feat: add basic rolling stats to a circuit

This replaces the existing cumulative behavior of a circuit's status, in
favor of snapshots every `X` milliseconds, where `X` defaults to 10000,
e.g. 10 seconds. This allows for rolling average calculations on
important statistics for Hystrix dashboard, and is in pursuit of
#32 (but does not
complete it).

lib/circuit.js

@@ -27,12 +27,16 @@ const NAME = Symbol('name');
  * opening.
  * @param options.resetTimeout The time in milliseconds to wait before setting
  * the breaker to `halfOpen` state, and trying the action again.
+ * @param options.rollingCountTimeout Sets the duration of the statistical
+ *  rolling window, in milliseconds. This is how long Opossum keeps metrics for
+ *  the circuit breaker to use and for publishing. Default: 10000
  * @fires CircuitBreaker#halfOpen
  */
 class CircuitBreaker extends EventEmitter {
   constructor (action, options) {
     super();
     this.options = options;
+    this.options.rollingCountTimeout = options.rollingCountTimeout || 10000;
     this.Promise = options.Promise;
     this[STATUS] = new Status(this);
     this[STATE] = CLOSED;

lib/status.js

@@ -9,38 +9,45 @@ const CIRCUIT_BREAKER = Symbol('circuit-breaker');
  */
 class Status {
   constructor (circuit) {
-    /**
-     * The number of times the breaker's action has failed
-     */
-    this.failures = 0;
-    /**
-     * The number of times a fallback function has been executed
-     */
-    this.fallbacks = 0;
-    /**
-     * The number of times the action for this breaker executed successfully
-     */
-    this.successes = 0;
-    /**
-     * The number of times this breaker been rejected because it was fired, but in the open state.
-     */
-    this.rejects = 0;
-    /**
-     * The number of times this circuit breaker has been fired
-     */
-    this.fires = 0;
-    /**
-     * The number of times this circuit breaker has timed out
-     */
-    this.timeouts = 0;
+    reset(this);
     this[CIRCUIT_BREAKER] = circuit;
     circuit.on('success', () => this.successes++);
     circuit.on('failure', () => this.failures++);
     circuit.on('fallback', () => this.fallbacks++);
     circuit.on('timeout', () => this.timeouts++);
     circuit.on('fire', () => this.fires++);
     circuit.on('reject', () => this.rejects++);
+    const interval = setInterval(
+      () => reset(this), circuit.options.rollingCountTimeout);
+    if (typeof interval.unref === 'function') interval.unref();
   }
 }
 
+function reset (status) {
+  /**
+   * The number of times the breaker's action has failed
+   */
+  status.failures = 0;
+  /**
+   * The number of times a fallback function has been executed
+   */
+  status.fallbacks = 0;
+  /**
+   * The number of times the action for this breaker executed successfully
+   */
+  status.successes = 0;
+  /**
+   * The number of times this breaker been rejected because it was fired, but in the open state.
+   */
+  status.rejects = 0;
+  /**
+   * The number of times this circuit breaker has been fired
+   */
+  status.fires = 0;
+  /**
+   * The number of times this circuit breaker has timed out
+   */
+  status.timeouts = 0;
+}
+
 module.exports = exports = Status;

test/test.js

@@ -294,6 +294,24 @@ test('CircuitBreaker status', (t) => {
     });
 });
 
+test('CircuitBreaker rolling counts', (t) => {
+  const breaker = cb(passFail, { rollingCountTimeout: 100 });
+  const deepEqual = (t, expected) => (actual) => t.deepEqual(actual, expected, 'expected status values');
+  Fidelity.all([
+    breaker.fire(10).then(deepEqual(t, 10)),
+    breaker.fire(20).then(deepEqual(t, 20)),
+    breaker.fire(30).then(deepEqual(t, 30))
+  ])
+    .then(() =>
+      t.deepEqual(breaker.status.successes, 3, 'breaker succeeded 3 times'))
+    .then(() => {
+      setTimeout(() => {
+        t.deepEqual(breaker.status.successes, 0, 'breaker reset stats');
+        t.end();
+      }, 100);
+    });
+});
+
 test('CircuitBreaker fallback event', (t) => {
   t.plan(1);
   const breaker = cb(passFail, {maxFailures: 0});