Read File then delete the file with node.js - node.js

Question

I am attempting to use nodeJS to do the following:
Start an executable file (executable file then creates a timestamp_Question.txt file), read the created .txt file and return the text to my front end. Accept user input and create a new timestamp_Answer.txt file with a new name containing the user input, and then delete the original txt file.
Everything seems to be working except the deletion of the original file which is getting the following error
Error: EBUSY: resource busy or locked, unlink 'c:\projects\pizzabox\95912_Questi
on.txt'
at Error (native)
Here are the various segments of my server code:
Starting the .exe file:
startProgram: function(req, res){
var date = new Date().toLocaleTimeString();
date = date.replace(/\D/g,'');
exec('C:/projects/pizzabox/server/webserver/Pizza_page_server.exe', [date], function(err, data) {
console.log(err)
});
res.send(date) /// front end needs date value to track file as date is attached to created txtfile
}
Front end requesting the contents of the txt created by the exe:
getDocument: function(req,res){
console.log('file requested', req.params.dateStamp);
var directory = 'C:/projects/pizzabox/'
var fileCode = req.params.dateStamp.toString()
var fileSuffix = "_Question.txt"
var file = directory+fileCode+fileSuffix
console.log('file is', file);
while(!fs.existsSync(file)){};
fs.readFile(file, 'utf8', function(err,data){
if (err) throw err;
console.log('read ', data);
res.send(data)
});
Front end submitting data to create new timestamp_Answer.txt file and deletion of old timestamp_Question.txt file:
sendValue: function(req,res){
console.log('got value', req.body.value, " questionNumber ", req.body.questionNumber, "timestamp ", req.body.timestamp);
fs.appendFile(req.body.timestamp+'_Answer.txt', "Question_Num:"+req.body.questionNumber+"\nAnswer_Val:"+req.body.value, function(err){
if(err) throw err;
console.log('file created!');
fs.unlink(req.body.timestamp+'_Question.txt', function(err){
if (err) throw err;
console.log('question file deleted');
})
})
}
Any reason why unlink wouldn't have access to the file? does fs.readFile() not close/exit the file after it completes?

Answer 1

It appears you have at least one race condition here. Just because fs.fileExistsSync() sees that a file exists, does not mean that the .exe is done writing to that file or has closed that file. So, it's very likely that your fs.readFile() gets called while the file is still open by the other process.
You probably want to rethink how all your operations are sequenced.
First off, the design you have now is a single user design. Your server can only ever be used by one single user at a time since you have no way of differentiating requests that are coming from different users.
Second off, you should move to a design that is more purposefully sequential rather than trying to use the existence of a file in the file system as evidence that some previous operation might now be done.
Third, off while(!fs.existsSync(file)){}; is never used in anything except a one-off script because it halts all processing of any type of request or timer or any event in the server until that file exists. Further, it probably hogs all the CPU. It's never used in a multi-user server.
You haven't really described the overall client/server flow you're trying to design or described whether this is a single-user system or intended to eventually be for multiple users so it's hard for us to make suggestions about exactly where to go for an architectural redesign.
You could probably make one simple improvement by not returning the date to the client until your .exe is done which hopefully will stop it from requesting its next step until the .exe is done:
startProgram: function(req, res){
var date = new Date().toLocaleTimeString();
date = date.replace(/\D/g,'');
exec('C:/projects/pizzabox/server/webserver/Pizza_page_server.exe', [date], function(err, data) {
if (err) {
console.log(err);
res.sendStatus(500);
} else {
// don't send this until the .exe is done
res.send(date) // front end needs date value to track file as date is attached to created txtfile
}
});
}