Do not create ZK paths if not needed (#16816)

Background:
ZK-based segment loading has been completely disabled in #15705 .
ZK `servedSegmentsPath` has been deprecated since Druid 0.7.1, #1182 .
This legacy path has been replaced by the `liveSegmentsPath` and is not used in the code anymore.

Changes:
- Never create ZK loadQueuePath as it is never used.
- Never create ZK servedSegmentsPath as it is never used.
- Do not create ZK liveSegmentsPath if announcement on ZK is disabled
- Fix up tests

docs/configuration/index.md

@@ -160,7 +160,6 @@ Druid interacts with ZooKeeper through a set of standard path configurations. We
 |`druid.zk.paths.announcementsPath`|Druid service announcement path.|`${druid.zk.paths.base}/announcements`|
 |`druid.zk.paths.liveSegmentsPath`|Current path for where Druid services announce their segments.|`${druid.zk.paths.base}/segments`|
 |`druid.zk.paths.coordinatorPath`|Used by the Coordinator for leader election.|`${druid.zk.paths.base}/coordinator`|
-|`druid.zk.paths.servedSegmentsPath`|Deprecated. Legacy path for where Druid services announce their segments.|`${druid.zk.paths.base}/servedSegments`|
 
 The indexing service also uses its own set of paths. These configs can be included in the common configuration.
 

docs/design/zookeeper.md

@@ -53,7 +53,7 @@ ${druid.zk.paths.coordinatorPath}/_COORDINATOR
 
 ## Segment "publishing" protocol from Historical and Realtime
 
-The `announcementsPath` and `servedSegmentsPath` are used for this.
+The `announcementsPath` and `liveSegmentsPath` are used for this.
 
 All [Historical](../design/historical.md) processes publish themselves on the `announcementsPath`, specifically, they will create an ephemeral znode at
 
@@ -64,13 +64,13 @@ ${druid.zk.paths.announcementsPath}/${druid.host}
 Which signifies that they exist. They will also subsequently create a permanent znode at
 
 ```
-${druid.zk.paths.servedSegmentsPath}/${druid.host}
+${druid.zk.paths.liveSegmentsPath}/${druid.host}
 ```
 
 And as they load up segments, they will attach ephemeral znodes that look like
 
 ```
-${druid.zk.paths.servedSegmentsPath}/${druid.host}/_segment_identifier_
+${druid.zk.paths.liveSegmentsPath}/${druid.host}/_segment_identifier_
 ```
 
 Processes like the [Coordinator](../design/coordinator.md) and [Broker](../design/broker.md) can then watch these paths to see which processes are currently serving which segments.

server/src/main/java/org/apache/druid/curator/ZkEnablementConfig.java

@@ -37,7 +37,7 @@ public class ZkEnablementConfig
   @JsonCreator
   public ZkEnablementConfig(@JsonProperty("enabled") Boolean enabled)
   {
-    this.enabled = enabled == null ? true : enabled.booleanValue();
+    this.enabled = enabled == null || enabled;
   }
 
   public boolean isEnabled()
@@ -48,6 +48,6 @@ public class ZkEnablementConfig
   public static boolean isEnabled(Properties properties)
   {
     String value = properties.getProperty(PROP_KEY_ENABLED);
-    return value == null ? true : Boolean.parseBoolean(value);
+    return value == null || Boolean.parseBoolean(value);
   }
 }

server/src/main/java/org/apache/druid/guice/ServerViewModule.java

@@ -41,10 +41,6 @@ public class ServerViewModule implements Module
   public static final String SERVERVIEW_TYPE_HTTP = "http";
   public static final String SERVERVIEW_TYPE_BATCH = "batch";
 
-  // this value should be consistent with the default implementation used in
-  // {@code ServerInventoryViewProvider} & {@code FilteredServerInventoryViewProvider}
-  public static final String DEFAULT_SERVERVIEW_TYPE = "http";
-
   @Override
   public void configure(Binder binder)
   {

server/src/main/java/org/apache/druid/server/coordination/ZkCoordinator.java

@@ -19,29 +19,25 @@
 
 package org.apache.druid.server.coordination;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.common.base.Throwables;
 import com.google.inject.Inject;
 import org.apache.curator.framework.CuratorFramework;
-import org.apache.curator.framework.recipes.cache.ChildData;
-import org.apache.curator.framework.recipes.cache.PathChildrenCache;
 import org.apache.curator.utils.ZKPaths;
-import org.apache.druid.java.util.common.concurrent.Execs;
 import org.apache.druid.java.util.common.lifecycle.LifecycleStart;
 import org.apache.druid.java.util.common.lifecycle.LifecycleStop;
 import org.apache.druid.java.util.emitter.EmittingLogger;
-import org.apache.druid.segment.loading.SegmentLoaderConfig;
+import org.apache.druid.server.initialization.BatchDataSegmentAnnouncerConfig;
 import org.apache.druid.server.initialization.ZkPathsConfig;
 
