Document on Component metadata. (#1141)

* Document  on Component metadata.

* Update component-schema.md

* Update setup-mqtt.md

* Update component-schema.md

Co-authored-by: Mark Fussell <mfussell@microsoft.com>

daprdocs/content/en/operations/components/component-schema.md

@@ -42,6 +42,31 @@ spec:
 | spec.ignoreErrors  | N        | Tells the Dapr sidecar to continue initialization if the component fails to load. Default is false  | `false`       
 | **spec.metadata**  | -        | **A key/value pair of component specific configuration. See your component definition for fields**|
 
+### Special metadata values
+
+Metadata values can contain a `{uuid}` tag that is replaced with a randomly generate UUID when the Dapr sidecar starts up. A new UUID is generated on every start up. It can be used, for example, to have a pod on Kubernetes with multiple application instances consuming a [shared MQTT subscription]({{< ref "setup-mqtt.md" >}}). Below is an example of using the `{uuid}` tag.
+
+```yaml
+apiVersion: dapr.io/v1alpha1
+kind: Component
+metadata:
+  name: messagebus
+spec:
+  type: pubsub.mqtt
+  version: v1
+  metadata:
+    - name: consumerID
+      value: "{uuid}"
+    - name: url
+      value: "tcp://admin:public@localhost:1883"
+    - name: qos
+      value: 1
+    - name: retain
+      value: "false"
+    - name: cleanSession
+      value: "false"
+```
+
 ## Further reading
 - [Components concept]({{< ref components-concept.md >}})
 - [Reference secrets in component definitions]({{< ref component-secrets.md >}})
@@ -49,4 +74,4 @@ spec:
 - [Supported pub/sub brokers]({{< ref supported-pubsub >}})
 - [Supported secret stores]({{< ref supported-secret-stores >}})
 - [Supported bindings]({{< ref supported-bindings >}})
-- [Set component scopes]({{< ref component-scopes.md >}})
+- [Set component scopes]({{< ref component-scopes.md >}})

daprdocs/content/en/operations/components/setup-pubsub/supported-pubsub/setup-mqtt.md

@@ -54,6 +54,7 @@ metadata:
   namespace: default
 spec:
   type: pubsub.mqtt
+  version: v1
   metadata:
   - name: url
     value: "tcps://host.domain[:port]"
@@ -71,6 +72,32 @@ spec:
     value: ''
 ```
 
+### Consuming a shared topic
+
+When consuming a shared topic, each consumer must have a unique identifier. By default, the application Id is used to uniquely identify each consumer and publisher. In self-hosted mode, running each Dapr run with a different application Id is sufficient to have them consume from the same shared topic. However on Kubernetes, a pod with multiple application instances shares the same application Id, prohibiting all instances from consuming the same topic. To overcome this, configure the component's `ConsumerID` metadata with a `{uuid}` tag, making each instance to have a randomly generated `ConsumerID` value on start up. For example:
+
+```yaml
+apiVersion: dapr.io/v1alpha1
+kind: Component
+metadata:
+  name: messagebus
+  namespace: default
+spec:
+  type: pubsub.mqtt
+  version: v1
+  metadata:
+    - name: consumerID
+      value: "{uuid}"
+    - name: url
+      value: "tcp://admin:public@localhost:1883"
+    - name: qos
+      value: 1
+    - name: retain
+      value: "false"
+    - name: cleanSession
+      value: "false"
+```
+
 {{% alert title="Warning" color="warning" %}}
 The above example uses secrets as plain strings. It is recommended to use a secret store for the secrets as described [here]({{< ref component-secrets.md >}}).
 {{% /alert %}}
@@ -149,4 +176,4 @@ You can then interact with the server using the client port: `tcp://mqtt-broker.
 ## Related links
 - [Basic schema for a Dapr component]({{< ref component-schema >}})
 - Read [this guide]({{< ref "howto-publish-subscribe.md#step-2-publish-a-topic" >}}) for instructions on configuring pub/sub components
-- [Pub/Sub building block]({{< ref pubsub >}})
+- [Pub/Sub building block]({{< ref pubsub >}})