In our previous post we talked about a general architecture of an RFID-based asset tracking system and, specifically, about an API server in the heart of such a system. Now we are going to take a look at the remote nodes collecting the data that comes from RFID readers and sending this data through some API to the database. Let's create our proper RFID gateway server software with Node.js.
The hardware part didn't change a lot since the previous article, though our diagram will be scoped on the peripherals now:
On the software side, our goal will be to leverage asynchronous design patterns forming the basis of Node.js API: event emitters and streams.
Since we can (and most likely would in the real life) have multiple RFID
readers connected to the same gateway, we have to be able to consume data from
several reader modules independently. Moreover, these modules can correspond /
talk to the different types of hardware while reporting to the same gateway.
Finally, in order to unload the main process and take advantage of multi-
processor / core architectures we are putting those reader modules in separate
child processes. These processes
are piping
the data to the Listener module of the parent process, which in its turn
analyses and transforms (if needed) the data and pipes it further to the
Uploader module. The latter is the one transmitting the results to the cloud
via HTTP or some other protocol.
We are assuming further that you already have Node.js and
npm installed and node / npm commands are
accessible. Let's create a new package for our RFID gateway by typing
npm init once we are in the target folder. After answering a few questions
(just use the defaults for now), we will have our package.json created.
Next, let's assume we have a file config.json containing some configuration
for our gateway in the root folder of our package. Then the entry point for
our application (./index.js) can look like this (the complete code of the
implementation is available
here):
'use strict';
/**
* Dependencies
*/
var config = require('./config.json');
var Gateway = require('./lib/gateway');
module.exports = new Gateway(config);
Obviously, this is a bit simplyfied, and in real life we should always check
if the configuration file exists, etc., but here we will omit these
verifications for sake of simplicity. Also, at this point we don't even care
about the exact content of config.json, as long as it contains all the data
necessary to connect to our RFID readers (e.g. ports and reader types) and to
transmit the information to the cloud (e.g. endpoints, authentication
credentials, etc.).
Gateway module is the core of our gateway's master process. Architectured as
an event emitter, it handles the system signals, deployment-related events,
creation of scanning and uploading objects, etc. Here is how it looks like
(this code goes to ./lib/gateway.js):
'use strict';
/**
* Dependencies
*/
var EventEmitter = require('events').EventEmitter;
var util = require('util')
var async = require('async');
var _ = require('lodash');
var Uploader = require('./uploader');
var Listener = require('./listener');
var defaults = require('./defaults.json');
module.exports = Gateway;
// Gateway module
function Gateway(config) {
if (!(this instanceof Gateway))
return new Gateway(config);
EventEmitter.call(this);
// Configuration validation, defaults, etc.
config = config || {};
_.defaults(config, defaults);
// Instantiate a listener and an uploader
var uploader = new Uploader(config['uploader']);
var listener = new Listener(config['listener']);
// Deployment-related events: online message
async.each([listener, uploader], function(stream, cb) {
stream.on('error', function(err) {
console.error(err);
process.send('shutdown');
});
stream.on('ready', function() {
cb();
});
}, function(err) {
if (err) {
console.error(err);
process.send('shutdown');
}
process.send('online');
});
// Deployment-related events: shutdown message
process.on('message', function(message) {
if (message === 'shutdown') {
async.each([listener, uploader], function(stream, cb) {
stream.emit('shutdown');
stream.on('close', function() {
cb();
});
}, function(err) {
process.exit((err) ? 1 : 0);
});
}
});
// Let's get it started
listener.pipe(uploader);
}
util.inherits(Gateway, EventEmitter);
Some key sections of this constructor include:
- configuration validation (feel free to validate more thoroughly based on your specific case), applying defaults;
- creation of
ListenerandUploaderinstances; - emitting
onlineevent once both listener and uploader are ready (this is necessary in order to provide zero-downtime deployment, with Naught, for example); - listening for the
shutdownmessage from our deployer in order to gracefully close all the connections and stop the process.
Finally, thanks to stream-based architecture we used, starting it all becomes as simple as piping our listener to our uploader:
listener.pipe(uploader);
Now, let's take a closer look at the components responsible for reading the data and transmitting it upstream.
