Avoid launching datanodes on labeld nodes (#9)

Author: kimoonkim

charts/hdfs-datanode-k8s/README.md

@@ -12,18 +12,24 @@ HDFS `datanodes` running inside a kubernetes cluster. See the other chart for
 
 ### Usage
 
-  1. Optionally, find the domain name of your k8s cluster that becomes part of
+  1. In some setup, the master node may launch a datanode. To prevent this,
+     label the master node with `hdfs-datanode-exclude`.
+  ```
+  $ kubectl label node YOUR-MASTER-NAME hdfs-datanode-exclude=yes
+  ```
+
+  2. Optionally, find the domain name of your k8s cluster that become part of
      pod and service host names. Default is `cluster.local`. See `values.yaml`
      for additional parameters to change. You can add them below in `--set`,
      as comma-separated entries.
 
-  2. Launch this helm chart, `hdfs-datanode-k8s`.
+  3. Launch this helm chart, `hdfs-datanode-k8s`.
 
   ```
   $ helm install -n my-hdfs-datanode hdfs-datanode-k8s
   ```
 
-  3. Confirm the daemons are launched.
+  4. Confirm the daemons are launched.
 
   ```
   $ kubectl get pods | grep hdfs-datanode-

charts/hdfs-datanode-k8s/templates/datanode-daemonset.yaml

@@ -24,6 +24,28 @@ spec:
     metadata:
       labels:
         name: hdfs-datanode
+      annotations:
+        # TODO: The following uses annotation to express affinity
+        # because Kubernetes 1.5 supports affinity only in annotation as alpha
+        # feature. In Kubernetes 1.6, this is now beta and supports regular
+        # YAML construct. Switch to the official syntax when we support 1.6.
+        scheduler.alpha.kubernetes.io/affinity: >
+          {
+            "nodeAffinity": {
+              "requiredDuringSchedulingIgnoredDuringExecution": {
+                "nodeSelectorTerms": [
+                  {
+                    "matchExpressions": [
+                      {
+                        "key": "hdfs-datanode-exclude",
+                        "operator": "DoesNotExist"
+                      }
+                    ]
+                  }
+                ]
+              }
+            }
+          }
     spec:
       hostNetwork: true
       hostPID: true