error handling in asynchronous node.js calls

Node 0.8 introduces a new concept called "Domains". They are very roughly analogousness to AppDomains in .net and provide a way of encapsulating a group of IO operations. They basically allow you to wrap your request processing calls in a context specific group. If this group throws any uncaught exceptions then they can be handled and dealt with in a manner which gives you access to all the scope and context specific information you require in order to successfully recover from the error (if possible).

This feature is new and has only just been introduced, so use with caution, but from what I can tell it has been specifically introduced to deal with the problem which the OP is trying to tackle.

Documentation can be found at: http://nodejs.org/api/domain.html

Checkout the uncaughtException handler in node.js. It captures the thrown errors that bubble up to the event loop.

http://nodejs.org/docs/v0.4.7/api/process.html#event_uncaughtException_

But not throwing errors is always a better solution. You could just do a return res.end('Unabled to load file xxx');

This is one of the problems with Node right now. It's practically impossible to track down which request caused an error to be thrown inside a callback.

You're going to have to handle your errors within the callbacks themselves (where you still have a reference to the request and response objects), if possible. The uncaughtException handler will stop the node process from exiting, but the request that caused the exception in the first place will just hang there from the user point of view.