Im new to using classes in JS and been trying to find the best practices for it and was wondering how requires work when using it in the class.
Say I wanted to create an IoT Connection class to use to make connection to the azure iot hub. In order to make connections i need some requires to get the function getClientFromConnectionString.
var Protocol = require('azure-iot-device-amqp').Amqp;
var clientFromConnectionString = require('azure-iot-device-amqp').clientFromConnectionString;
class IoT_Connection {
constructor(_deviceName, _securityKey1, _securityKey2) {
this.deviceName = _deviceName;
this.securityKey1 = _securityKey1;
this.securityKey2 = _securityKey2;
}
}
module.exports = IoT_Connection;
var conn = new IoT_Connection("z", "x", "y");
What happens when i create a new instance of the class? Does the require load only once or would be a problem if i was making hundreds of connections every few minutes?
Is there a better approach to this problem?
Yes; whenever your run your nodejs server your code is interpreted by javascript engine(in this case the very V8 engine) it will interpret your code. The file which is require by any of the starting file(server.js or app.js) will be interpreted and the require statements will be executed only once.
So,
var clientFromConnectionString = require('azure-iot-device-amqp').clientFromConnectionString;
will make only one connection.
To understand better of 'how module.exports and require work' create an empty file and do some console.log('foo') in there and require it in your existing code.
Related
I have a project for work where I am basically creating a form of CMS to which we will add applications as time moves forward.
The issue we're having is getting those applications loaded in (and more specifically modified) on run-time within the server.
The reason we're requiring this form of "hot loading" is because we don't want the server to restart whenever a change has been made, and more specifically, we'd like to add the new applications through an admin panel.
Nodemon is a useful tool for development, but for our production environment we want to be able to replace an existing application (or module/plugin if you will) without having to restart the server (whether it's manually or through nodemon, the server needs to be running at all time).
You could compare this to how CMS' like Drupal, Yoomla, or Wordpress do things, but for our needs, we decided that Node was the better way to go for many reasons.
Code wise, I am looking for something like this, but that will work:
let applications = []
//add a new application through the web interface calling the appropiate class method, within the method the following code runs:
applications.push(require('path/to/application');
//when an application gets modified:
applications.splice(index,1);
applications.push('path/to/application');
But I require existing instances of said application to be adjusted as well.
Example:
// file location: ./Applications/application/index.js
class application {
greet() {
console.log("Hello");
}
}
module.exports = application;
the app loader would load in said application:
class appLoader {
constructor() {
this.List = new Object();
}
Add(appname) {
this.List[appname] = require(`./Applications/${appname}/index`);
}
Remove(appname) {
delete require.cache[require.resolve(`./Applications/${appname}/index`)]
delete this.List[appname];
}
Reload(appname) {
this.Remove(appname);
this.Add(appname);
}
}
The running code:
const AppLoader = require('appLoader');
const applications = new AppLoader();
applications.add('application'); // adds the application created above
var app = new applications.List['application']();
app.greet();
// Change is made to the application file, .greet() now outputs "Hello World" instead of "Hello"
//do something to know it has to reload, either by fs.watch, or manual trigger
applications.Reload('application');
app.greet();
The expected behavior is:
Hello
Hello World
In reality, I'm getting:
Hello
Hello
If anyone can help me figure out a way to dynamically load in applications like this, but also remove/reload them during run-time, it would be greatly appreciated!
Edit: if there is a way to run my application code without the use of require that would allow a dynamic load/reload/remove, that is also a welcome solution
Ok, thanks to #jfriend00 I realized I need to fix something else with my code, so his comments can still be useful for other people. As to my issue of unloading required modules or reloading them without a server restart, I figured out a relatively elegant way of making it happen.
Let me start by showing you all my test class and app.js and I'll explain what I did and how it works.
Class.js:
"use strict";
class Class {
constructor() {
// this.file will be put in comments post run-time, and this.Output = "Hey" will be uncommented to change the source file.
var date = new Date()
this.Output = date.getHours() + ":" + date.getMinutes() + ":" + date.getSeconds() + "." + date.getMilliseconds();
this.file = global.require.fs.readFileSync('./file.mov');
//this.Output = "Hey";
}
}
module.exports = Class;
app.js:
'use strict';
global.require = {
fs: require('fs')
};
const arr = [];
const mod = './class.js'
let Class = [null];
Class[0] = require(mod);
let c = [];
c.push(new Class[0]());
console.log(c[0].Output);
console.log(process.memoryUsage());
setTimeout(() => {
delete require.cache[require.resolve(mod)];
delete Class[0];
Class[0] = require(mod);
console.log(Class)
delete c[0];
c[0] = new Class[0]();
console.log(c[0].Output);
console.log(process.memoryUsage());
}, 10000);
Now let me explain here for a bit, and mind you, this is testing code so the naming is just horrid.
