loop()

if __name__ == "__main__":
    run()

5.1 The server module

Server-side applications should create and run a server.run task. This runs forever and takes the following args:

1. expected A set of expected client ID strings.
2. verbose=False If True output diagnostic messages.
3. port=8123 TCP/IP port for connection. Must match clients.
4. timeout=2000 Timeout for outage detection in ms. Must match the timeout of all Client instances.

The expected arg causes the server to produce a warning message if an unexpected client connects, or if multiple clients have the same ID (this will cause tears before bedtime).

The module is based on the Connection class. A Connection instance provides a communication channel to a specific client. The Connection instance for a given client is a singleton and is acquired by issuing

conn = await server.client_conn(client_id)

This will pause until connectivity has been established. It can be issued at any time: if the Connection has already been instantiated, that instance will be returned. The Connection constructor should not be called by applications.

The Connection instance

Methods (asynchronous):

1. readline No args. Pauses until data received. Returns a line.
2. write Args: buf, qos=True, wait=True. buf holds a line of text.
   If qos is set, the system guarantees delivery. If it is clear messages may (rarely) be lost in the event of an outage.__ The wait arg determines the behaviour when multiple concurrent writes are launched with qos set. See Quality of service.

Methods (synchronous):

1. status Returns True if connectivity is present. The connection state may also be retrieved using function call syntax (via .__call__).
2. __getitem__ Enables the Connection of another client to be retrieved using list element access syntax. Will throw a KeyError if the client is unknown (has never connected).

Class Method (synchronous):

1. close_all No args. Closes all sockets: call on exception (e.g. ctrl-c).

Bound variable:

1. nconns Maintains a count of (re)connections for information or monitoring of outages.

The Connection class is awaitable. If

await connection_instance

is issued, the coroutine will pause until connectivity is (re)established.

Applications which always await the write method do not need to check or await the server status: write will pause until it can complete. If write is launched using create_task it is essential to check status otherwise during an outage unlimited numbers of coroutines will be created.

The server buffers incoming messages but it is good practice to have a coro which spends most of its time waiting for incoming data.

Server module coroutines:

1. run Args: expected verbose=False port=8123 timeout=2000 This is the main coro and starts the system. expected is a set containing the ID's of all clients.
   verbose causes debug messages to be printed.
   port is the port to listen to.
   timeout is the number of ms that can pass without a keepalive until the connection is considered dead.
2. client_conn Arg: client_id. Pauses until the sepcified client has connected. Returns the Connection instance for that client.
3. wait_all Args: client_id=None peers=None. See below.

The wait_all coroutine is intended for applications where clients communicate with each other. Typical user code cannot proceed until a given set of clients have established initial connectivity.

wait_all, where a client_id is specified, behaves as client_conn except that it pauses until further clients have also connected. If a client_id is passed it will returns that client's Connection instance. If None is passed the assumption is that the current client is already connected and the coro returns None.

The peers argument defines which clients it must await: it must either be None or a set of client ID's. If a set of client_id values is passed, it pauses until all clients in the set have connected. If None is passed, it pauses until all clients specified in run's expected set have connected.

It is perhaps worth noting that the user application can impose a timeout on this by means of asyncio.wait_for.

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6. Ensuring resilience

There are two principal ways of provoking LmacRxBlk errors and crashes.

1. Failing to close sockets when connectivity is lost.
2. Feeding excessive amounts of data to a socket after connectivity is lost: this causes an overflow to an internal ESP8266 buffer.

These modules aim to address these issues transparently to application code, however it is possible to write applications which violate 2.

There is a global TIMEOUT value defined in local.py which should be the same for the server and all clients. Each end of the link sends a keepalive (KA) packet (an empty line) at a rate guaranteed to ensure that at least one KA will be received in every TIMEOUT period. If it is not, connectivity is presumed lost and both ends of the interface adopt a recovery procedure.

