ESP8266 over the Web: Getting started with IoT System Architecture

Project Goal

The use scenario of this project.

• Connect an Air Quality sensor with ESP8266
• Programming ESP8266 in Lua programming language
• Setup a CoAP server at your host PC
• Print the data on the screen when CoAP server receives the data from ESP8266

Prerequisites

1. Node.js. Note: Node should be with a version above 4.3
2. ESP8266 or NodeMCU development kit
3. An air quality sensor or any other sensor which is the type of analog in

Figure-1: Connect an air quality sensor to a NodeMCU development kit.

Quickstart

Figure-2: The use cases that we're going to achieve in the next two lessons.

Device Side

• Connect your ESP8266 to WiFi AP
• Run coap-air-quality.lua in this project

Server Side

• Run server.js in this project at your host PC

$ npm install
$ export HOST=<server-ip-address>
$ export PORT=<listening-port>
$ npm start

Getting Started

For IoT developers, Devify is a software framework which makes it easy to build IoT server using CoAP and WebSocket. It minimizes the time to write a Hello, World IoT server. The following explains how esp8266-coap-server.js works.

First, it requires devify-server library and get a CoAP server instance.

var server = require('devify-server').coapBroker;

Then, it invokes start() method to start the server.

server.start();

Use onmessage callback

The server.js demo doesn't print any thing on the screen. To print data received from NodeMCU at the terminal console, you need to implement the onmessage callback function.

To print the data sent from NodeMCU, give a onmessage function callback to start().

var onmessage = function(message) {
	// Parse strings to JSON object.
	var obj = JSON.parse(message.data);
	// Print strings.
	console.log('<DATA> ' + message.data);
};

server.start({
	onmessage: onmessage,
});

Once there is an incoming message from NodeMCU, the onmessage function will be called with one parameter of Object type. The data from ESP8266 is stored at the data property. The data from ESP8266 is in JSON string format. To read this data, it should be parsed into Object type.

var obj = JSON.parse(message.data);

This demo is simple. However, it's not the best practice to get data from NodeMCU. The best practice is explained at Understanding Resource-Oriented.

NodeMCU

The full source list is as the following.

-- Configure the ESP as a station (client)
wifi.setmode(wifi.STATION)  
wifi.sta.config("<SSID>", "<PASSWORD>")  
wifi.sta.autoconnect(1)

-- Create a CoAP client
cc = coap.Client()

-- Make a POST request
uri="coap://192.168.0.100:8000/object/12345678/send"

-- Setup a timer to send ADC data
tmr.alarm(0, 8000, 1, function() 
    buf = 
          "{" ..
          "\"quality\":" ..
          adc.read(0) ..
          "}"
    
    cc:post(uri, buf)
    print(buf)
end)

Please modify the uri variable to the real CoAP server URI. Basically, the IP address and the listening port need to be fixed.

The buf variable stores the JSON string (aka JSON document) which includes the air quality data. Finally, the JSON string will be sent to your server over the web via COAP.

Understanding Resource-Oriented

The best practice to achieve the purpose of getting data from NodeMCU is to use coap://192.168.0.100:8000/object/12345678/viewer.

Devify framework will automatically generate this URI for your NodeMCU device. The URI is the resource name of NodeMCU data.

Write Node.js code to read the resource of this URI coap://192.168.0.100:8000/object/12345678/viewer is easy.

var coap = require('coap');

var clientWriable = coap.request('coap://192.168.0.100:8000/object/12345678/viewer');

clientWriable.on('response', function(res) {
    res.pipe(process.stdout)
});

clientWriable.end();

Please save the code under your project folder (esp_air_iot/ at this tutorial) as the file name coap-view-data.js. Then, execute coap-view-data.js by running node coap-view-data.js.

Understanding the URI Design

Devify has a simplified design of WoT framework.

• Take advantage of broker pattern architecture
• Thing modeling (thing description in JSON)
• URI based communications (supoprt CoAP/WebSocket)

The URI style is as the following.

coap://127.0.0.1:8000/object/<ObjectID>/send

• object is the resource name
• <ObjectID> is the unique ID of the resource. Please assign a string for your device.
• send or viewer is the property of this resource. Use send to send data to CoAP server, and viewer to view data from CoAP server

This URI style is defined in the underlying of Devify framework.

Better IoT Programming

Flowchain open source project for flow-based IoT programming is available at Flowchain.

This chapter explains two different programming models.

• One is the monolithic application (aka single process) with conventional if..else... programming model.
• The other is the concept of resource-oriented using URI model.

Flowchain opensource project brings the vision of A new way to write IoT application server code. For better IoT programming with flow-based paradigm, please read Flowchain.

License

MIT