add a doc for existing kafka

docs/README.md

@@ -7,6 +7,7 @@
 - [Running on azure container services](kubeless-on-azure-container-services.md)
 - [Monitoring](monitoring.md)
 - [Autoscaling](autoscaling.md)
+- [Use existing Kafka cluster](use-existing-kafka.md)
 
 ## Development
 

docs/use-existing-kafka.md

@@ -0,0 +1,64 @@
+# How to deploy Kubeless PubSub function with an existing Kafka cluster in Kubernetes
+
+In Kubeless [release page](https://github.com/kubeless/kubeless/releases), we provide along with Kubeless manifests a collection of Kafka and Zookeeper statefulsets which helps user to quickly deploying PubSub function. These statefulsets are deployed in `kubeless` namespace. However, if you have a Kafka cluster already running in the same Kubernetes cluster, this doc will walk you through how to deploy Kubeless PubSub function with it.
+
+Let's assume that you have Kafka cluster running at `pubsub` namespace like below:
+
+```
+$ kubectl -n pubsub get po
+NAME      READY     STATUS    RESTARTS   AGE
+kafka-0   1/1       Running   0          7h
+zoo-0     1/1       Running   0          7h
+
+$ kubectl -n pubsub get svc
+NAME        TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)             AGE
+kafka       ClusterIP   10.55.253.151   <none>        9092/TCP            7h
+zookeeper   ClusterIP   10.55.248.146   <none>        2181/TCP            7h
+```
+
+And Kubeless already running at `kubeless` namespace:
+
+```
+$ kubectl -n kubeless get po
+NAME                                   READY     STATUS    RESTARTS   AGE
+kubeless-controller-58676964bb-l79gh   1/1       Running   0          5d
+```
+
+Kubeless provide several [PubSub runtimes](https://hub.docker.com/r/kubeless/),which has suffix `event-consumer`, specified for languages that help you to quickly deploy your function with PubSub mechanism. Those runtimes are configured to read Kafka configuration at two environment variables:
+
+- KUBELESS_KAFKA_SVC: which points to kafka service name in Kubernetes cluster.
+- KUBELESS_KAFKA_NAMESPACE: which declares the namespace that Kafka is running on.
+
+In this example, when deploying function we will declare two environment variables `KUBELESS_KAFKA_SVC=kafka` and `KUBELESS_KAFKA_NAMESPACE=pubsub`.
+
+We now try to deploy a provided function in `examples` folder with command as below:
+
+```
+$ kubeless function deploy pubsub-python --trigger-topic s3-python --runtime python2.7 --handler pubsub.handler --from-file examples/python/pubsub.py --env KUBELESS_KAFKA_SVC=kafka --env KUBELESS_KAFKA_NAMESPACE=pubsub
+```
+
+The `pubsub-python` function will just print out messages it receive from `s3-python` topic. Checking if the function is up and running:
+
+```
+$ kubectl get po
+NAME                             READY     STATUS        RESTARTS   AGE
+pubsub-python-5445bdcb64-48bv2   1/1       Running       0          4s
+```
+
+Now we need to create `s3-python` topic and try to publish some messages. You can do it on your own kafka client. In this example, I will try to use the bundled binaries in the kafka container:
+
+```
+# create s3-python topic
+$ kubectl -n pubsub exec -it kafka-0 -- /opt/bitnami/kafka/bin/kafka-topics.sh --create --zookeeper zookeeper.pubsub:2181 --replication-factor 1 --partitions 1 --topic s3-python
+
+# send test message to s3-python topic
+$ kubectl -n pubsub exec -it kafka-0 -- /opt/bitnami/kafka/bin/kafka-console-producer.sh --broker-list localhost:9092 --topic s3-python
+> hello world
+```
+
+Open another terminal and check for the pubsub function log to see if it receives the message:
+
+```
+$ kubectl logs -f pubsub-python-5445bdcb64-48bv2
+hello world
+```