Is there a way to share and edit global variables between two javascript node runs?

Question

The main function file main.js has:

var nLastPingTime = 0,
    nLastPingNumber = 0;

    module.exports = {

      compareData: function(nPingTime, nLastPingNumber){
        nLastPingTime = nPingTime;
        nLastPingNumber = nLastPingNumber;
      }

    };

Now two other files dataGenOne.js and dataGenTwo.js look something like this:

const mainDataHolder = require('./main.js');

//Gets data from some API's here

mainDataHolder.compareData(nPingTime, nLastPingNumber);

Then to start we run:

node dataGenOne.js

and

node dataGenTwo.js

The problem is that the main.js file doesn't share nLastPingTime and nLastPingNumber mutually between both sets of data.

For example when looking at nLastPingNumber, its the number from dataGenOne.js specifically and not from dataGenTwo.js at all (or vise versa).

I believe this is because they are running on two separate threads.

Is there anyway to achieve what I'm trying to do? The alternative could be to connect database or write to a file but if possible I would rather not do that.

They're separate processes--unless they have a way to talk to each other I don't see what your options are. It'd be like this no matter what you wrote it in. The "best" way to do this depends on what you're actually trying to accomplish. — Dave Newton
– Dave Newton, Commented Jun 15, 2018 at 17:55
What exactly are you trying to do? What do you hope to accomplish by sharing the data? Why do you not want to go through other data sharing channels? Please give as much context as you can. Right now this sounds like an XY problem. Since they are separate processes, there are ways to sync the data, but not through the conventional JavaScript context since they don't share contexts. — zero298
– zero298, Commented Jun 15, 2018 at 18:06
Basically its pinging data from two different API's (stocks and cryptos) and then compares the data in the main.js file. So I wouldn't want them in the same process because if one API lags it can slow down the whole process. — Marc Alexander
– Marc Alexander, Commented Jun 15, 2018 at 18:09
It sounds to me like maybe you just need a shared JSON config file that each script could load. Then, each script could load the same settings and if those settings were changed, the next run of each script would see the changed settings. Global variables are NOT shared in any way between separate node processes. — jfriend00
– jfriend00, Commented Jun 15, 2018 at 18:15

Adam H · Accepted Answer · 2018-06-15 18:15:50Z

1

To achieve what you are attempting to do, have two node processes communicate, you are going to have create process, lets call it spawn, that spawns both of the processes (let's call them p1 & p2) and then handles communication between p1 & p2.

So spawn would be a very simple process that would just wire the events for p1 & p2 and then forward those events to the other process. I don't have a working example of this but if you take a look here you should be able to piece that together pretty quickly.

answered Jun 15, 2018 at 18:15

Adam H

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Marc Alexander · Accepted Answer · 2018-06-28 18:50:31Z

Adam H pointed me in the right direct. The correct way to do this is in fact child_processes.

Below are the code changes:

The main function file main.js now has:

var cp = require('child_process');
var childDataOne = cp.fork('./dataGenOne.js', [process.argv[2]], { silent: true });
var childDataTwo = cp.fork('./dataGenTwo.js', [process.argv[3]], { silent: true });

childDataOne.stdout.on('data', function(data) {
    console.log('parent: ' + data);
    compareData(data);
    //Here is where the output goes
});

childDataTwo.stdout.on('data', function(data) {
    console.log('parent: ' + data);
    compareData(data);
    //Here is where the output goes
});

Now two other files dataGenOne.js and dataGenTwo.js changed to something like this:

process.stdin.resume();
var passingString = nPingTime + "," + nLastPingNumber;
process.stdout.write(passingString);

To start running we only have to do:

node main.js param1 param2

Instead of running dataGenOne.js and dataGenTwo.js individually.

This correctly allows the child processes to pass data back to the parent process. main.js is listening with stdout.on and the two dataGen child processes are passing the data with stdout.write.

user9947879 · Accepted Answer · 2018-06-15 18:34:52Z

So to avoid the complexity of storing these variables somewhere, merge the processes, but reorganize your code to make it easier to navigate.

main.js (the compare function?) - remove the variables from the top but make sure the compare function returns the latest ping values along with the comparison data i.e.
```
 return { 
     data, 
     lastPingTime, 
     lastPingNumber 
 }
```

move the api stuff into separate files so you can do this

 var dataSetOne = require('./dataOne');
 var dataSetTwo = require('./dataTwo');
 var datasets = [dataSetOne, DataSetTwo];

 // initialize the values
 var lastPingTime = 0;
 var lastPingNumber = 0;

 // loop through the datasets
 for (var i = 0, len = datasets.length; i < len; i++) {
      let currentDataSet = datasets[i];
      const results = comparePrices(lastPingTime, lastPingumber, aAsks, aBids);
      // update the ping info here
      lastPingTime = results.lastPingTime;
      lastPingNumber = results.lastPingNumber;
 }

And if you have a lot of datasets, make an 'index.js' file that does all those requires and just returns the datasets array.

Hope that helps!

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