Multiple backend non blocking calls from NodeJS gives slow response - performance

Question

I have a specific use case which Im trying to solve using Node. The response time that I receive from NodeJS is not the one I expect.
The application is an express.js web application. The flow is as below
a. Request reaches the server.
b. Based on the parameter, backend REST Service is invoked.
c. The response of the REST Service has links to multiple other objects.
d. Navigate each of the link and agrregate the data.
e. This data is formatted (not much) and send to the client.
The actual test data-
The response from C has got 100 links and hence I make 100 parallel calls (Im using async.map). Each of the backend service responds in less than 30 msecs. But the overall response time for 100 requests is 4 seconds. This is considerably high.
What I have observed is :
The time difference between the first backend request and the last backend request is around 3 seconds. I believe that this is due to the fact that Node is single threaded and it takes 3 seconds to place all of the 100 http requests.
The code that I use to make parallel calls is given below
var getIndividualRecord = function(entity,callback1)
{
httpExecutor.executeRequest( entity.link.url, callback1);
}
var aggregateData = function(err, results)
{
callback(null, results);
}
async.map(childObjects, getIndividualRecord, aggregateData);
The childObjects is an array with 100 records. httpExecutor makes a REST invocation using request module.
Is there something wrong Im doing or is this a wrong use case for Node?

Answer 1

You're assumption is correct: node is single threaded, so while your HTTP requests happen in a nonblocking manner (requests are made right after the other without even waiting for the response from the server), they don't truly happen in simultaneously.
So, yes, it's probable it takes Node 3 seconds to get through all these requests and process them.
There are a few ways "around" this, which might work depending on your situation:
Could you use Node's cluster module to spawn multiple node apps and each do a portion of the work? Then you would be doing things simultaneously (since you have N Node processes going on).
Use a background queue mechanism (aka: Resque, Beanstalk) and have a background worker (or a process spawned with Cluster) to distribute the work (to Node worker processes waiting around to pick things off this queue)
Refactor your web app a little bit to deal with the fact that parts will take a while. Perhaps render most of the page then onload make an ajax request that fires off the 3 second route and then puts the results in some DOM element when the AJAX request comes back.

Answer 2

i have similar scenario and similar observation.
in my case I run node app using pm2. in app there are 2 sub servers (let's call them A and B). pm2 spawns 2 processes per each server. from a client i call server A, it calculates simple thing and call server B in async manner. when server B responds server A sends data back to client.
very simple scenario but when I used jmeter to create 1000 threads (where each thread makes 50 calls)to call server A I got average response around 4 sec (for 50000 calls).
server B responds after 50ms and I think this is the problem. during first 50ms nodejs processes lots of incoming requests and then it cannot quickly process responses from server B and incoming calls.
I would expect that application code is executed in single thread but there supposed to be background threads to deal with all the rest. it seems this is not the case.