gistfile1.js

async.parallel([
    function(callback){
                db.order_book.aggregate(
                [
                    { $match: {currency: task.params.currency, pricing: task.params.pricing}},
                    { $group: {_id: "$price", amount: {$sum: "$amount"}}},
                    { $sort: {_id: -1}}
                ], callback
        );

    },
    function(callback){
            db.order_book.aggregate(
                [
                    { $match: {currency: task.params.pricing, pricing: task.params.currency}},
                    { $group: {_id: "$price", amount: {$sum: "$amount"}}},
                    { $sort: {_id: 1}}
                ], callback
        );
    }

    ],
    function(err,results){
        console.log(results[0]);
        console.log(results[1]);
        update_task(task,function(err,res){
            if(err)
                console.log(err);
        });
    });
}