Why is node.js only processing six requests at a time?

Node js can handle thousands of incoming requests - yes!

But when it comes down to ougoing requests every request has to deal with a dns lookup and dns lookup's, disk reads etc are handled by the libuv which is programmed in C++. The default value of threads for each node process is 4x threads.

If all 4x threads are busy with https requests ( dns lookup's ) other requests will be queued. That is why no matter how brilliant your code might be : you sometimes get 6 or sometimes less concurrent outgoing requests per second completed.

Learn about dns cache to reduce the amount of dns look up's and increase libuv size. If you use PM2 to manage your node processes they do have a well documentation on their side on environment variables and how to inject them. What you are looking for is the environment variable UV_THREADPOOL_SIZE = 4

You can set the value anywhere between 1 or max limit of 1024. But keep in mind libuv limit of 1024 is across all event loops.

There are no TCP connection limits imposed by Node itself. (The whole point is that it's highly concurrent and can handle thousands of simultaneous connections.) Your OS may limit TCP connections.

It's more likely that you're either hitting some kind of limitation of your backend server, or you're hitting the builtin HTTP library's connection limit, but it's hard to say without more details about that server or your Node implementation.

Node's built-in HTTP library (and obviously any libraries built on top of it, which are most) maintains a connection pool (via the Agent class) so that it can utilize HTTP keep-alives. This helps increase performance when you're running many requests to the same server: rather than opening a TCP connection, making a HTTP request, getting a response, closing the TCP connection, and repeating; new requests can be issued on reused TCP connections.

In node 0.10 and earlier, the HTTP Agent will only open 5 simultaneous connections to a single host by default. You can change this easily: (assuming you've required the HTTP module as http)

http.globalAgent.maxSockets = 20; // or whatever

node 0.12 sets the default maxSockets to Infinity.

You may want to keep some kind of connection limit in place. You don't want to completely overwhelm your backend server with hundreds of HTTP requests under a second – performance will most likely be worse than if you just let the Agent's connection pool do its thing, throttling requests so as to not overload your server. Your best bet will be to run some experiments to see what the optimal number of concurrent requests is in your situation.

However, if you really don't want connection pooling, you can simply bypass the pool entirely – sent agent to false in the request options:

http.get({host:'localhost', port:80, path:'/', agent:false}, callback);

In this case, there will be absolutely no limit on concurrent HTTP requests.

How are you getting data from the central server? "Node does not limit connections" is not entirely accurate when making HTTP requests with the http module. Client requests made in this way use the http.globalAgent instance of http.Agent, and each http.Agent has a setting called maxSockets which determines how many sockets the agent can have open to any given host; this defaults to 5.

So, if you're using http.request or http.get (or a library that relies on those methods) to get data from your central server, you might try changing the value of http.globalAgent.maxSockets (or modify that setting on whatever instance of http.Agent you're using).

See:

• http.Agent documentation
• agent.maxSockets documentation
• http.globalAgent documentation
• Options you can pass to http.request, including an agent parameter to specify your own agent

It's the limit on number of concurrent connections in the browser:

How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers?

I have upvoted the other answers, as they helped me diagnose the problem. The clue was that node's socket limit was 5, and I was getting 6 at a time. 6 is the limit in Chrome, which is what I was using to test the server.