add blog: java controller framework (#16135)

content/en/blog/_posts/2019-08-28-cloud-native-java-controller-sdk.md

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+
+---
+layout: blog
+title: "Develop a Kubernetes controller in Java"
+date: 2019-08-28
+slug: Develop-A-Kubernetes-Controller-in-Java
+---
+
+**Authors:** Min Kim (Ant Financial), Tony Ado (Ant Financial)
+
+The official [Kubernetes Java SDK](https://github.com/kubernetes-client/java) project 
+recently released their latest work on providing the Java Kubernetes developers 
+a handy Kubernetes controller-builder SDK which is helpful for easily developing 
+advanced workloads or systems.  
+
+## Overall
+
+Java is no doubt one of the most popular programming languages in the world but
+it's been difficult for a period time for those non-Golang developers to build up 
+their customized controller/operator due to the lack of library resources in the 
+community. In the world of Golang, there're already some excellent controller 
+frameworks, for example, [controller runtime](https://github.com/kubernetes-sigs/controller-runtime),
+[operator SDK](https://github.com/operator-framework/operator-sdk). These
+existing Golang frameworks are relying on the various utilities from the 
+[Kubernetes Golang SDK](https://github.com/kubernetes/client-go) proven to 
+be stable over years. Driven by the emerging need of further integration into 
+the platform of Kubernetes, we not only ported many essential toolings from the Golang 
+SDK into the kubernetes Java SDK including informers, work-queues, leader-elections, 
+etc. but also developed a controller-builder SDK which wires up everything into 
+a runnable controller without hiccups.
+
+
+## Backgrounds
+
+Why use Java to implement Kubernetes tooling? You might pick Java for:
+
+- __Integrating legacy enterprise Java systems__: Many companies have their legacy 
+systems or frameworks written in Java in favor of stability. We are not able to 
+move everything to Golang easily.
+
+- __More open-source community resources__: Java is mature and has accumulated abundant open-source 
+libraries over decades, even though Golang is getting more and more fancy and 
+popular for developers. Additionally, nowadays developers are able to develop 
+their aggregated-apiservers over SQL-storage and Java has way better support on SQLs.
+
+
+## How to use?
+
+Take maven project as example, adding the following dependencies into your dependencies: 
+
+```xml
+<dependency>
+    <groupId>io.kubernetes</groupId>
+    <artifactId>client-java-extended</artifactId>
+    <version>6.0.1</version>
+</dependency>
+```
+
+Then we can make use of the provided builder libraries to write your own controller. 
+For example, the following one is a simple controller prints out node information 
+on watch notification, see complete example [here](https://github.com/kubernetes-client/java/blob/master/examples/src/main/java/io/kubernetes/client/examples/ControllerExample.java):
+
+```java
+...
+    Reconciler reconciler = new Reconciler() {
+      @Override
+      public Result reconcile(Request request) {
+        V1Node node = nodeLister.get(request.getName());
+        System.out.println("triggered reconciling " + node.getMetadata().getName());
+        return new Result(false);
+      }
+    };
+    Controller controller =
+        ControllerBuilder.defaultBuilder(informerFactory)
+            .watch(
+                (workQueue) -> ControllerBuilder.controllerWatchBuilder(V1Node.class, workQueue).build())
+            .withReconciler(nodeReconciler) // required, set the actual reconciler
+            .withName("node-printing-controller") // optional, set name for controller for logging, thread-tracing
+            .withWorkerCount(4) // optional, set worker thread count
+            .withReadyFunc( nodeInformer::hasSynced) // optional, only starts controller when the cache has synced up
+            .build();
+```
+
+If you notice, the new Java controller framework learnt a lot from the design of 
+[controller-runtime](https://github.com/kubernetes-sigs/controller-runtime) which
+successfully encapsulates the complex components inside controller into several 
+clean interfaces. With the help of Java Generics, we even move on a bit and simply 
+the encapsulation in a better way.
+
+As for more advanced usage, we can wrap multiple controllers into a controller-manager 
+or a leader-electing controller which helps deploying in HA setup. In a word, we can 
+basically find most of the equivalence implementations here from Golang SDK and
+more advanced features are under active development by us.
+
+## Future steps
+
+The community behind the official Kubernetes Java SDK project will be focusing on
+providing more useful utilities for developers who hope to program cloud native
+Java applications to extend Kubernetes. If you are interested in more details, 
+please look at our repo [kubernetes-client/java](https://github.com/kubernetes-client/java).
+Feel free to share also your feedback with us, through Issues or [Slack](http://kubernetes.slack.com/messages/kubernetes-client/).
+