Setting Your AWS Node Application's Port

When working locally, using an arbitrary port number is fine, but you need to get that property when deploying your Node application to AWS.

Technically, you can code-up a Node web server in less than ten lines of code. Most likely, your application will require a few more lines of code than that. But my point here is: getting a basic Node web server running is not terribly difficult.

Example # 1

var http = require('http');

var server = http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.end('Hello!');
})

//using an arbitrary port
server.listen(3000);

var http = require('http');

var server = http.createServer(function (req, res) {

res.writeHead(200, {'Content-Type': 'text/html'});

res.end('Hello!');

})

//using an arbitrary port

server.listen(3000);

In example # 1, I used port # 3000, but I could have used virtually any valid port number. When working locally this is for the most part a non-issue. As long as no other application is using the port you want to use, you chose one and then use it. Easy. This example does little more than say “Hello!”, but the point I’m trying to make is that your main JS file ends with the server.listen method, and you need to pass it a port number.

But when you attempt to deploy this code to your AWS Elastic Beanstalk instance, you will get a “503 Bad Gateway” error in your browser. The reason for this is: you don’t know which port should be used when calling the server.listen method. The great thing about AWS is that it provides a layer of abstraction for those kinds of details. In other words; AWS takes care of details such as which port to listen on. The downside here is that you have no way of knowing exactly which port that will be when you deploy your code.

Example # 2

var http = require('http');

//if process.env.PORT is set, use that. if not, then use 3000
var port = process.env.PORT || 3000;

var server = http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.end('Hello!');
})

//safely use the right port
server.listen(port);

var http = require('http');

//if process.env.PORT is set, use that. if not, then use 3000

var port = process.env.PORT || 3000;

var server = http.createServer(function (req, res) {

res.writeHead(200, {'Content-Type': 'text/html'});

res.end('Hello!');

})

//safely use the right port

server.listen(port);

In example # 2, we set a variable named: port. We attempt to assign the value of process.env.PORT to that variable. If that value is falsely, then we set it to 3000. The reason this works is; if our code is running on our AWS instance, then process.env.PORT will automatically be set and we will listen on that port. If we are running our code locally, then process.env.PORT will be undefined (or “falsely”). So, then our port variable will have a value of 300. This way, our code can run successfully on our AWS instance, or locally.