Core synchronous queue with an example worker

src/services/calc-queue/exceptions/AlreadyInProgress.js

@@ -0,0 +1,11 @@
+
+module.exports = function AlreadyInProgress(extra) {
+  Error.captureStackTrace(this, this.constructor)
+  this.name = this.constructor.name
+  this.message = 'calculation request is already in progress'
+  if (typeof extra !== 'undefined') {
+    this.extra = extra
+  }
+}
+
+require('util').inherits(module.exports, Error)