Models of concurrency in nodejs

Question

Does anyone know what the memory and threading models are in nodejs?

In particular, is ii++ atomic? Does it behave as if ii were volatile in Java 1.5, in Java 1.4, in C, or not at all?

Answer 1

It is useful to understand how node and V8 interact. Node handles waiting for I/O or timers from the operating system. When node wakes up from I/O or a timer, it generally has some JavaScript callbacks to invoke. When node runs these callbacks, control is passed into V8 until V8 returns back to node.

So, if you do var ii = 1; ii++;, you will never find that ii is anything other than 2. All JavaScript runs until completion, and then control is passed back to node. If you do doSomething(); doSomething(); that will always run doSomething twice, and it will not return to node's event loop until the second invocation of doSomething returns. This means you can completely lock up node from a simple error like this:

for (var i=0 ; i >= 0 ; i++) {} 

It doesn't mater how many I/O callbacks you have registered, timers set to go off, or sockets waiting to be read. Until V8 returns from that infinite loop, node does no more work.

This is part of what makes programming in node so nice. You never have to worry about locking. There are no race conditions or critical sections. There is only one thread where your JavaScript code runs.