Expressjs路由请求和响应是异步的吗？

Question

I have that doubt, really is not a problem but i want to know if my actual structure to handle requests and responses is non blocking.

Some of my code looks like this:

get: (req, res) ->
    permission = req.user.username

    if not permission
      return res.json new Unauthorized("#{req.user.username} no tiene permisos")

    Client.find()
      .exec (err, clients) ->
        if err then res.json new Internal(err.message, err.stack)
        if not clients then res.json new NotFound('No encontrado')

        res.json new Ok(clients, 'OK')

And of course is handled in a router archive, but i don't exactly know when my code is async and when it's not.

So, can someone explain me if this is an async non blocking code? If it's not, please, some bit of documentation or libraries? I tryed to use promisejs but it throws me some errors in the execution (i think that's because im pretty noob).

That's all, thanks in advance!

Answer 1

Mongoose's exec() is an async function in your code. As a rule of thumb, expensive operations are asynchronous, while cheap operations are not*. Expensive means an operation that takes some ms, such as reading a remote file, accessing a (possibly) remote database or even reading a local file. Some examples of expensive, async operations:

• request : load a external website.
• fs : reading files from your system. Most of the functions are asynchronous, but as reading a file is not prohibitively expensive, they also have a synchronous version.
• mongoose : as @Molda explained, your mongoose call is asynchronous. Most of the mongoose operations are.

Furthermore, asynchronous functions are characterized by them breaking the flow of the file, so they normally include a callback where the execution flow continues. In node these callbacks accept two or more arguments as a convention, being the first argument an error and the other ones the data being retrieved. Examples for the previous ones:

request('httsp://google.com/', function(err, response, body){
  console.log(body);   // print the website's html on the terminal
});

fs.readFile('/config.js', function(err, message){
  console.log(message);   // print the contents of config.js on the terminal
});

User.find({ id: 25 }, function(err, user){
  console.log(user);  // print user's 25 data on the terminal
});

Edit: as an alternative you could simplify your code by one line by including the callback in find() , making this function async:

Client.find {}, (err, clients) ->
  if err then res.json new Internal(err.message, err.stack)
  if not clients then res.json new NotFound('No encontrado')

  res.json new Ok(clients, 'OK')

*please note that expensive and cheap are highly relative. For instance, parsing a huge file in JSON can be really expensive and they are synchronous, while doing a small file read is relatively cheap.