error handling in asynchronous node.js calls

Question

I'm new to node.js although I'm pretty familiar with JavaScript in general. My question is regarding "best practices" on how to handle errors in node.js.

Normally when programming web servers, FastCGI servers or web pages in various languages I'm using Exceptions with blocking handlers in a multi-threading environment. When a request comes in I usually do something like this:

function handleRequest(request, response) {   try {      if (request.url=="whatever")       handleWhateverRequest(request, response);     else       throw new Error("404 not found");    } catch (e) {     response.writeHead(500, {'Content-Type': 'text/plain'});     response.end("Server error: "+e.message);   } }  function handleWhateverRequest(request, response) {   if (something)      throw new Error("something bad happened");   Response.end("OK"); } 

This way I can always handle internal errors and send a valid response to the user.

I understand that with node.js one is supposed to do non-blocking calls which obviously leads to various number of callbacks, like in this example:

var sys    = require('sys'),     fs     = require('fs');  require("http").createServer(handleRequest).listen(8124);  function handleRequest(request, response) {    fs.open("/proc/cpuinfo", "r",     function(error, fd) {       if (error)         throw new Error("fs.open error: "+error.message);        console.log("File open.");        var buffer = new require('buffer').Buffer(10);       fs.read(fd, buffer, 0, 10, null,         function(error, bytesRead, buffer) {            buffer.dontTryThisAtHome();  // causes exception            response.end(buffer);         }); //fs.read      }); //fs.open  } 

This example will kill the server completely because exceptions aren't being catched. My problem is here that I can't use a single try/catch anymore and thus can't generally catch any error that may be raised during the handling of the request.

Of course I could add a try/catch in each callback but I don't like that approach because then it's up to the programmer that he doesn't forget a try/catch. For a complex server with lots of different and complex handlers this isn't acceptable.

I could use a global exception handler (preventing the complete server crash) but then I can't send a response to the user since I don't know which request lead to the exception. This also means that the request remains unhandled/open and the browser is waiting forever for a response.

Does someone have a good, rock solid solution?

Answer 1

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

Answer 2

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');