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Section 50.1: Processing long running queries with Node

Main logic of this example:

  • Client sends request to the server.

  • Server starts the routine in separate node instance and sends immediate response back with related task ID.

  • Client continiously sends checks to a server for status updates of the given task ID.

  • Project structure:

project
│
|    package.json
│
|    index.html
│
├───js
│     main.js
│     jquery-1.12.0.min.js
│
└───srv
    │
    |    app.js
    ├─── models
    │
    |    task.js
    |     
    └─── tasks
          data-processor.js
  • app.js:
let express = require('express');
let app = express();
let http = require('http').Server(app);
let mongoose = require('mongoose');
let bodyParser = require('body-parser');
let childProcess = require('child_process');
let Task = require('./models/task');

app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use(express.static(__dirname + '/../'));

app.get('/', function(request, response){
  response.render('index.html');
});

//route for the request itself
app.post('/long-running-request', function(request, response){
  //create new task item for status tracking
  let t = new Task({ status: 'Starting ...' });
  
  t.save(function(err, task){
    //create new instance of node for running separate task in another thread
    taskProcessor = childProcess.fork('./srv/tasks/data-processor.js');

    //process the messages comming from the task processor
    taskProcessor.on('message', function(msg){
      task.status = msg.status;
      task.save();
    }.bind(this));

    //remove previously openned node instance when we finished
    taskProcessor.on('close', function(msg){
      this.kill();
    });

    //send some params to our separate task
    let params = {
      message: 'Hello from main thread'
    };

    taskProcessor.send(params);
    response.status(200).json(task);
  });
});

//route to check is the request is finished the calculations
app.post('/is-ready', function(request, response){
  Task
    .findById(request.body.id)
    .exec(function(err, task){
      response.status(200).json(task);
    });
});

mongoose.connect('mongodb://localhost/test');
http.listen('1234');
  • task.js:
let mongoose = require('mongoose');
let taskSchema = mongoose.Schema({
  status: {
    type: String
  }
});

mongoose.model('Task', taskSchema);
module.exports = mongoose.model('Task');
  • data-processor.js:
process.on('message', function(msg){
  init = function(){
    processData(msg.message);
  }.bind(this)();

  function processData(message){
    //send status update to the main app
    process.send({ status: 'We have started processing your data.' });

    //long calculations ..
    setTimeout(function(){
      process.send({ status: 'Done!' });
      //notify node, that we are done with this task
      process.disconnect();
    }, 5000);
  }
});

process.on('uncaughtException',function(err){
  console.log("Error happened: " + err.message + "\n" + err.stack + ".\n");
  console.log("Gracefully finish the routine.");
});
  • index.html:
<!DOCTYPE html>
<html>
  <head>
    <script src="./js/jquery-1.12.0.min.js"></script>
    <script src="./js/main.js"></script>
  </head>
  <body>
    <p>Example of processing long-running node requests.</p>
    <button id="go" type="button">Run</button>
    <br />
    <p>Log:</p>
    <textarea id="log" rows="20" cols="50"></textarea>
  </body>
</html>
  • main.js:
$(document).on('ready', function(){
  $('#go').on('click', function(e){
    //clear log
    $("#log").val('');

    $.post("/long-running-request", {some_params: 'params' })
      .done(function(task){
        $("#log").val( $("#log").val() + '\n' + task.status);

        //function for tracking the status of the task
        function updateStatus(){
          $.post("/is-ready", {id: task._id })
            .done(function(response){
              $("#log").val( $("#log").val() + '\n' + response.status);
              if(response.status != 'Done!'){
                checkTaskTimeout = setTimeout(updateStatus, 500);
              }
            });
        }
        
        //start checking the task
        let checkTaskTimeout = setTimeout(updateStatus, 100);
      });
  });
});
  • package.json:
{
"name": "nodeProcessor",
  "dependencies": {
    "body-parser": "^1.15.2",
    "express": "^4.14.0",
    "html": "0.0.10",
    "mongoose": "^4.5.5"
  }
}

Disclaimer: this example is intended to give you basic idea. To use it in production environment, it needs improvements.