If an application always awaits a write with qos==True there is no risk of Feeding excess data to a socket: this is because the coroutine does not return until the remote endpoint has acknowledged reception.

On the other hand if multiple messages are sent within a timeout period with qos==False there is a risk of buffer overflow in the event of an outage.

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7. Quality of service

In the presence of a stable WiFi link TCP/IP should ensure that packets sent are received intact. In the course of extensive testing with the ESP8266 we found that (very rarely) packets were lost. It is not known whether this behavior is specific to the ESP8266. Another mechanism for message loss is the case where a message is sent in the interval between an outage occurring and it being detected. This is likely to occur on all platforms.

The client and server modules avoid message loss by the use of acknowledge packets: if a message is not acknowledged within a timeout period it is retransmitted. This implies duplication where the acknowledge packet is lost. Receive message de-duplication is employed to provide a guarantee that the message will be delivered exactly once. While delivery is guaranteed, timeliness is not. Messages are inevitably delayed for the duration of a WiFi or server outage where the write coroutine will pause for the duration.

Guaranteed delivery involves a tradeoff against throughput and latency. This is managed by optional arguments to .write, namely qos=True and wait=True.

7.1 The qos argument

Message integrity is determined by the qos argument. If False message delivery is not guaranteed. A use-case for disabling qos is in applications such as remote control. If the user presses a button and nothing happens they would simply repeat the action. Such messages are always sent immediately: the application should limit the rate at which they can be sent, particularly on ESP8266 clients, to avoid risk of buffer overflow.

With qos set, the message will be delivered exactly once.

Where successive qos messages are sent there may be a latency issue. By default the transmission of a qos message will be delayed until reception of its predecessor's acknowledge. Consequently the write coroutine will pause, introducing latency. This serves two purposes. Firstly it ensures that messages are received in the order in which they were sent (see below).

Secondly consider the case where an outage has occurred but has not yet been detected. The first message is written, but no acknowledge is received. Subsequent messages are delayed, precluding the risk of ESP8266 buffer overflows. The interface resumes operation after the outage has cleared.

7.2 The wait argument

This default can be changed with the wait argument to write. If False a qos message will be sent immediately, even if acknowledge packets from previous messages are pending. Applications should be designed to limit the number of such qos messages sent in quick succession: on ESP8266 clients buffer overflows can occur.

In testing in 2019 the ESP32 was not resilient under these circumstances; this appears to have been fixed in current firmware builds. Nevertheless setting wait=False potentially risks resilience. If used, applications should be tested to verify quality of service in the presence of WiFi outages.

If messages are sent with wait=False there is a chance that they may not be received in the order in which they were sent. As described above, in the event of qos message loss, retransmission occurs after a timeout period has elapsed. During that timeout period the application may have successfully sent another non-waiting qos message resulting in out of order reception.

The demo programs qos/c_qos_fast.py (client) and qos/s_qos_fast.py issue four write operations with wait=False in quick succession. This number is probably near the maximum on an ESP8266. Note the need explicitly to check for connectivity before issuing the write: this is to avoid spawning large numbers of coroutines during an outage.

In summary specifying wait=False should be considered an "advanced" option requiring testing to prove that resilence is maintained.

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8. Performance

8.1 Latency and throughput

The interface is intended to provide low latency: if a switch on one node controls a pin on another, a reasonably quick response can be expected. The link is not designed for high throughput because of the buffer overflow issue discussed in section 6. This is essentially a limitation of the ESP8266 device: more agressive use of the wait arg may be possible on platforms such as the Pyboard D.

In practice latency on the order of 100-200ms is normal; if an outage occurs latency will inevitably persist for the duration.

TIMEOUT

This defaults to 2s. On Client it is a constructor argument, on the server it is an arg to server.run. Its value should be common to all clients and the sever. It determines the time taken to detect an outage and the frequency of keepalive packets. This time was chosen on the basis of measured latency periods on WiFi networks. It may be increased at the expense of slower outage detection. Reducing it may result in spurious timeouts with unnecessary WiFi reconnections.

