Documentation for hystrix dashboard support feature (issue #1244)

docs/root/operations/admin.rst

@@ -174,3 +174,28 @@ The fields are:
   .. http:get:: /runtime?format=json
 
   Outputs /runtime in JSON format. This can be used for programmatic access of runtime values.
+
+.. http:get:: /hystrix_event_stream
+
+  Start streaming
+  `event stream <https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events>`_
+  to Hystrix dashboard. Detalied description of Hystrix and its dashboard can be found
+  `here <https://github.com/Netflix-Skunkworks/hystrix-dashboard>`_.
+
+  The dashboard should set the stream source to this admin endpoint.
+
+  Notes on the usage of Hystrix dashboard to visualize Envoy statistics:
+     Not all the data presented in the Hystrix dashboard is relevant in Envoy.
+
+    * Success, Failure, Timeout, Error rate, Number of host, are shown in the dashboard.
+    * Hystrix's *Short circuit* is similar to Envoy's outlier detection, in that it is triggered by
+      error responses. In Envoy the result of high error rate is the host being ejected from the load
+      balancer pool, but it does not cause rejected requests, and therefore we set *Short circuited*
+      to 0, and circuit breaker is set to *Forced Closed*.
+    * *Bad request* does not exist in Envoy, and therefore is set to 0.
+    * Requests rejected as a result of exceeding max number of connections or queue size (in Envoy,
+      referred as *short circuited*) are presented as *Rejected*, which better suits Hystrix's
+      terminology.
+    * Latency information is currently not available.
+    * Window size is set to 10 seconds, with 10 buckets of 1 second each. Currently, the value is
+      not configurable.