将响应发送到客户端然后在Node.js中运行长时间/繁重的操作的快速方法是什么

Question

I'm trying to figure out, what is the best/fast way to send the response from expressjs and then do log or do long actions in the server, without delaying the response to the client.

I have the following code, but I see that the response to the client is send only after the loop is finished. I though that the response will be send because I'm triggering res.send(html); and then calling longAction

function longAction () {
    for (let i = 0; i < 1000000000; i++) {}
    console.log('Finish');
}

function myfunction (req, res) {
    res.render(MYPATH, 'index.response.html'), {title: 'My Title'}, (err, html) => {
        if (err) {
            re.status(500).json({'error':'Internal Server Error. Error Rendering HTML'});
        }
        else {
            res.send(html);
            longAction();            
        }
    });
}


router.post('/getIndex', myfunction);

What is the best way to send the response and then run my long/heavy actions? Or What I'm missing?

Answer 1

I'm trying to figure out, what is the best/fast way to send the response from expressjs and then do log or do long actions in the server, without delaying the response to the client.

The best way to do this is to only call longAction() when express tells you that the response has been sent. Since the response object is a stream, you can use the finish event on that stream to know when all data from the stream has been flushed to the underlying OS.

From the writable stream documentation :

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

Here's how you could use that in your specific code:

function myfunction (req, res) {
    res.render(MYPATH, 'index.response.html'), {title: 'My Title'}, (err, html) => {
        if (err) {
            res.status(500).json({'error':'Internal Server Error. Error Rendering HTML'});
        }
        else {
            res.on('finish', () => {
                longAction();            
            });
            res.send(html);
        }
    });
}

---

For a little more explanation on the finish event, you can start by looking at the Express code for res.send() and see that is ends up calling res.end() to actually send the data. If you then look at the documentation for .end() on a stream writable, it says this:

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream. If provided, the optional callback function is attached as a listener for the 'finish' event.

So, since Express doesn't expose access to the callback that .end() offers, we just listen to the finish event ourselves to be notified when the stream is done sending its last bit of data.

---

Note, there is also a typo in your code where re.status(500) should be res.status(500) .

Answer 2

Use setImmediate :

var test = function(){
    for (let i = 0; i < 1000000000; i++) {}
    console.log('Finish');
}

router.get("/a", function(req, res, next){
    setImmediate(test);
    return res.status(200).json({});
});

Your long function will be executed at the end of the current event loop cycle. This code will execute after any I/O operations (in this sample, first I/O operation is the res.status(200)) in the current event loop and before any timers scheduled for the next event loop.

Answer 3

Thanks for the answers:

After checking I think the best approach is using listening to finish event

res.on('finish', () => {
    // Do another stuff after finish got the response
});