userland implementation of https://github.com/joyent/node/issues/5243

The domains mechanism is a useful tool for adding context to errors raised in asynchronous call chains (or, if you like living dangerously / tempting the wrath of @isaacs, to recover from errors without restarting your services). It also almost serves the purposes of developers who want to annotate async call chains with metadata or extra state (examples: logging, tracing / profiling, generic instrumentation), but due to the needs of error-handling, it doesn't quite generalize enough to be truly useful in this regard. There are modules that allow developers to do similar things when they have full control over their stacks (CrabDude/trycatch and Gozala/reducers, among many others), but none of these modules are a good fit for developers writing tooling meant to be dropped transparently into user code.

See also joyent/node#3733.

Here is a sketch at what the user-visible API might look like. My original attempt at this used a slightly modified version of the domains API with some special-purpose logic for dealing with nested contexts, but allowing multiple distinct namespaces is actually simpler and trades memory for execution time. It also makes it possible to special-case behavior for specific namespaces (i.e. my hope would be that domains would just become a specialized namespace, and _tickDomainCallback and _nextDomainTick would be all that would be required to deal with namespaces), although that isn't included here.

Here's an example of how the API might be used:

var context = require('context');
// multiple contexts in use 
var tracer = context.createNamespace('tracer');
function Trace(harvester) {
  this.harvester = harvester;
Trace.prototype.runHandler = function (callback) {
  var trace = tracer.createContext();
  trace.on('end', function () {
    var transaction = trace.get('transaction');
    this.harvester.emit('finished', transaction);
Trace.prototype.annotateState = function (namevalue) {
  var active = tracer.active;
  active.set(name, value);