Add Kubernetes services to FAQ

docs/gitbook/faq.md

@@ -51,3 +51,55 @@ curl -H 'X-Canary: insider' http://app.example.com
 curl -b 'canary=always' http://app.example.com
 ```
 
+### Kubernetes services
+
+How is an application exposed inside the cluster?
+
+Assuming the app name is podinfo you can define a canary like:
+
+```yaml
+apiVersion: flagger.app/v1alpha3
+kind: Canary
+metadata:
+  name: podinfo
+  namespace: test
+spec:
+  targetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: podinfo
+  service:
+    # container port (required)
+    port: 9898
+    # port name can be http or grpc (default http)
+    portName: http
+```
+
+Based on the canary spec service, Flagger generates the following Kubernetes ClusterIP service:
+
+* `<name>.<namespaces>.vc.cluster.local` with selector `app=<name>-primary`
+* `<name>-primary.<namespaces>.vc.cluster.local` with selector `app=<name>-primary`
+* `<name>-canary.<namespaces>.vc.cluster.local` with selector `app=<name>`
+
+This ensures that traffic coming from a namespace outside the mesh to `podinfo.test:9898`
+will be routed to the latest stable release of your app. 
+
+
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  name: podinfo
+spec:
+  type: ClusterIP
+  selector:
+    app: podinfo-primary
+  ports:
+  - name: http
+    port: 9898
+    protocol: TCP
+    targetPort: http
+```
+
+The `podinfo-canary.test:9898` address is available only during the 
+canary analysis and can be used for conformance testing or load testing.