Add Member Roles doc

docs/articles/clustering/node-roles.md

@@ -0,0 +1,24 @@
+---
+uid: node-roles
+title: Node Roles
+---
+
+# Node Roles
+
+Not all nodes of a cluster need to perform the same function. For example, there might be one sub-set which runs the web front-end, one which runs the data access layer and one for the number-crunching. Choosing which actors to start on each node, for example cluster-aware routers, can take node roles into account to achieve this distribution of responsibilities.
+
+The node roles are defined in the configuration property named `akka.cluster.roles` and typically defined in the start script as a system property or environment variable.
+
+The roles are part of the membership information in `MemberEvent` that you can subscribe to. The roles of the own node are available from the `SelfMember` and that can be used for conditionally starting certain actors:
+
+```csharp
+var selfMember = Cluster.Get(_actorSystem).SelfMember;
+if (selfMember.HasRole("backend")) 
+{
+  context.ActorOf(Backend.Prop(), "back");
+} 
+else if (selfMember.HasRole("front")) 
+{
+  context.ActorOf(Frontend.Prop(), "front");
+}
+```

docs/articles/clustering/toc.yml

@@ -21,4 +21,6 @@
 - name: Distributed Data
   href: distributed-data.md
 - name: Split Brain Resolver
-  href: split-brain-resolver.md
+  href: split-brain-resolver.md
+- name: Node Roles
+  href: node-roles.md