Repeatedly calling Rest Endpoint from NodeJS

Question

I am very new to Node family. I have a service which has two endpoint exposed. One is a post method - which takes a payload (which gets processed asynchronously) and sends an acknowledgement to the caller immediately. Another one is a get method - Which is used to check the status of the earlier request.

For example:

Lets assume two end points

(1) http://localhost:8080/myservice/process/11

PayLoad - Any JSON Object
Response: "Request Received. Under Process"

(2) http://localhost:8080/myservice/checkstatus/11

Response: "In-Progress" or "Completed"

From Node module I have to call first end point and then the endpoint will respond with an acknowledgement. Then upon receiving the ack, I need to keep on calling the second GET end point unless it has a response "Completed".

I am not able to understand how can I repeatedly call the endpoint. Any small code snippet will help me understanding the same.

Thanks in advance.

Answer 1

You can use setInterval for this :

Please Note : Below is a pseudo code and should be used just for reference. Copy pasting would not result in a working solution.

service.post('url-1', payload).then(function(response){
   var timer = setInterval(function(){
        service.get('url-2').then(function(resp){
           if(resp.data == 'Completed') clearInterval(timer);
        });
   }, 1000);
});

Answer 2

This will poll the service every half second until it gets a response that matches the endingCondition (whatever you want that to be). Notice that native Promises do not have a 'denodeify' function although its easy enough to write your own or use third-party lib. The done value is not strictly necessary and could be refactored out if desired, but allows for more introspection. Also note that this is a design smell: as noted in the comments on the question it'd be better to simply return the response from the server when the async process completes.

let request = require('request');
let promise = require('promise');
let done = false;
let poll = promise.denodeify(request.get).bind(request);
let p = poll(serviceURL);
let handle = setInterval(() => {
  p.then(res => {
    if (res === endingCondition) {
      done = true;
      clearInterval(handle);
    }

    if (!done) {
      p = poll(serviceURL);
    } 
  });
}, 500);

Answer 3

You can use SynJS for this type of tasks, below is working code:

var SynJS = require('synjs');
var request = require('request');

function requestWrapper(context, url, method, postData) {
    console.log('get started:', url, method, postData);
    var result = { done: false };
    request({
        uri: url,
        method: method,
        postData: postData,

    }, function(error, response, body){
        result.done = true;
        result.body = body;
        result.response = response; 
        console.log('get finished:', url, method, postData);
        SynJS.resume(context); // <-indicates that long running functoin is done
    });
    return result;
}

// synchronous function
function myApiSubmitter(modules) {
    var postRes = modules.requestWrapper(_synjsContext, "https://www.google.com/search","POST","asdasd");
    SynJS.wait(postRes.done); // <- waits for SynJS.resume, and checks for postRes.done, continues if postRes.done is true

    for(var i=0; i<5; i++) {
        SynJS.wait(500); // <- waits for 500ms

        var getRes = modules.requestWrapper(_synjsContext, "https://www.google.com/","GET",i);
        SynJS.wait(getRes.done); // <- waits for SynJS.resume, and checks for getRes.done, continues if getRes.done is true

        if(getRes.body.indexOf('some_success_message')>=0) // <- some arbitrary condition to break the cycle
            break;
    }
};

var modules = {
        SynJS:  SynJS,
        requestWrapper: requestWrapper,
};

SynJS.run(myApiSubmitter,null,modules,function () {
    console.log('done');
});

It would produce following output:

get started: https://www.google.com/search POST asdasd
get finished: https://www.google.com/search POST asdasd
get started: https://www.google.com/ GET 0
get finished: https://www.google.com/ GET 0
get started: https://www.google.com/ GET 1
get finished: https://www.google.com/ GET 1
get started: https://www.google.com/ GET 2
get finished: https://www.google.com/ GET 2
get started: https://www.google.com/ GET 3
get finished: https://www.google.com/ GET 3
get started: https://www.google.com/ GET 4
get finished: https://www.google.com/ GET 4
done