This is how I went to work:
Step 1
I needed a way to separate required modules (like fs, or websocket, express, etc.) so it wouldn't mess with the whole delete require_cache() part of the code, my solution was making those globally required:
global.required = {
fs: require('fs')
}
Step 2
Figure out a way to make sure the Garbage Collector removes the unloaded code, I achieved this by putting my requires and class declarations inside of a variable so that I could use the delete functionality within Node/Javascript. (I used let in my test code because I was testing another method beforehand, haven't tested if const would work again).
I also made a variable that contains the path string for the file (in this case './Class.js' but for my explanation below I'll just write it in as is)
let Class = [null] //this declares an array that has an index '0'
Class[0] = require('./Class');
let c = [new Class[0]()] // this declares an array that has the class instantiated inside of index '0'
As for the garbage collection, I'm simply able to do the following:
delete Class[0];
delete c[0];
After this I am able to redo the declaration of the required class and subsequently the class itself and keep my code working without requiring a restart.
Take in mind that his takes a lot of work to implement in an actual project, but you could split it up by adding an unload() method to a class to unload underlying custom classes. But my initial testing shows that this works like a charm!
Edit: I feel required to note that without jfriend00's comments I'd never have figured out this solution
Output
When the project start, it outputs the current time and the process.memoryUsage()
13:49:13.540
{ rss: 50343936,
heapTotal: 7061504,
heapUsed: 4270696,
external: 29814377 }
during the 10 second wait, I change the Class.js file to not read the file.mov and say "Hey" instead of the time, after the 10s timout this is the output:
Hey
{ rss: 48439296,
heapTotal: 7585792,
heapUsed: 4435408,
external: 8680 }
I'm running against a wall here, maybe it's just a small problem where I can't see the solution due to my inexperience with NodeJS.
Right now I'm constructing a BT device which will be controlled by a master application and I have settled for the prototyping on a Raspberry PI 3 with NodeJS using the Bleno module.
So far everything worked fine, the device gets found and I can set and get values over Bluetooth. But to separate the different "programs" which the device could execute from the Bluetooth logic (because of loops etc.) I have opted to extract these into external NodeJS files.
The idea here was to use the NodeJS fork module and control the starting and stoppping of those processes through the main process.
But herein my problems start. I can fork the different JavaScript files without problem and these get executed, but I can't get them to stop and I don't know how to fix it.
Here's the code (simplified):
var util = require('util');
var events = require('events');
var cp = require('child_process');
...
var ProgramTypeOne = {
NONE: 0,
ProgramOne: 1,
...
};
...
var currentProgram=null;
...
function BLEDevice() {
events.EventEmitter.call(this);
...
this.currentProgram=null;
...
}
util.inherits(BLELamp, events.EventEmitter);
BLELamp.prototype.setProgram = function(programType, programNumber) {
var self = this;
var result=0;
if(programType=="ProgramTypeOne "){
if(programNumber==1){
killProgram();
this.currentProgram=cp.fork('./programs/programOne');
result=1;
}else if(programNumber==2){
...
}
self.emit('ready', result);
};
...
module.exports.currentProgram = currentProgram;
...
function killProgram(){
if(this.currentProgram!=null){
this.currentProgram.kill('SIGTERM');
}
}
There seems to be a problem with the variable currentProgram which, seemingly, never gets the childprocess from the fork call.
As I have never worked extensivley with JavaScript, except some small scripts on websites, I have no idea where exactly my error lies.
I think it has something to do with the handling of class variables.
The starting point for me was the Pizza example of Bleno.
Hope the information is enough and that someone can help me out.
Thanks in advance!
Since killProgram() is a standalone function outside of the scope of BLELamp, you need to call killProgram with the correct scope by binding BLELamp as this. Using apply (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply) should resolve it. The following I would expect would fix it (the only line change is the one invoking killProgram):
BLELamp.prototype.setProgram = function(programType, programNumber) {
var self = this;
var result=0;
if(programType=="ProgramTypeOne "){
if(programNumber==1){
killProgram.apply(this);
this.currentProgram=cp.fork('./programs/programOne');
result=1;
}else if(programNumber==2){
...
}
self.emit('ready', result);
};
As a side note, your code is kind of confusing because you have a standalone var currentProgram then a couple prototypes with their own bound this.currentPrograms. I would change the names to prevent confusion.
I have an object called NetFlowStorage that contains methods to access a specific elasticsearch index. My constructor looks like:
function NetFlowStorage() {
this.host = 'localhost:9200';
this.shards = '4';
this.replicas = '0';
this.index_name = 'flow_track2';
this.client = null;
}
Inside of the object I have a method called connect which, when called, will make the connection and store the elasticsearch client object in the this.client property (if there isn't one already there). This way all of the object methods can get access to the elasticsearch client by using this.client
First question, is this an appropriate pattern? If not, what is preferable?
Second question (and the one that drove me here), how would I mock calls to things like this.client.index({}) I'm just starting to mess around with unit testing and mocks under node/js so I don't really have a preference in terms of framework (currently using mocha/chai/sinon)
Full code is here if you want to see in more detail.
For something like this I would use dependency injection.
