Nodejs Synchronous For each loop

问题:

I want to do a for each loop but have it run synchronously. Each iteration of the loop will do an http.get call and that will return json for it to insert the values into a database. The problem is that the for loop runs asynchronously and that causes all of the http.gets to all run at once and my database doesn't end up inserting all of the data.I am using async-foreach to try to do what I want it to do, but I don't have to use it if I can do it the right way.

mCardImport = require('m_cardImport.js');
var http = require('http');
app.get('/path/hi', function(req, res) {

mCardImport.getList(function(sets) {
  forEach(sets, function(item, index, arr) {
    theUrl = 'http://' + sets.set_code + '.json';
    http.get(theUrl, function(res) {

      var jsonData = '';
      res.on('data', function(chunk) {
        jsonData += chunk;
      });

      res.on('end', function() {
        var theResponse = JSON.parse(jsonData);
        mCardImport.importResponse(theResponse.list, theResponse.code, function(theSet) {
          console.log("SET: " + theSet);
        });
      });
    });
  });
});
});

and my model

exports.importResponse = function(cardList, setCode, callback) {

mysqlLib.getConnection(function(err, connection) {

forEach(cardList, function(item, index, arr) {

  var theSql = "INSERT INTO table (name, code, multid, collector_set_num) VALUES "
   + "(?, ?, ?, ?) ON DUPLICATE KEY UPDATE id=id";
  connection.query(theSql, [item.name, setCode, item.multid, item.number], function(err, results) {
    if (err) {
      console.log(err);
    };
  });
});
});
callback(setCode);
};

回答1:

With recursion the code is pretty clean. Wait for the http response to come back then fire off next attempt. I don't think a for-each is the best approach.

var urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];

var processItems = function(x){
  if( x < urls.length ) {
    http.get(urls[x], function(res) {

      // add some code here to process the response

      processItems(x+1);
    });
  }
};

processItems(0);

One final note: this is only one approach to solving this problem. A solution using promises would also work well and would be compatible with many promise-oriented libraries that help manage the scheduling of async operations.

回答2:

To loop and synchronously chain asynchronous actions, the cleanest solution is probably to use a promise library (promises are being introduced in ES6, this is the way to go).

Using Bluebird, this could be

Var p = Promise.resolve();
forEach(sets, function(item, index, arr) {
    p.then(new Promise(function(resolve, reject) {
         http.get(theUrl, function(res) {
         ....
         res.on('end', function() {
              ...
              resolve();
         }
    }));
});
p.then(function(){
   // all tasks launched in the loop are finished
});

回答3:

I found out that I wasn't releasing my mysql connections after I was done with each call and this tied up the connections causing it to fail and appear to be an issue with synchronization.

After explicitly calling connection.release(); it caused my code to work 100% correctly even in an asynchronous fashion.

Thanks for those who posted to this question.

回答4:

"use strict";

var Promise = require("bluebird");
var some = require('promise-sequence/lib/some');

var pinger = function(wht) {
    return new Promise(function(resolve, reject) {
        setTimeout(function () { 

            console.log('I`ll Be Waiting: ' + wht);
            resolve(wht);

        }, Math.random() * (2000 - 1500) + 1500);
    });
}

var result = [];
for (var i = 0; i <= 12; i++) {
    result.push(i);
}

some(result, pinger).then(function(result){
  console.log(result);
});