Add DTR health check explanations

Author: whoshuu

datacenter/dtr/2.2/guides/admin/configure/use-a-load-balancer.md

@@ -4,8 +4,9 @@ description: Learn how to configure a load balancer to balance user requests acr
 keywords: docker, dtr, load balancer
 ---
 
-Once you’ve joined multiple DTR replicas nodes for high-availability, you can
-configure your own load balancer to balance user requests across all replicas.
+Once you’ve joined multiple DTR replicas nodes for
+[high-availability](set-up-high-availability.md), you can configure your own
+load balancer to balance user requests across all replicas.
 
 ![](../../images/use-a-load-balancer-1.svg)
 
@@ -23,9 +24,36 @@ Make sure you configure your load balancer to:
 
 * Load balance TCP traffic on ports 80 and 443
 * Not terminate HTTPS connections
-* Use the `/health` endpoint on each DTR replica, to check if
-the replica is healthy and if it should remain on the load balancing pool or
-not
+* Use the `/health` endpoint (note the lack of an `/api/v0/` in the path) on each
+DTR replica, to check if the replica is healthy and if it should remain on the
+load balancing pool or not
+
+## Health check endpoints
+
+The `/health` endpoint returns a JSON object for the replica being queried with
+`"Healthy"` as one of the keys. Any response other than a 200 HTTP status code
+and `"Healthy":true` means the replica is unsuitable for taking requests. If
+the API server is still up, the returned JSON object will have an `"Error"` key
+with more details. More specifically, these issues can be in any of these
+services:
+
+* Storage container (registry)
+* Authorization (garant)
+* Metadata persistence (rethinkdb)
+* Content trust (notary)
+
+Note that this endpoint is for checking the health of a *single* replica. To get
+the health of every replica in a cluster, querying each individual replica is
+the preferred way to do it in real time.
+
+The `/api/v0/meta/cluster_status` endpoint returns a JSON object for the entire
+cluster *as observed* by the replica being queried. Health status for the
+replicas is available in the `"replica_health"` key. These statuses are taken
+from a cache which is updated by each replica individually.
+
+In addition, this endpoint returns a dump of the rethink system tables
+which can be rather large (~45 KB) for a status endpoint.
+
 
 ## Where to go next