[KYUUBI #453] Refine ServiceDiscovery in Serverable API

![yaooqinn](https://badgen.net/badge/Hello/yaooqinn/green) [![Closes #453](https://badgen.net/badge/Preview/Closes%20%23453/blue)](https://github.com/yaooqinn/kyuubi/pull/453) ![22](https://badgen.net/badge/%2B/22/red) ![11](https://badgen.net/badge/-/11/green) ![1](https://badgen.net/badge/commits/1/yellow) ![Target Issue](https://badgen.net/badge/Missing/Target%20Issue/ff0000) ![Feature](https://badgen.net/badge/Label/Feature/) [<img width="16" alt="Powered by Pull Request Badge" src="https://user-images.githubusercontent.com/1393946/111216524-d2bb8e00-85d4-11eb-821b-ed4c00989c02.png">](https://pullrequestbadge.com/?utm_medium=github&utm_source=yaooqinn&utm_campaign=badge_info)<!-- PR-BADGE: PLEASE DO NOT REMOVE THIS COMMENT -->

<!--
Thanks for sending a pull request!

Here are some tips for you:
  1. If this is your first time, please read our contributor guidelines: https://kyuubi.readthedocs.io/en/latest/community/contributions.html
  2. If the PR is related to an issue in https://github.com/yaooqinn/kyuubi/issues, add '[KYUUBI #XXXX]' in your PR title, e.g., '[KYUUBI #XXXX] Your PR title ...'.
  3. If the PR is unfinished, add '[WIP]' in your PR title, e.g., '[WIP][KYUUBI #XXXX] Your PR title ...'.
-->

### _Why are the changes needed?_
<!--
Please clarify why the changes are needed. For instance,
  1. If you add a feature, you can talk about the use case of it.
  2. If you fix a bug, you can clarify why it is a bug.
-->

A service discovery should be a default module of Severable API and shall be initialized after `FroundendService` for a resolved serviceUri.

it's a reflection only fix to enhance this logic

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/tools/testing.html#running-tests) locally before make a pull request

Closes #453 from yaooqinn/refine.

077dca0 [Kent Yao] Refine ServiceDiscovery in Serverable API

Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>

externals/kyuubi-spark-sql-engine/src/main/scala/org/apache/kyuubi/engine/spark/SparkSQLEngine.scala

@@ -28,7 +28,7 @@ import org.apache.kyuubi.config.KyuubiConf
 import org.apache.kyuubi.engine.spark.SparkSQLEngine.countDownLatch
 import org.apache.kyuubi.ha.HighAvailabilityConf._
 import org.apache.kyuubi.ha.client.{EngineServiceDiscovery, RetryPolicies, ServiceDiscovery}
-import org.apache.kyuubi.service.Serverable
+import org.apache.kyuubi.service.{Serverable, Service}
 import org.apache.kyuubi.util.SignalRegister
 
 private[spark] final class SparkSQLEngine(name: String, spark: SparkSession)
@@ -37,16 +37,16 @@ private[spark] final class SparkSQLEngine(name: String, spark: SparkSession)
   def this(spark: SparkSession) = this(classOf[SparkSQLEngine].getSimpleName, spark)
 
   override private[kyuubi] val backendService = new SparkSQLBackendService(spark)
-  private val discoveryService = new EngineServiceDiscovery(this)
+  override protected def supportsServiceDiscovery: Boolean = {
+    ServiceDiscovery.supportServiceDiscovery(conf)
+  }
+
+  override protected val discoveryService: Service = new EngineServiceDiscovery(this)
 
   override def initialize(conf: KyuubiConf): Unit = {
     val listener = new SparkSQLEngineListener(this)
     spark.sparkContext.addSparkListener(listener)
     super.initialize(conf)
-    if (ServiceDiscovery.supportServiceDiscovery(conf)) {
-      addService(discoveryService)
-      discoveryService.initialize(conf)
-    }
   }
 
   override def start(): Unit = {

kyuubi-common/src/main/scala/org/apache/kyuubi/service/Serverable.scala

@@ -28,12 +28,19 @@ abstract class Serverable(name: String) extends CompositeService(name) {
 
   private[kyuubi] val backendService: AbstractBackendService
   private lazy val frontendService = new FrontendService(backendService, OOMHook)
+  protected def supportsServiceDiscovery: Boolean
+  protected val discoveryService: Service
 
   def connectionUrl: String = frontendService.connectionUrl
 
   override def initialize(conf: KyuubiConf): Unit = synchronized {
+    this.conf = conf
     addService(backendService)
     addService(frontendService)
+    if (supportsServiceDiscovery) {
+      // Service Discovery depends on the frontend service to be ready
+      addService(discoveryService)
+    }
     super.initialize(conf)
   }
 

kyuubi-common/src/test/scala/org/apache/kyuubi/service/NoopServer.scala

@@ -32,4 +32,8 @@ class NoopServer extends Serverable("noop") {
   override protected def stopServer(): Unit = {
     throw new KyuubiException("no need to stop me")
   }
+
+
+  override protected val discoveryService: Service = backendService
+  override protected val supportsServiceDiscovery: Boolean = false
 }

kyuubi-main/src/main/scala/org/apache/kyuubi/server/KyuubiServer.scala

@@ -80,7 +80,10 @@ class KyuubiServer(name: String) extends Serverable(name) {
   def this() = this(classOf[KyuubiServer].getSimpleName)
 
   override private[kyuubi] val backendService: AbstractBackendService = new KyuubiBackendService()
-  private val discoveryService = new KyuubiServiceDiscovery(this)
+  override protected def supportsServiceDiscovery: Boolean = {
+    ServiceDiscovery.supportServiceDiscovery(conf)
+  }
+  override protected val discoveryService = new KyuubiServiceDiscovery(this)
 
   override def initialize(conf: KyuubiConf): Unit = synchronized {
     val kinit = new KinitAuxiliaryService()
@@ -91,11 +94,8 @@ class KyuubiServer(name: String) extends Serverable(name) {
     }
 
     super.initialize(conf)
-    if (ServiceDiscovery.supportServiceDiscovery(conf)) {
-      addService(discoveryService)
-      discoveryService.initialize(conf)
-    }
   }
 
   override protected def stopServer(): Unit = KyuubiServer.zkServer.stop()
+
 }