Merge pull request #267 from kragniz/cassandra-docs

Automatic merge from submit-queue.

Add doc on multi-az cassandra deployments

```release-note
NONE
```

Author: jetstack-ci-bot

docs/cassandra/multi-az.md

@@ -0,0 +1,87 @@
+Cassandra Across Multiple Availability Zones
+============================================
+
+With rack awareness
+-------------------
+
+Navigator supports running Cassandra with
+[rack and datacenter-aware replication](https://docs.datastax.com/en/cassandra/latest/cassandra/architecture/archDataDistributeReplication.html).
+To deploy this, you must run a `nodePool` in each availability zone, and mark each as a separate Cassandra rack.
+
+The
+[`nodeSelector`](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#nodeselector)
+field of a nodePool allows scheduling the nodePool to a set of nodes matching labels.
+This should be used with a node label such as
+[`failure-domain.beta.kubernetes.io/zone`](https://kubernetes.io/docs/reference/labels-annotations-taints/#failure-domainbetakubernetesiozone).
+
+The `datacenter` and `rack` fields mark all Cassandra nodes in a nodepool as being located in that datacenter and rack.
+This information can then be used with the
+[`NetworkTopologyStrategy`](http://cassandra.apache.org/doc/latest/architecture/dynamo.html#network-topology-strategy)
+keyspace replica placement strategy.
+If these are not specified, Navigator will select an appropriate name for each: `datacenter` defaults to a static value, and `rack` defaults to the nodePool's name.
+
+As an example, the nodePool section of a CassandraCluster spec for deploying into GKE in europe-west1 with rack awareness enabled:
+
+```yaml
+  nodePools:
+  - name: "np-europe-west1-b"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "europe-west1-b"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-b"
+    persistence:
+      enabled: true
+      size: "5Gi"
+      storageClass: "default"
+  - name: "np-europe-west1-c"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "europe-west1-c"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-c"
+    persistence:
+      enabled: true
+      size: "5Gi"
+      storageClass: "default"
+  - name: "np-europe-west1-d"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "europe-west1-d"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-d"
+    persistence:
+      enabled: true
+      size: "5Gi"
+      storageClass: "default"
+```
+
+Without rack awareness
+----------------------
+
+Since the default rack name is equal to the nodepool name,
+simply set the rack name to the same static value in each nodepool to disable rack awareness.
+
+A simplified example:
+
+```yaml
+  nodePools:
+  - name: "np-europe-west1-b"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "default-rack"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-b"
+  - name: "np-europe-west1-c"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "default-rack"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-c"
+  - name: "np-europe-west1-d"
+    replicas: 3
+    datacenter: "europe-west1"
+    rack: "default-rack"
+    nodeSelector:
+      failure-domain.beta.kubernetes.io/zone: "europe-west1-d"
+```