qdomain - Promises from domains

Small experiment combining node.js domain error handling with promises using the Q library.

Using domains it is possible to capture unhandled asynchronous exceptions. This module wraps that magic to promises.

var qdomain = require("qdomain");

qdomain(function(defer){

  setTimeout(function(){
    throw new Error("async error");
  }, 100);

}).then(function(){
  // nothing...
}, function(err){

  // We will get the thrown async error here!

});

qdomain takes a callback and returns a promise. It can be resolved or rejected using following methods:

1. Throw an exception on this tick, the next or whenever to reject it. That's the magic of domains!
2. Call resolve or reject on the given defer object - It's full Q defer object
3. Return another promise

This makes it easy to capture stream errors while piping for example

qdomain(function(defer){
  fs.createReadStream("somefile")
  .pipe(transform1())
  .pipe(transform2())
  .pipe(transform3())
  .pipe(fs.createWriteStream("output-file"))
  .on("close", defer.resolve);
}).fail(function(err){
  // Any IO errors or transform errors will be handled here
});

Note about domain disposing

When the promise is resolved or rejected the domain associated with it will be disposed automatically. Which means all IO and timers must be completed before resolving the promise or they will be canceled! This has few nice features:

• No more errors can be thrown from the qdomain callback
• You can be sure that all IO started from the callback has been completed (or canceled) when the promise is resolved or rejected.

Install

npm install qdomain