Adding more information about /state endpoint (#4456)

* Adding more information about /state endpoint

* addressing comments

- removing non-empty blank lines
- converting multiple paragraphs to a list

* Adding /health endpoint docs

(cherry picked from commit 9423d20)

Author: prashant-shahi

wiki/content/deploy/index.md

@@ -1225,6 +1225,24 @@ By default the Alpha listens on `localhost` for admin actions (the loopback addr
 
 {{% notice "tip" %}}Set max file descriptors to a high value like 10000 if you are going to load a lot of data.{{% /notice %}}
 
+### More about /health endpoint
+
+The `/health` endpoint of Dgraph Alpha returns HTTP status 200 with a JSON consisting of basic information about the running worker.
+
+Here’s an example of JSON returned from `/health` endpoint:
+
+```json
+{
+  "version": "v1.1.1",
+  "instance": "alpha",
+  "uptime": 75011100974
+}
+```
+
+- `version`: Version of Dgraph running the Alpha server.
+- `instance`: Name of the instance. Always set to `alpha`.
+- `uptime`: Time in nanoseconds since the Alpha server is up and running.
+
 ## More about Dgraph Zero
 
 Dgraph Zero controls the Dgraph cluster. It automatically moves data between
@@ -1242,13 +1260,13 @@ Like Alpha, Zero also exposes HTTP on 6080 (+ any `--port_offset`). You can quer
 to see useful information, like the following:
 
 * `/state` Information about the nodes that are part of the cluster. Also contains information about
-  size of predicates and groups they belong to.
+size of predicates and groups they belong to.
 * `/assign?what=uids&num=100` This would allocate `num` uids and return a JSON map
 containing `startId` and `endId`, both inclusive. This id range can be safely assigned
 externally to new nodes during data ingestion.
 * `/assign?what=timestamps&num=100` This would request timestamps from Zero.
-  This is useful to fast forward Zero state when starting from a postings
-  directory, which already has commits higher than Zero's leased timestamp.
+This is useful to fast forward Zero state when starting from a postings
+directory, which already has commits higher than Zero's leased timestamp.
 * `/removeNode?id=3&group=2` If a replica goes down and can't be recovered, you
 can remove it and add a new node to the quorum. This endpoint can be used to
 remove a dead Zero or Dgraph Alpha node. To remove dead Zero nodes, pass
@@ -1261,15 +1279,146 @@ You should not use the same `idx` of a node that was removed earlier.
 {{% /notice %}}
 
 * `/moveTablet?tablet=name&group=2` This endpoint can be used to move a tablet to a group. Zero
-  already does shard rebalancing every 8 mins, this endpoint can be used to force move a tablet.
+already does shard rebalancing every 8 mins, this endpoint can be used to force move a tablet.
 
 
 These are the **POST** endpoints available:
 
 * `/enterpriseLicense` Use endpoint to apply an enterprise license to the cluster by supplying it
 as part of the body.
 
+### More about /state endpoint
+
+The `/state` endpoint of Dgraph Zero returns a JSON document of the current group membership info:
 
+- Instances which are part of the cluster.
+- Number of instances in Zero group and each Alpha groups.
+- Current leader of each group.
+- Predicates that belong to a group.
+- Estimated size in bytes of each predicate.
+- Enterprise license information.
+- Max Leased transaction ID.
+- Max Leased UID.
+- CID (Cluster ID).
+
+Here’s an example of JSON returned from `/state` endpoint for a 6-node Dgraph cluster with three replicas:
+
+```json
+{
+  "counter": "15",
+  "groups": {
+    "1": {
+      "members": {
+        "1": {
+          "id": "1",
+          "groupId": 1,
+          "addr": "alpha1:7080",
+          "leader": true,
+          "lastUpdate": "1576112366"
+        },
+        "2": {
+          "id": "2",
+          "groupId": 1,
+          "addr": "alpha2:7080"
+        },
+        "3": {
+          "id": "3",
+          "groupId": 1,
+          "addr": "alpha3:7080"
+        }
+      },
+      "tablets": {
+        "counter.val": {
+          "groupId": 1,
+          "predicate": "counter.val"
+        },
+        "dgraph.type": {
+          "groupId": 1,
+          "predicate": "dgraph.type"
+        }
+      },
+      "checksum": "1021598189643258447"
+    }
+  },
+  "zeros": {
+    "1": {
+      "id": "1",
+      "addr": "zero1:5080",
+      "leader": true
+    },
+    "2": {
+      "id": "2",
+      "addr": "zero2:5080"
+    },
+    "3": {
+      "id": "3",
+      "addr": "zero3:5080"
+    }
+  },
+  "maxLeaseId": "10000",
+  "maxTxnTs": "10000",
+  "cid": "3602537a-ee49-43cb-9792-c766eea683dc",
+  "license": {
+    "maxNodes": "18446744073709551615",
+    "expiryTs": "1578704367",
+    "enabled": true
+  }
+}
+```
+
+Here’s the information the above JSON document provides:
+
+- Group 0
+  - members
+    - zero1:5080, id: 1, leader
+    - zero2:5080, id: 2
+    - zero3:5080, id: 3
+- Group 1
+    - members
+        - alpha1:7080, id: 1, leader
+        - alpha2:7080, id: 2
+        - alpha3:7080, id: 3
+    - predicates
+        - dgraph.type
+        - counter.val
+- Enterprise license
+    - Enabled
+    - maxNodes: unlimited
+    - License expires on Friday, January 10, 2020 4:59:27 PM GMT-08:00 (converted from epoch timestamp)
+- Other data:
+    - maxTxnTs
+        - The current max lease of transaction timestamps used to hand out start timestamps
+          and commit timestamps.
+        - This increments in batches of 10,000 IDs. Once the max lease is reached, another
+          10,000 IDs are leased. In the event that the Zero leader is lost, then the new
+          leader starts a brand new lease from maxTxnTs+1 . Any lost transaction IDs
+          in-between will never be used.
+        - An admin can use the Zero endpoint HTTP GET `/assign?what=timestamps&num=1000` to
+          increase the current transaction timestamp (in this case, by 1000). This is mainly
+          useful in special-case scenarios, e.g., using an existing p directory to a fresh
+          cluster in order to be able to query the latest data in the DB.
+    - maxLeaseId
+        - The current max lease of UIDs used for blank node UID assignment.
+        - This increments in batches of 10,000 IDs. Once the max lease is reached, another
+          10,000 IDs are leased. In the event that the Zero leader is lost, the new leader
+          starts a brand new lease from maxLeaseId+1. Any UIDs lost in-between will never
+          be used for blank-node UID assignment.
+        - An admin can use the Zero endpoint HTTP GET `/assign?what=uids&num=1000` to
+          reserve a range of UIDs (in this case, 1000) to use externally (Zero will NEVER
+          use these UIDs for blank node UID assignment, so the user can use the range
+          to assign UIDs manually to their own data sets.
+    - CID
+        - This is a unique UUID representing the *cluster-ID* for this cluster. It is generated
+          during the initial DB startup and is retained across restarts.
+    - Group checksum
+        - This is the checksum verification of the data per Alpha group. This is used internally
+          to verify group memberships in the event of a tablet move.
+
+{{% notice "note" %}}
+"tablet", "predicate", and "edge" are synonymous terms today. The future plan to
+improve data scalability is to shard a predicate into separate tablets that could
+be assigned to different groups.
+{{% /notice %}}
 
 ## TLS configuration