-import javax.annotation.Nullable;
 import java.io.IOException;
-import java.util.concurrent.ExecutorService;
 
 /**
- * We are gradually migrating to {@link org.apache.druid.server.http.SegmentListerResource} for driving segment
+ * Creates paths for announcing served segments on Zookeeper.
- * loads/drops on data server processes.
  *
- * However, this class is still the default mechanism as of this writing (2020-12-03).
+ * @deprecated as Druid has already migrated to HTTP-based segment loading and
+ * will soon migrate to HTTP-based inventory view using {@code SegmentListerResource}.
+ *
+ * @see org.apache.druid.server.http.SegmentListerResource
  */
 @Deprecated
 public class ZkCoordinator
@@ -50,36 +46,25 @@ public class ZkCoordinator
 
   private final Object lock = new Object();
 
-  private final DataSegmentChangeHandler dataSegmentChangeHandler;
-  private final ObjectMapper jsonMapper;
   private final ZkPathsConfig zkPaths;
   private final DruidServerMetadata me;
   private final CuratorFramework curator;
+  private final BatchDataSegmentAnnouncerConfig announcerConfig;
 
-  @Nullable
-  private volatile PathChildrenCache loadQueueCache;
   private volatile boolean started = false;
-  private final ExecutorService segmentLoadUnloadService;
 
   @Inject
   public ZkCoordinator(
-      SegmentLoadDropHandler loadDropHandler,
-      ObjectMapper jsonMapper,
       ZkPathsConfig zkPaths,
       DruidServerMetadata me,
       CuratorFramework curator,
-      SegmentLoaderConfig config
+      BatchDataSegmentAnnouncerConfig announcerConfig
   )
   {
-    this.dataSegmentChangeHandler = loadDropHandler;
-    this.jsonMapper = jsonMapper;
     this.zkPaths = zkPaths;
     this.me = me;
     this.curator = curator;
-    this.segmentLoadUnloadService = Execs.multiThreaded(
+    this.announcerConfig = announcerConfig;
-        config.getNumLoadingThreads(),
-        "ZKCoordinator--%d"
-    );
   }
 
   @LifecycleStart
@@ -92,40 +77,17 @@ public class ZkCoordinator
 
       log.info("Starting zkCoordinator for server[%s]", me.getName());
 
-      final String loadQueueLocation = ZKPaths.makePath(zkPaths.getLoadQueuePath(), me.getName());
+      if (announcerConfig.isSkipSegmentAnnouncementOnZk()) {
-      final String servedSegmentsLocation = ZKPaths.makePath(zkPaths.getServedSegmentsPath(), me.getName());
+        log.info("Skipping zkPath creation as segment announcement on ZK is disabled.");
-      final String liveSegmentsLocation = ZKPaths.makePath(zkPaths.getLiveSegmentsPath(), me.getName());
+        started = true;
+        return;
+      }
 
-      loadQueueCache = new PathChildrenCache(
+      final String liveSegmentsLocation = ZKPaths.makePath(zkPaths.getLiveSegmentsPath(), me.getName());
-          curator,
+      log.info("Creating zkPath[%s] for announcing live segments.", liveSegmentsLocation);
-          loadQueueLocation,
-          true,
-          true,
-          Execs.singleThreaded("ZkCoordinator")
-      );
 
       try {
-        curator.newNamespaceAwareEnsurePath(loadQueueLocation).ensure(curator.getZookeeperClient());
-        curator.newNamespaceAwareEnsurePath(servedSegmentsLocation).ensure(curator.getZookeeperClient());
         curator.newNamespaceAwareEnsurePath(liveSegmentsLocation).ensure(curator.getZookeeperClient());
-
-        loadQueueCache.getListenable().addListener(
-            (client, event) -> {
-              final ChildData child = event.getData();
-              switch (event.getType()) {
-                case CHILD_ADDED:
-                  childAdded(child);
-                  break;
-                case CHILD_REMOVED:
-                  log.info("zNode[%s] was removed", event.getData().getPath());
-                  break;
-                default:
-                  log.info("Ignoring event[%s]", event);
-              }
-            }
-
-        );
-        loadQueueCache.start();
       }
       catch (Exception e) {
         Throwables.propagateIfPossible(e, IOException.class);
@@ -136,54 +98,6 @@ public class ZkCoordinator
     }
   }
 
