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Live Changes Within Node.js Scripts Without Stop/Start

Imagine that you are working within a node.js project. Here is a simple script:

var CONF = ['item1', 'item2', 'item3'];

var last;
setInterval(function () {
    var next = CONF.indexOf(last) + 1;
    last = (CONF[next] == undefined) ? CONF[0] : CONF[next];
    console.log(last);
}, 1000);

If we run this script, we will see in the console one element of CONF each second. Simple, isn’t it? Now imagine we want to add one new element to the list (let’s say item4). We can easily change the script, stop the execution and run again. But imagine that we cannot stop/start the script as many times as we want. What can we do?. We can store the CONF data into one external storage (Redis, for example), but today we are going to do much easier. We are going to modify CONF in execution time. The idea is to open a TCP socket to let us change CONF with a simple protocol.

If we change the script to:
var net = require('net');
var CONF_PORT = 9730;
var CONF = ['item1', 'item2', 'item3'];

var last;
setInterval(function () {
    var next = CONF.indexOf(last) + 1;
    last = (CONF[next] == undefined) ? CONF[0] : CONF[next];
    console.log(last);
}, 1000);

var serverConf = net.createServer(function (confSocket) {
    confSocket.on("data", function (data) {
        var dataAsString = data.toString().trim();
        switch (dataAsString.substr(0, 1)) {
            case '+':
                var userVariable = dataAsString.substr(1);
                if (CONF.indexOf(userVariable) < 0) {
                    CONF.push(userVariable);
                    confSocket.write("+ " + userVariable + " added\n");
                } else {
                    confSocket.write("+ " + userVariable + " already added\n");
                }
                break;
            case '-':
                var userVariable = dataAsString.substr(1);
                if (CONF.indexOf(userVariable) >= 0) {
                    CONF.splice(CONF.indexOf(userVariable), 1)
                    confSocket.write("- " + userVariable + " deleted\n");
                } else {
                    confSocket.write("- " + userVariable + " don't exists\n");
                }
                break;
            case '=':
                for (var i in CONF) {
                    confSocket.write(CONF[i] + "\n");
                }
                break;
        }
        confSocket.write("\n");
        confSocket.write("Number of elements: " + CONF.length + "\n");
        confSocket.end("\n");
    });
});
serverConf.listen(CONF_PORT);

You can see the script in action here:




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