The life cycle of the request/response object in Express.js when using callbacks

Question

The life cycle of the request/response object in Express.js when using callbacks

Feel free to correct me if this question has already been asked. (I've done as much research as I can handle before asking)

I'm really trying to wrap my head around the life cycle of request and response objects.

Take a look at the following code snippet:

app.get('/', function(req, res) {

  var setCookie = function(err, idCookie) { //callback after cookieSearch in DB
    // ... do something with the result of findCookieInDatabase() (handle error, etc.);
    res.sendfile('index.html');
  }

  var cookie = parseCookie(req.get('Cookie')); //parse and format cookie

  findCookieInDatabase(cookie, afterCookieSearch); //tries to find cookie is DB


  // .. do some content return stuff, etc.
}

(Please be aware that the actual code does much more, including checking if 'Cookie' even exists etc.)

I know that req and res objects are created and eventually have to be garbage collected. (At least I hope so)

When findCookieInDatabase() is called with setCookie as an argument, I understand that setCookie is just a string (containing the function) until the callback(setCookie) statement is reached in findCookieInDatabase().

I also realize that I could be completely mistaken in my assumption above, which might be due to my lack of understanding of javascript callbacks. (I've searched extensively on this topic but only found endless tutorials on how to use callbacks, nothing about what goes on behind the scenes)

So here's my question: How does javascript (or node.js) determine how long to keep 'res' alive and when it's safe to garbage collect it?

Does the line res.sendfile in setCookie serve as an active reference because it's invoked through findCookieInDatabase()?

Does javascript actually monitor all references and keep req and/or res alive for as long as any part of any callbacks or other functions are still running?

Any assistance would be greatly appreciated. Thank you for reading.

javascript express callback garbage-collection lifecycle

Answer 1

Answer №1

If you are feeling overwhelmed by your code and assumptions, it might be time to revisit some JavaScript fundamentals. Expand your knowledge by exploring these recommended books:

Check out Speaking JavaScript by Axel Rauschmayer
Dive into JavaScript: The Good Parts by Douglas Crockford (note: not everything Douglas Crockford says is gospel, but this book is essential for aspiring JavaScript developers)
Explore Learning Node by Shelley Powers

Don't forget to also take a look at my own book, Web Development with Node and Express. Now that we've covered the reading material, let's delve into solving the core of your issue.

When an HTTP request reaches Node, it generates the req and res objects (originally instances of http.IncomingMessage and

http.ServerResponse</code). These objects are designed to exist throughout the duration of the HTTP request cycle. Following the client's request, various activities occur, culminating in invoking a method on <code>res

to return an HTTP response to the client, after which these objects become unnecessary.

Due to Node's asynchronous nature, multiple instances of req and res can coexist, differentiated only by their respective scopes. This concept may seem intricate; however, in practice, it's generally inconsequential because when writing code, you treat each instance uniquely without concerning yourself about managing multiple requests as frameworks like Express handle such complexities.

Javascript employs a garbage collector whereby objects get deallocated once their reference count diminishes to zero. As long as there exists a reference to a particular object such as req, that object remains intact. Consider this rudimentary example involving a flawed practice within an Express program:

An array named “allRequests” purposely maintains all user requests.
app.use(function(req, res, next) {
    allRequests.push(req);
    next();
});

Nevertheless, failing to remove items from allRequests could lead to memory exhaustion while processing traffic.

In Express, asynchronous functions play a pivotal role by executing callbacks post tasks completion. If said callback holds references to req or res objects, these objects remain unchanged until the function finishes its duties and the callback executes (alongside exiting other relevant scopes). For instance:

A brief demonstration showcasing delays:
app.get('/fast', function(req, res) {
    res.send('fast!');
});

app.get('/slow', function(req, res) {
    setTimeout(function() {
        res.send('sloooooow');
    }, 3000);
});

Accessing /slow results in a 3-second delay, whereas repeatedly visiting /fast showcases uninterrupted functionality. Each request in Express generates distinct req and res objects, maintaining segregation. Yet, the associated res object linked to /slow is retained due to its pending callback execution.

Remember, understanding the foundational concepts is crucial; nonetheless, grappling with counting references and managing garbage collection in Javascript is often needless. Take a breather and trust that these mechanisms operate seamlessly in the background.

I trust this elucidates your concerns.

Answer 2

If you are feeling overwhelmed by your code and assumptions, it might be time to revisit some JavaScript fundamentals. Expand your knowledge by exploring these recommended books:

Check out Speaking JavaScript by Axel Rauschmayer
Dive into JavaScript: The Good Parts by Douglas Crockford (note: not everything Douglas Crockford says is gospel, but this book is essential for aspiring JavaScript developers)
Explore Learning Node by Shelley Powers

Don't forget to also take a look at my own book, Web Development with Node and Express. Now that we've covered the reading material, let's delve into solving the core of your issue.

When an HTTP request reaches Node, it generates the req and res objects (originally instances of http.IncomingMessage and

http.ServerResponse</code). These objects are designed to exist throughout the duration of the HTTP request cycle. Following the client's request, various activities occur, culminating in invoking a method on <code>res

to return an HTTP response to the client, after which these objects become unnecessary.

Due to Node's asynchronous nature, multiple instances of req and res can coexist, differentiated only by their respective scopes. This concept may seem intricate; however, in practice, it's generally inconsequential because when writing code, you treat each instance uniquely without concerning yourself about managing multiple requests as frameworks like Express handle such complexities.

Javascript employs a garbage collector whereby objects get deallocated once their reference count diminishes to zero. As long as there exists a reference to a particular object such as req, that object remains intact. Consider this rudimentary example involving a flawed practice within an Express program:

An array named “allRequests” purposely maintains all user requests.
app.use(function(req, res, next) {
    allRequests.push(req);
    next();
});

Nevertheless, failing to remove items from allRequests could lead to memory exhaustion while processing traffic.

In Express, asynchronous functions play a pivotal role by executing callbacks post tasks completion. If said callback holds references to req or res objects, these objects remain unchanged until the function finishes its duties and the callback executes (alongside exiting other relevant scopes). For instance:

A brief demonstration showcasing delays:
app.get('/fast', function(req, res) {
    res.send('fast!');
});

app.get('/slow', function(req, res) {
    setTimeout(function() {
        res.send('sloooooow');
    }, 3000);
});

Accessing /slow results in a 3-second delay, whereas repeatedly visiting /fast showcases uninterrupted functionality. Each request in Express generates distinct req and res objects, maintaining segregation. Yet, the associated res object linked to /slow is retained due to its pending callback execution.

Remember, understanding the foundational concepts is crucial; nonetheless, grappling with counting references and managing garbage collection in Javascript is often needless. Take a breather and trust that these mechanisms operate seamlessly in the background.

I trust this elucidates your concerns.

The life cycle of the request/response object in Express.js when using callbacks

Answer №1

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