Can node.js code result in race conditions?

Question

From what I read, race conditions occur when different threads try to change a shared variable, which can result in a value that\'s not possible with any serial order of exe

Answer 1

Yes. It can.

Race condition in Nodejs is feasible when you use cluster module to initialise multiple workers.

The case

var cluster = require('cluster');
var fs = require('fs');
if(cluster.isMaster){
    for(var i=0;i<4;i++){
        cluster.fork();
    }
}else{
    fs.watch('/path/to/file',function(){
        var anotherFile = '/path/to/anotherFile';
        fs.readFile(anotherFile,function(er,data){
             if(er){
                 throw er;
             }
             data = +data+1;
             fs.writeFile(anotherFile,data,function(er){
                 if(er){
                     throw er;
                 }
                 fs.readFile(anotherFile,function(er,newData){
                     if(er){
                         throw er;
                     }
                     console.log(newData); //newData is now undetermined
                 });
             });
        });
    });
}

Whenever you change the watched file, 4 workers will execute the handler at the same time. This behaviour causes the undetermined newData.

The solution

if(cluster.isMaster){
    var lock = {};
    var timer = setInterval(function(){
        if(Object.keys(cluster.workers).length >= 4){
            return clearInterval(timer);
        }
        //note that this lock won't 100% work if workers are forked at the same time with loop.
        cluster.fork().on('message',function(id){
             var isLocked = lock[id];
             if(isLocked){
                 return console.log('This task has already been handled');
             }
             lock[id] = 1;
             this.send('No one has done it yet');
        });
    },100);
}else{
     process.on('message',function(){
        //only one worker can execute this task
        fs.watch('/path/to/file',function(){
            var anotherFile = '/path/to/anotherFile';
            fs.readFile(anotherFile,function(er,data){
                 if(er){
                     throw er;
                 }
                 data = +data+1;
                 fs.writeFile(anotherFile,data,function(er){
                     if(er){
                        throw er;
                     }
                     fs.readFile(anotherFile,function(er,newData){
                         if(er){
                             throw er;
                         }
                         console.log(newData); //newData is now determined
                     });
                 });
            });
        });
     });
     //ask the master for permission
     process.send('watch');
}