datasource.js

// somewhere in your data source:

someMethod: function () {
  SC.Request.getUrl('http://your.url.here').json().notify(this, this._myCallbackMethod).send();  
},

_myCallbackMethod: function (result) {
  if (SC.ok(result)){ 
    // successfully retrieved
  }
  else {
    //error
  }
}