Add README.md for wordpress to show variable reference usage

examples/README.md

@@ -29,3 +29,5 @@ go get github.com/kubernetes-sigs/kustomize
 
  * [breakfast](breakfast.md) - Customize breakfast for
    Alice and Bob.
+   
+ * [wordpress](wordpress/README.md) - Create wordpress with mysql

examples/wordpress/README.md

@@ -0,0 +1,145 @@
+# Demo: WordPress
+
+In this tutorial, you will learn - how to use `kustomize` to declare a variable reference and substitute it in container's command.
+
+In the WordPress example, we want 
+
+- to connect WordPress with a MySQL database
+- access the service name of MySQL database from WordPress container
+
+First make a place to work:
+<!-- @makeDemoHome @test -->
+```
+DEMO_HOME=$(mktemp -d)
+MYSQL_HOME=$DEMO_HOME/mysql
+mkdir -p $MYSQL_HOME
+WORDPRESS_HOME=$DEMO_HOME/wordpress
+mkdir -p $WORDPRESS_HOME
+```
+
+### Download resources
+
+Download the resources and `kustomization.yaml` for WordPress.
+
+<!-- @downloadResources @test -->
+```
+CONTENT="https://raw.githubusercontent.com\
+/kubernetes-sigs/kustomize\
+/master/examples/wordpress/wordpress"
+
+curl -s -o "$WORDPRESS_HOME/#1.yaml" \
+  "$CONTENT/{deployment,service,kustomization}.yaml"
+```
+
+Download the resources and `kustomization.yaml` for MySQL.
+
+<!-- @downloadResources @test -->
+```
+CONTENT="https://raw.githubusercontent.com\
+/kubernetes-sigs/kustomize\
+/master/examples/wordpress/mysql"
+
+curl -s -o "$MYSQL_HOME/#1.yaml" \
+  "$CONTENT/{deployment,service,secret,kustomization}.yaml"
+```
+
+### Create kustomization.yaml
+We create a new kustomization, which has two bases: mysql and wordpress.
+
+<!-- @createKustomization @test -->
+```
+cat <<EOF >$DEMO_HOME/kustomization.yaml
+bases:
+  - wordpress
+  - mysql
+namePrefix: demo-
+EOF
+```
+
+### Download patch for WordPress
+In the new kustomization, we apply a patch for wordpress deployment. The patch does two things
+- Add an initial container to show the mysql service name
+- Add environment variable for mysql database so that wordpress can connect to it
+
+<!-- @downloadPatch @test -->
+```
+CONTENT="https://raw.githubusercontent.com\
+/kubernetes-sigs/kustomize\
+/master/examples/patch.yaml"
+
+curl -s -o "$DEMO_HOME/#1.yaml" \
+  "$CONTENT/{patch}.yaml"
+```
+The patch has following content
+> ```
+> apiVersion: apps/v1beta2
+> kind: Deployment
+> metadata:
+>   name: wordpress
+> spec:
+>   template:
+>     spec:
+>       initContainers:
+>       - name: init-command
+>         image: debian
+>         command:
+>         - "echo $(WORDPRESS_SERVICE)"
+>         - "echo $(MYSQL_SERVICE)"
+>       containers:
+>       - name: wordpress
+>         env:
+>         - name: WORDPRESS_DB_HOST
+>           value: mysql
+>         - name: WORDPRESS_DB_PASSWORD
+>           valueFrom:
+>             secretKeyRef:
+>               name: mysql-pass
+>               key: password
+> ```
+In the init container's command, there are two variables as `$(WORDPRESS_SERVICE)` and `$(MYSQL_SERVICE)`, which are for the service names of wordpress and mysql. If we define those variables in `kustomization.yaml`, they will be substituted in `kustomize build` to the correct names.
+
+
+### Add Variable Reference
+Let us define the variables needed in the init container's command. 
+
+<!-- @addVarRef @test -->
+```
+cat <<EOF >>$DEMO_HOME/kustomization.yaml
+vars:
+  - name: WORDPRESS_SERVICE
+    objref:
+      kind: Service
+      name: wordpress
+      apiVersion: v1
+    fieldref:
+      fieldpath: metadata.name
+  - name: MYSQL_SERVICE
+    objref:
+      kind: Service
+      name: mysql
+      apiVersion: v1
+EOF
+```
+`WORDPRESS_SERVICE` is from the field `metadata.name` of Service `wordpress`. If we don't specify `fieldref`, the default is `metadata.name`. So `MYSQL_SERVICE` is from the field `metadata.name` of Service `mysql`.
+
+### Substitution
+We can see the substitution by `kustomize build`
+
+<!-- @kustomizeBuild @test -->
+```
+kustomize build $DEMO_HOME
+```
+
+We will see the init Container's command as
+
+> ```
+> (truncated)
+> ...
+>      initContainers:
+>      - command:
+>        - echo demo-wordpress
+>        - echo demo-mysql
+>        image: debian
+>        name: init-command
+>
+> ```