meteor - Synchronizing multiple async queries before returning?

Question

So I have a Meteor method that is supposed to tell the server to send multiple API requests to 3rd party APIs, and then combine the results of these queries into one array,

Answer 1

You have to use the asynchronous version of HTTP.get and collect the results using Futures.

I made up a simple example using setTimeouts to simulate HTTP requests so that you understand the principle, I advise you start from this code and replace the dummy setTimeout with your HTTP get request.

The example is a test server method which takes a number of tasks (n) as a parameter, it then launches n tasks that each compute the square of their index in index seconds.

// we use fibers which is a dependency of Meteor anyway
var Future = Npm.require("fibers/future");

Meteor.methods({
    test: function(n) {
        // build a range of tasks from 0 to n-1
        var range = _.range(n);
        // iterate sequentially over the range to launch tasks
        var futures = _.map(range, function(index) {
            var future = new Future();
            console.log("launching task", index);
            // simulate an asynchronous HTTP request using a setTimeout
            Meteor.setTimeout(function() {
                // sometime in the future, return the square of the task index
                future.return(index * index);
            }, index * 1000);
            // accumulate asynchronously parallel tasks
            return future;
        });
        // iterate sequentially over the tasks to resolve them
        var results = _.map(futures, function(future, index) {
            // waiting until the future has return
            var result = future.wait();
            console.log("result from task", index, "is", result);
            // accumulate results
            return result;
        });
        //
        console.log(results);
        return results;
    }
});

Type > Meteor.call("test",3,function(error,result){console.log(result);}); in your browser console. This will output [0,1,4] after 3 seconds.

In your server console, this will output :

// immediately :
launching task 0
launching task 1
launching task 2
// after 1 second :
result from task 0 is 0
// after 2 seconds :
result from task 1 is 1
// after 3 seconds :
result from task 2 is 4
[ 0, 1, 4 ]

The HTTP.get asynchronous version is detailed in Meteor docs :

http://docs.meteor.com/#http_call

If you want to understand better the whole Future concept, refer to the fibers docs :

https://github.com/laverdet/node-fibers