I just started to code in node.js for a little while. Now here is one of my questions about it:
In HTTP apps, given the request-response model, the single app thread is blocked until all the back end tasks are done and response is returned to the client, so the performance improvement seems to be limited only to fine-tuning back end things like parallelizing IO requests. (Well, this improvement matters when it comes to many heavy and independent IO operations being involved, but usually the condition also implies that by redesigning the data structure you could eliminate a large number of IO request and, possibly, end up with even better performance than just issuing parallelized operations.)
If that is true, how could it produce superior performance than those frameworks based on Java (or PHP, python, etc.) do?
I also referred to an article Understanding the node.js event loop, which also explains that situation:
It really is a single thread running: you can’t do any parallel code execution; doing a “sleep” for example will block the server for one second:
while(new Date().getTime() < now + 1000) {
// do nothing
}
…however, everything runs in parallel except your code.
I personally verified that by putting exact the "sleep" code into one IO callback closure, and tried submitting a request leading to this callback, then submitted another one. Both requests will trigger a console log when it is processed. And my observation is that the later was blocked until the former returned a response.
So, does it imply that only in socket mode, where both sides can emit events and push messages to each other at any time, would the full power of its asynchronous processing capability be utilized?
I'm a little confused about that. Any comment or advice is welcome. Thanks!
update
I ask this question because some performance evaluation cases are reported, for instance Node.js is taking over the Enterprise – whether you like it or not, and LinkedIn Moved from Rails to Node: 27 Servers Cut and Up to 20x Faster. Some radical opinion claims that J2EE will be totally replaced: J2EE is Dead: Long-live Javascript Backed by JSON Services.
NodeJS uses libuv, so IO operations are non-blocking. Yes, your Node app uses 1 thread, however, all the IO requests are pushed to an event queue. Then when the request is made, it is obvious that its response will not be read from socket, file etc. at zero-time. So, whatever is ready in the queue is popped and it is handled. In the mean time, your requests can be answered, there might be chunks or full data to be read, however they are just waiting in the queue to be processed. This goes on until there is no event remains, or the open sockets are closed. Then the NodeJS can finally end its execution.
As you see, NodeJS is not like other frameworks, pretty different. If you have a long going and Non-IO operation, so it is blocking, like matrix operations, image&video processing, you can spawn another processes and assign them the job, use message passing, the way you like TCP, IPC.
The main point of NodeJS is to remove unncesseary context switches which brings significant overhead when not used properly. In NodeJS, why would you want context switches? All the jobs are pushed to event queue and they are probably small in computation, since all they do to make multiple IO/s, (read from db, update db, write to client, write to bare TCP socket, read from cache), it is not logical to stop them in the middle and switch to another job. So with the help of libuv, whichever IO is ready can be executed right now.
For reference please look at libuv documentation: http://nikhilm.github.io/uvbook/basics.html#event-loops
As far as my experience(though brief) goes with node.js, i agree the performance of node.js server can not be compared with other webservers like tomcat etc as stated in node.js doc somewhere
It really is a single thread running: you can’t do any parallel code execution; doing a “sleep” for example will block the server for one second:
So we used it not as alternative to full fledged webserver like tomcat but just to distrubute some load from tomcat where we can take of single thread model. So it has to be trade-off somewhere
Also see http://www.sitepoint.com/node-js-is-the-new-black/ Thats the beautiful article about node.js