Illustrate exception handling with a simple proxy middleware

armor.js

/**
 * Pattern for performing error handling given standard node
 * callback patterns (ie. error passed as first argument).
 */
function Armorer(nextCallback) {
	this.nextCallback=nextCallback;
	this.cleanupCallbacks=[];
}
Armorer.prototype={
	onCleanup: function(callback) {
		this.cleanupCallbacks.push(callback);
		return this;
	},
	
	/**
	 * Given an eventEmitter, attach to the error event such that
	 * the retreat is called if it is ever raised.
	 */
	enlist: function(eventEmitter, cleanupCallback) {
		eventEmitter.on('error', this.retreat.bind(this));
		if (cleanupCallback) {
			this.cleanupCallbacks.push(cleanupCallback);
		}
		return this;
	},
	
	/**
	 * Handle the given exception, invoking all exception handlers
	 * in the process.
	 */
	retreat: function(exception) {
		this.cleanup();
		return this.nextCallback(exception);
	},
	
	/**
	 * Initiate cleanup
	 */
	cleanup: function() {
		for (var i=0; i<this.cleanupCallbacks.length; i++) {
			var cb=this.cleanupCallbacks[i];
			cb();
		}
	},
	
	/**
	 * Traps any exceptions coming out of callback and calls
	 * retreat() if they happen.
	 */
	guard: function(callback) {
		try {
			return callback();
		} catch (e) {
			console.log('Exception handled in guarded callback: ' + e);
			return this.retreat(e);
		}
	},
	
	/**
	 * Returns a function that will invoke callback with full exception
	 * handling.
	 */
	trap: function(callback) {
		var me=this;
		return function() {
			try {
				return callback.apply(this, arguments);
			} catch (e) {
				console.log('Exception handled in guarded callback: ' + e);
				return me.retreat(e);
			}
		};
	}
};
 
function armor(nextCallback) {
	return new Armorer(nextCallback);
}

Function.prototype.armor=function(armorer) {
	return armorer.trap(this);
};

module.exports=armor;

proxy.js

/**
 * Simple proxy intended to be used as middleware to direct parts
 * of an HTTP namespace to other servers.  
 *
 * Copyright (c) 2011, Stella Laurenzo
 */
var util=require('util'),
	url=require('url'),
	http=require('http'),
	armor=require('./armor');

function makeAppendDestination(baseUrl) {
	return function(request) {
		//console.log('Request: ' + util.inspect(request));
		return baseUrl + request.url;
	};
}
 
module.exports=function(destination) {
	if (typeof destination==='string')
		destination=makeAppendDestination(destination);
	
	return function(req, res, next) {
		var newUrl=destination(req),
			parsed=url.parse(newUrl),
			port=parsed.port;
			
		//console.log('url: ' + util.inspect(parsed));
		if (!port) {
			if (parsed.protocol==='http:') port=80;
			else if (parsed.protocol==='https:') port=443;
			else port=80;
		}
		
		var client=http.createClient(port, parsed.hostname),
			clientRequest=client.request(req.method, 
				parsed.pathname + (parsed.search||''),
				req.headers);
		
		// Error handling
		var a=armor(next)
			.enlist(client)
			.enlist(clientRequest, clientRequest.end.bind(clientRequest));
		
		// Patch through the response
		clientRequest.on('response', function(pxres) {
			pxres.on('data', function(chunk) {
				res.write(chunk, 'binary');
			}.armor(a));
			pxres.on('end', function() {
				res.end();
			}.armor(a));
			res.writeHead(pxres.statusCode, pxres.headers);
		}.armor(a));
		
		// Patch through the request
		req.on('data', function(chunk) {
			clientRequest.write(chunk, 'binary');
		}.armor(a));
		req.on('end', function() {
			clientRequest.end();
		}.armor(a));
	};
};