Don't understand the callback and non-blocking example - Node.js

Question

In the book Hands-on node, the author gives an example of blocking I\O,

var post = db.query("select * from posts where id = 1");
doSomethingWithPost(post)
doSomethingElse();

The author says nothing is executed till line 1 is finished executing db query

And, then he shows non-blocking code

callback = function(post){
doSomethingWithPost(post)
}

db.query("select * from posts where id = 1",callback);
doSomethingElse();

Isn't this also blocking till the query is executed?

So, doSomethingElse won't be executed till query is completed.

score 6 · Answer 1 · answered Jul 11 '11 at 06:21

6

You should always read non-blocking functions like doRealStuff( params, callback ) as "put doRealStuff, params and callback in the queue, do callback() when reached queue end". This also may help avoid doing mistakes like

for (var i=0; i < 1000000; i++)
{
    // I want to do many http requests now
    do_request(test_url);
}

// not a single byte is sent here because event loop was blocked by for loop

answered Jul 11 '11 at 06:21

Andrey Sidorov

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1

there is also good async explaining phrase floating around from Nodebeginner: "In node, everything runs in parallel, except your code" - http://nodebeginner.org/index.html – Andrey Sidorov Jul 12 '11 at 02:31

score 2 · Answer 2 · answered Jul 11 '11 at 04:50

The author is absolutely correct. If the second example is non-blocking, the code execution will trigger the query and then continue executing the rest of the code. The callback function will be called after the query is complete, at some undetermined point in the future. doSomethingElse(); will be called immediately.

What actually makes this example blocking vs non-blocking is not clear in the examples you have provided. It will be something internal to the DB implementation. Perhaps by passing in a callback parameter you are indicating that the request should be non-blocking.

Hopefully that helps, tyler.

I still do not get..say if `query(String, Callback) { //implementaion for retreiving records for the the given query string ...make take time to execute...On basis of resuts fetched call the callback}` . To me this is also blocking..unless database is queried callback won't execute..would wait..RIGHT!! — prvn, Oct 03 '16 at 12:24

score 2 · Answer 3 · answered Jul 11 '11 at 04:54

Ryan Dahl's intro is a pretty good overview, but this beginner's tutorial is excellent, it explains in detail and in a friendly way the premise of non/blocking operations in Javascript (and so Node).

But in a nutshell, the callback parameter in the 2nd example you've posted isn't executed until db.query finishes, it is "held on to for later", which means code after the db.query() call can continue to be executed.

Don't understand the callback and non-blocking example - Node.js

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