8.2 Client RAM utilisation

On ESP8266 with a current (June 2020) daily build the demo reports over 20KB free. Free RAM of 25.9KB was achieved with compiled firmware with frozen bytecode as per Installation.

8.3 Platform reliability

In extensive testing the Pyboard D performed impeccably: no failures of any kind were observed in weeks of testing through over 1000 outages.

ESP32 was prone to occasional spontaneous reboots. It would typically run for a few days through multiple WiFi outages before rebooting.

ESP8266 still occasionally crashes and it is recommended to use the watchdog feature to reboot it should this occur.

It would take a very long time to achieve more than a subjective impression of the effectof usage options on failure rate. The precautionary principle suggests maximising free ram with frozen bytecode on ESP8266 and avoiding concurrent qos==1 writes on ESPx platforms.

Contents

9. Extension to the Pyboard

This extends the resilient link to MicroPython targets lacking a network interface; for example the Pyboard V1.x. Connectivity is provided by an ESP8266 running a fixed firmware build: this needs no user code.

The interface between the Pyboard and the ESP8266 uses I2C and is based on the existing I2C module.

Resilient behaviour includes automatic recovery from WiFi and server outages; also from ESP8266 crashes.

See documentation.

10. How it works

10.1 Interface and client module

The client module was designed on the expectation that client applications will usually be simple: acquiring data from sensors and periodically sending it to the server and/or receiving data from the server and using it to control devices. Developers of such applications probably don't need to be concerned with the operation of the module.

There are ways in which applications can interfere with the interface's operation either by blocking or by attempting to operate at excessive data rates. Such designs can produce an erroneous appearance of poor WiFi connectivity.

Outages are detected by a timeout of the receive tasks at either end. Each peer sends periodic keepalive messages consisting of a single newline character, and each peer has a continuously running read task. If no message is received in the timeout period (2s by default) an outage is declared.

From the client's perspective an outage may be of the WiFi or the server. In practice WiFi outages are more common: server outages on a LAN are typically caused by the developer testing new code. The client assumes a WiFi outage. It disconnects from the network for long enough to ensure that the server detects the outage. It then attempts repeatedly to reconnect. When it does so, it checks that the connection is stable for a period (it might be near the limit of WiFi range).

If this condition is met it attempts to reconnect to the server. If this succeeds the client runs. Its status becomes True when it first receives data from the server.

A client or server side application which blocks or hogs processor time can prevent the timely transmission of keepalive messages. This will cause the server to declare an outage: the consequence is a sequence of disconnect and reconnect events even in the presence of a strong WiFi signal.

10.2 Server module

Server-side applications communicate via a Connection instance. This is unique to a client. It is instantiated when a specified client first connects and exists forever. During an outage its status becomes False for the duration. The Connection instance is retrieved as follows, with the client_conn method pausing until initial connectivity has been achieved:

import server
# Class details omitted
    self.conn = await server.client_conn(self.client_id)

Each client must have a unique ID. When the server detects an incoming connection on the port it reads the client ID from the client. If a Connection instance exists for that ID its status is updated, otherwise a Connection is instantiated.

The Connection has a continuously running coroutine ._read which reads data from the client. If an outage occurs it calls the ._close method which closes the socket, setting the bound variable ._sock to None. This corresponds to a False status. The ._read method pauses until a new connection occurs. The aim here is to read data from ESP8266 clients as soon as possible to minimise risk of buffer overflows.

The Connection detects an outage by means of a timeout in the ._read method: if no data or keepalive is received in that period an outage is declared, the socket is closed, and the Connection status becomes False.

The Connection has a ._keepalive method. This regularly sends keepalive messages to the client. Application code which blocks the scheduler can cause this not to be scheduled in a timely fashion with the result that the client declares an outage and disconnects. The consequence is a sequence of disconnect and reconnect events even in the presence of a strong WiFi signal.