CODEWITHSUNDEEP
javascriptnode.jsamazon-web-servicescallbackwhile-loop
Fox
Fox

Reputation: 9444

Pausing the nodejs function for a response from other function

I am playing with the Node.JS library provided by Amazon to manage the EC2 and Autosclaing group. I have written a program to delete an instance so that the autoscaling group will create a new instance. But before I delete any additional instances, I need to make sure that the new one is generated by amazon and is running. For that I need to pause my program and keep checking till I get a positive response. So this is how it works ---

(I usually have 3-4 instances and have to replace with 3-4 instances of new type.)

So my program is --

 updateServer(){
      Which has the code to retrieve the instances from the server.
      foreach(instance in the list of instances){
           replace(); (with new one.)
      }
 }

replace() {
   delete the old one.
   while(new instance is not generated) {
      check();
   }
}

check(){
  return the status.
}

Now the question is I need this to happen in sequence. How can I pause the program? Because in my case, the foreach loop executes and the program ends and it does not enter that check loop. Can you please give me ideas.

Thanks

Links -- http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/frames.html

Upvotes: 0

Views: 256

Answers (3)

Darren White
Darren White

Reputation: 625

The problem you are describing here is one one 'polling' and while talking of 'pauses' is normally an anti-pattern in node's asynchronous programming model, polling is possibly the one case where it has valid application.

However for reasons of code maintainability - in this case the ability to add other tasks later (such as other checks), you should also handle the polling asynchronously.

Here are some different approaches that should solve your problem.

1. Don't handle the problem in node.js at all - invoke the node application from the host's crontab and design the program to exit. This isn't very node-ish though but its certainly robust solution.

2. Use npm to install the node timer module [https://github.com/markussieber/timer]. Using timer you would pass check() function as an argument so it would call it back periodically. This is more slippery but scales in that you can have lots of checks running which for a scaleable EC2 deployment is what is probably called for.

timer = require('timer');   //import the time module    
function check(){           //check function now polls
    //add an if(something==true) statement here to allow you to exit if necessary
    console.log("Checking stuff");
    timer.timer(200, check); //calls this function repeatedly every 200ms
}

check(); //starts the code polling

3. Having said that, the functionality you are trying to implement sounds to me like it is the same as that provide by Amazon Autoscaling [http://aws.amazon.com/autoscaling/] and Amazon Elastic Beanstalk [http://aws.amazon.com/elasticbeanstalk/]. You might also have some luck with those.

Happy node hacking :)

Upvotes: 1

ralphtheninja
ralphtheninja

Reputation: 133008

You should never pause in node. Take a look at async (https://github.com/caolan/async). What you need is probably async.forEachSeries(), which allows you to do things in series, but asyncronously.

In your case it would start up a server and when that is started a callback is called that makes the series continue on the next server etc.

Here's some sample code from node-levelup https://github.com/rvagg/node-levelup/blob/master/test/benchmarks/index.js#L60-L84

Upvotes: 0

mayorbyrne
mayorbyrne

Reputation: 507

nodejs isn't made to pause, really. you might be looking for something like an eventEmitter, so you can call a function when you emit an event. http://nodejs.org/api/events.html

Upvotes: 0

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