You want to decouple the NetFlowStorage class from the actual elasticsearch client:
function NetFlowStorage(esClient) {
this.host = 'localhost:9200';
this.shards = '4';
this.replicas = '0';
this.index_name = 'flow_track2';
// if you don't wanna share connections across several instances
// you can instantiate the client here otherwise you can pass the
// client instance
this.client = esClient; // or new esClient({ host: this.host })
}
This way you won't even need the elasticsearch as part of the node module and even share a connection across more than one instance (or not?)
This decoupling will also make it easier to mock the esClient as you would inject the mock elasticsearch client in the test itself.
I think that you should pass a config object and a connection object to the method.
So if you would use Jasmine for testing for example you could pass a spy
var client = {index:function(){}}
spyOn(client, 'index');
....
expect(client.index)toHaveBeenCalled();
and to pass it at some point with injection or a singleton to the SUT
I am used to cross platform dev. On the C/C++ side it's simple, but I'm stuck with a problem when using javascript. I want to be able to reuse code from my web-service on the client-side. Currently node.js requires me to write my program in a most unpleasant way but I can handle it:
//MyClass.js
function MyClass()
{
this.test = function(){}
}
module.exports.MyClass = MyClass;
//server.js
var m = new require('MyClass.js').MyClass();
m.test();
//client.js
$.getScript("MyClass.js", function(){
var m = new MyClass();
m.test();
});
To that point it is all fine, but I have an issue when I need to load classes from MyClass.js. How can I make it work across all the platforms? Is there a way to achieve reusability without processing the files?
Node requires you to do nothing. If you aren't a fan of the way the Node module system works, simply don't use it.
//MyClass.js
MyClass = function ()
{
this.test = function(){}
}
//server.js
require('./MyClass.js');
var m = new MyClass();
m.test();
Now your application is compatible with what you have going on client-side. Just bear in mind that you are now creating classes in the global namespace, which is one reason for using Node's module layout.
I suggest also looking into some of the ways to use a Node-style require on the client. There are many scripts available, such as RequireJS or Browserify.
my problem is not about "memory leakage", but about "memory purge" of node.js (expressjs) app.
My app should maintain some objects in memory for the fast look-up's during the service. For the time being (one or two days) after starting the app, everthing seemed fine, until suddenly my web client failed to look-up the object bacause it has been purged (undefined). I suspect Javascript GC (garbage collection). However, as you can see in the psedu-code, I assigned the objects to the node.js "global" variable properties to prevent GC from purging them. Please give me some clue what caused this problem.
Thanks much in advance for your kind advices~
My node.js environments are node.js 0.6.12, expressjs 2.5.8, and VMWare cloudfoundry node hosting.
Here is my app.js pseudo-code :
var express = require("express");
var app = module.exports = express.createServer();
// myMethods holds a set of methods to be used for handling raw data.
var myMethods = require("myMethods");
// creates node.js global properties referencing objects to prevent GC from purging them
global.myMethods = myMethods();
global.myObjects = {};
// omited the express configurations
// creates objects (data1, data2) inside the global.myObjects for the user by id.
app.post("/createData/:id", function(req, res) {
// creates an empty object for the user.
var myObject = global.myObjects[req.prams.id] = {};
// gets json data.
var data1 = JSON.parse(req.body.data1);
var data2 = JSON.parse(req.body.data2);
// buildData1 & buildData2 functions transform data1 & data2 into the usable objects.
// these functions return the references to the transformed objects.
myObject.data1 = global.myMethods.buildData1(data1);
myObject.data2 = global.myMethods.buildData2(data2);
res.send("Created new data", 200);
res.redirect("/");
});
// returns the data1 of the user.
// Problem occurs here : myObject becomes "undefined" after one or two days running the service.
app.get("/getData1/:id", function(req, res) {
var myObject = global.myObjects[req.params.id];
if (myObject !== undefined) {
res.json(myObject.data1);
} else {
res.send(500);
}
});
// omited other service callback functions.
// VMWare cloudfoundry node.js hosting.
app.listen(process.env.VCAP_APP_PORT || 3000);
Any kind of cache system (whether is roll-your-own or a third party product) should account for this scenario. You should not rely on the data always being available on an in-memory cache. There are way too many things that can cause in-memory data to be gone (machine restart, process restart, et cetera.)
In your case, you might need to update your code to see if the data is in cache. If it is not in cache then fetch it from a persistent storage (a database, a file), cache it, and continue.
Exactly like Haesung I wanted to keep my program simple, without database. And like Haesung my first experience with Node.js (and express) was to observe this weird purging. Although I was confused, I really didn't accept that I needed a storage solution to manage a json file with a couple of hundred lines. The light bulb moment for me was when I read this
If you want to have a module execute code multiple times, then export a function, and call that function.
which is taken from http://nodejs.org/api/modules.html#modules_caching. So my code inside the required file changed from this
var foo = [{"some":"stuff"}];
export.foo;
to that
export.foo = function (bar) {
var foo = [{"some":"stuff"}];
return foo.bar;
}
And then it worked fine :-)
Then I suggest to use file system, I think 4KB overhead is not a big deal for your goals and hardware. If you familiar with front-end javascript, this could be helpful https://github.com/coolaj86/node-localStorage