node.js - Concurrent request handling in Nodejs -

i have issue of concurrent request, modifies db.

what doing is. 1 request fetch data user-1 , calculate data user-1 modified field-1 in record, , save.

next request fetch data user-1 , calculate data user-1 modified field-1 in record, , save.

both request operates simultaneously. last request update wrong data.

function calculate() {    var needupdate = false;    user = new userlib(user_id);    var old_config = user.config;    if (old_config[req.id]) {      old_config[req.id].value = 0;      needupdate = true;    }    if (req.delete == void(0) || req.delete == false) {      delete req.delete;      old_config[req.id].value = old_config[req.id].value + 1;      needupdate = true;    }    if (needupdate) {      return user.save();    }    return true;  }

we getting both requests @ same time.

  var express = require('express');  var app = express();    app.get('/update', function(req, res) {      res.writehead(200, {      'content-type': 'text/html'    });        calculate(req);      function calculate(req) {        var needupdate = false;        user = new userlib(user_id);      var old_config = user.config;        if (old_config[req.id]) {        old_config[req.id].value = 0;        needupdate = true;      }        if (req.delete == void(0) || req.delete == false) {        delete req.delete;        old_config[req.id].value = old_config[req.id].value + 1;        needupdate = true;      }        if (needupdate) {        user.save();      }    }      res.end('done');  });  first reuest following parameter {    user_id: 1,    id: 1,    value: 5,    delete: false  }    anothere request follwing parmter {    user_id: 1,    id: 1,    delete: true  }

if want operate on each request simultaneously, i'd suggest use bluebird.map can handle each request want, concurrency, , final result.

for example:

let users = ['foo', 'bar']; //fetching users want  bluebird.map(users, (user) => {    return user.calculate()    .then((res) => res.shouldupdate ? user.save() : promise.resolve()) }, {concurrency: 2}) .then((results) => {   //results array both resolved promises below }) 

you may interested in bluebird.join calculate, , join resulting data more 1 promise.

second example fetch same user twice in same promise:

//both promises bluebird.all([fetchuser1, fetchuser2]) .spread(function(user1, user2) {    //check if should update    return user1.delete !== user2.delete ? user.delete() : null }) .then(() => {}) 

bluebird.spread documentation