You may have heard that Node.js is good for I/O-bound applications. And the commonly mentioned counterpart is the CPU-bound application. You may have wondered what do these terms actually mean.
"What do the terms 'CPU bound' and 'I/O bound' mean?"
Let's find out.
Bound implies performance bottleneck
Computation is said to be bound by something, when that resource is the bottleneck for achieving performance increase. When trying to figure out if your program is CPU, memory or I/O bound you can think the following. By increasing which resource would your program perform better? Does increasing CPU performance increase the performance of your program? Memory, hard disk speed or network connection? All of these questions lead you to right to the source, of which resource your program is being held up on.
This is the case for typical Node.js web server application. Majority of the time is spent waiting for network, filesystem and perhaps database I/O to complete. Increasing hard disk speed or network connection improves the overall performance.
In its most basic form Node.js is best suited for this type of computing. All I/O in Node.js is non-blocking and it allows other requests to be served while waiting for a particular read or write to complete.
An example of CPU bound application would be a service that calculates SHA-1 checksums. Majority of the time is spent crunching the hash - doing large amount of bitwise xors and shifts for the input string.
This kind of application leads to trouble in Node.js. If the application spends too much time performing CPU intensive task all other requests are being held up. Node.js runs a single threaded event loop to concurrently advance many computations, for example serving multiple incoming HTTP requests. This works well as long as all event handlers are small and yet wait for more events themselves. But if you perform CPU intensive calculation your concurrent web server Node.js application will come to a screeching halt. Other incoming requests will wait as only one request is being served at a time - not a very good service.
There are strategies to coping with CPU intensive tasks. You can separate the calculation to elsewhere - forking a child process or using cluster module, using low level worker thread from libuv or creating a separate service. If you still want to do it in the main thread, the least you can do is give the execution back to the event loop frequently with setImmediate().
Not every Node.js program needs high level of concurrency. When running popular task runner grunt for example, it doesn't really matter if something is CPU intensive or not. Sure it may take time, but in the end it's only serving one user: the one sitting in front of the monitor.
In the end
A typical healthy Node.js server application is I/O bound. That is what Node.js was designed for and handles well using the single-threaded event loop. CPU bound tasks cause trouble if not handled correctly - by yielding execution frequently back to the event loop or moving it to another thread, process or service. There also exists other classifications that we did not touch here such as memory-bound or cache-bound.