As we saw on the software architecture diagram, Listener module object
is a transform stream, which means it is writable and readable (and can
be piped from and to). Since the current version of Node.js streams (as
of v0.10, so called "streams2") operates on strings and buffers by
default, we need to put our Listener stream in the object mode if we
want it to emit generic JavaScript values. So, we have to call its
parent constructor like this:
Transform.call(this, {objectMode: true});
Next, we need to fork child processes for different RFID readers
attached to our gateway (as listed in the configuration file). Using
async library, we make sure all the devices are activated (see
attach function) and then emit ready event on our Listener
object. The whole module we will have in ./lib/listener.js is listed
below:
'use strict';
/**
* Dependencies
*/
var Transform = require('stream').Transform;
var util = require('util');
var fork = require('child_process').fork;
var async = require('async');
var split2 = require('split2');
module.exports = Listener;
function Listener(config) {
var that = this;
config = config || {};
config['devices'] = config['devices'] || [];
Transform.call(this, {objectMode: true});
async.each(config['devices'], function(deviceConfig, cb) {
that.attach(deviceConfig, function(err) {
cb(err);
});
}, function(err) {
if (err)
return that.emit('error', err);
that.emit('ready');
});
}
util.inherits(Listener, Transform);
Listener.prototype._transform = function(message, enc, next) {
// We can do some preprocessing here
this.push(message, enc);
next();
};
Listener.prototype.attach = function(config, done) {
var that = this;
var device = fork(config.module, [], {
silent: true
});
device.send({
event: 'connect',
config: config
});
device.on('message', function(message) {
switch (message.event) {
case 'ready':
done();
break;
case 'error':
that.emit('error', message.error);
break;
default:
break;
}
});
device.stdout.pipe(split2(JSON.parse)).pipe(this);
};
Here, attach method receives a configuration object
containing all the information necessary to connect to the
reader and start scanning, including the module property,
which is a path to the reader driver module to be forked.
Note, that we are forking this child process with silent
option enabled. It means that stdout stream of this
process will be piped to device.stdout, which, in its
turn, we are piping to our very Listener stream through
another transform stream, split2(JSON.parse). What the
latter does, it takes the chunks coming from this stdout
stream (which is not in the object mode, i.e. can be
sending fragmented output), assembles them into complete
lines and parses into JSON objects that are being pushed
further.
This processing procedure defines the following principles that have to be used while architecturing reader modules:
- reader module has to print the scanning results as
stringified JSON objects, one per line, to its
stdout; - if an error occurres, the reader process should send a
message object with
eventproperty equal toerror(this message will be transmitted upstream to the gateway module).
Also, as you can notice, _transform function (necessary for
implementation of a transform stream) doesn't do a lot in our
case simply pushing further all the messages it receives, but
at the same time it could be used to do some preprocessing
(and/or error handling). For example, we could cache the
messages, analyze the new input and push it only in case of
some specific changes we are tracking.
Finally, after all these manipulations we have our data
flowing into the Uploader object, which is a writable
stream with the object mode enabled. That means it
receives JavaScript objects sent by the reader driver
modules. Since the implementation of a specific approach
to sending the data to remote servers falls beyond the
scope of this article, we will just log the incoming
messages to the console. The code below goes to
./lib/uploader.js:
'use strict';
/**
* Dependencies
*/
var Writable = require('stream').Writable;
var util = require('util');
module.exports = Uploader;
function Uploader(config) {
Writable.call(this, {objectMode: true});
}
util.inherits(Uploader, Writable);
Uploader.prototype._write = function(message, enc, next) {
console.info('Sending: ' + JSON.stringify(message));
// Sending to the cloud here
next();
};
One of the most important subjects we haven't looked
into yet is the reader modules. The main reason is the
fact that any specific implementation of such a module
would rely heavily on the hardware being used. In other
words, since there is no standard way for
RFID readers to connect to gateways, every reader
manufacturer (or even devices from the same
manufacturer) requires its own reader module to be
written. We are planning to examine some examples of
such modules in greater depth in our future posts, but
for now let's just use the
mockout
package to simulate messages from a reader module.
There is a couple of last things we need to do before we will be able to start our gateway.
First, we need an example config file. This will go to
./config.json:
{
"listener": {
"devices": [
{
"path": "/some/path1",
"module": "./node_modules/mockout"
},
{
"path": "/some/path2",
"module": "./node_modules/mockout"
},
{
"path": "/some/path3",
"module": "./node_modules/mockout"
}
]
},
"uploader": {
}
}
Also, let's put default configuration values in
./config/defaults.json:
{
"listener": {
"devices": []
},
"uploader": {}
}
Finally, we have to install the packages used in the implementation of our RFID gateway:
npm install --save [email protected] [email protected] [email protected] [email protected]
This will also add listed packages to the dependencies
section of our ./package.json file.
And that is it. If we run node index.js now,
we will see some output similar to the following:
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM2","module":"./node_modules/mockout"}}}
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM1","module":"./node_modules/mockout"}}}
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM3","module":"./node_modules/mockout"}}}
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM2","module":"./node_modules/mockout"}}}
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM1","module":"./node_modules/mockout"}}}
Sending: {"sortOf":"anObject","initializedWith":{"event":"connect","config":{"path":"tmr:///dev/ttyACM3","module":"./node_modules/mockout"}}}
...
This means that we have three child processes piping
the data from RFID readers through our Listener to
the Uploader, which is ready to send it wherever
we want.
So, does this implementation cover completely the software for peripheral nodes of our RFID system? Or course, no. It provides a robust way to organize the flow of data from the hardware to the point when it's ready to be sent to remote servers (which is pretty powerful result for only about two hundred lines of code), but specific implementation of reader modules (for different RFID manufacturers) and uploading procedures (which, for example, can use special protocols like MQTT or CoAP) worth a closer look and can be a topic of our further articles.
Copyright (c) 2014 Innobec Technologies Inc.