-  private void childAdded(ChildData child)
-  {
-    segmentLoadUnloadService.submit(() -> {
-      final String path = child.getPath();
-      DataSegmentChangeRequest request = new SegmentChangeRequestNoop();
-      try {
-        final DataSegmentChangeRequest finalRequest = jsonMapper.readValue(
-            child.getData(),
-            DataSegmentChangeRequest.class
-        );
-
-        finalRequest.go(
-            dataSegmentChangeHandler,
-            () -> {
-              try {
-                curator.delete().guaranteed().forPath(path);
-                log.info("Completed request [%s]", finalRequest.asString());
-              }
-              catch (Exception e) {
-                try {
-                  curator.delete().guaranteed().forPath(path);
-                }
-                catch (Exception e1) {
-                  log.error(e1, "Failed to delete zNode[%s], but ignoring exception.", path);
-                }
-                log.error(e, "Exception while removing zNode[%s]", path);
-                throw new RuntimeException(e);
-              }
-            }
-        );
-      }
-      catch (Exception e) {
-        // Something went wrong in either deserializing the request using jsonMapper or when invoking it
-        try {
-          curator.delete().guaranteed().forPath(path);
-        }
-        catch (Exception e1) {
-          log.error(e1, "Failed to delete zNode[%s], but ignoring exception.", path);
-        }
-
-        log.makeAlert(e, "Segment load/unload: uncaught exception.")
-           .addData("node", path)
-           .addData("nodeProperties", request)
-           .emit();
-      }
-    });
-  }
-
   @LifecycleStop
   public void stop()
   {
@@ -193,21 +107,7 @@ public class ZkCoordinator
         return;
       }
 
-      try {
+      started = false;
-        loadQueueCache.close();
-      }
-      catch (Exception e) {
-        throw new RuntimeException(e);
-      }
-      finally {
-        loadQueueCache = null;
-        started = false;
-      }
     }
   }
-
-  public boolean isStarted()
-  {
-    return started;
-  }
 }

server/src/main/java/org/apache/druid/server/initialization/ZkPathsConfig.java

@@ -31,15 +31,10 @@ public class ZkPathsConfig
   @JsonProperty
   private String announcementsPath;
   @JsonProperty
-  @Deprecated
-  private String servedSegmentsPath;
-  @JsonProperty
   private String liveSegmentsPath;
   @JsonProperty
   private String coordinatorPath;
   @JsonProperty
-  private String loadQueuePath;
-  @JsonProperty
   private String connectorPath;
 
   public String getBase()
@@ -57,12 +52,12 @@ public class ZkPathsConfig
     return (null == announcementsPath) ? defaultPath("announcements") : announcementsPath;
   }
 
+  /**
+   * Path to announce served segments on.
+   *
+   * @deprecated Use HTTP-based segment discovery instead.
+   */
   @Deprecated
-  public String getServedSegmentsPath()
-  {
-    return (null == servedSegmentsPath) ? defaultPath("servedSegments") : servedSegmentsPath;
-  }
-
   public String getLiveSegmentsPath()
   {
     return (null == liveSegmentsPath) ? defaultPath("segments") : liveSegmentsPath;
@@ -78,11 +73,6 @@ public class ZkPathsConfig
     return defaultPath("overlord");
   }
 
-  public String getLoadQueuePath()
-  {
-    return (null == loadQueuePath) ? defaultPath("loadQueue") : loadQueuePath;
-  }
-
   public String getConnectorPath()
   {
     return (null == connectorPath) ? defaultPath("connector") : connectorPath;
@@ -116,9 +106,7 @@ public class ZkPathsConfig
         this.getConnectorPath().equals(otherConfig.getConnectorPath()) &&
         this.getLiveSegmentsPath().equals(otherConfig.getLiveSegmentsPath()) &&
         this.getCoordinatorPath().equals(otherConfig.getCoordinatorPath()) &&
-        this.getLoadQueuePath().equals(otherConfig.getLoadQueuePath()) &&
+        this.getPropertiesPath().equals(otherConfig.getPropertiesPath())) {
-        this.getPropertiesPath().equals(otherConfig.getPropertiesPath()) &&
-        this.getServedSegmentsPath().equals(otherConfig.getServedSegmentsPath())) {
       return true;
     }
     return false;
@@ -130,10 +118,8 @@ public class ZkPathsConfig
     int result = base != null ? base.hashCode() : 0;
     result = 31 * result + (propertiesPath != null ? propertiesPath.hashCode() : 0);
     result = 31 * result + (announcementsPath != null ? announcementsPath.hashCode() : 0);
-    result = 31 * result + (servedSegmentsPath != null ? servedSegmentsPath.hashCode() : 0);
     result = 31 * result + (liveSegmentsPath != null ? liveSegmentsPath.hashCode() : 0);
     result = 31 * result + (coordinatorPath != null ? coordinatorPath.hashCode() : 0);
-    result = 31 * result + (loadQueuePath != null ? loadQueuePath.hashCode() : 0);
     result = 31 * result + (connectorPath != null ? connectorPath.hashCode() : 0);
     return result;
   }

server/src/test/java/org/apache/druid/initialization/ZkPathsConfigTest.java

@@ -56,10 +56,8 @@ public class ZkPathsConfigTest extends JsonConfigTesterBase<ZkPathsConfig>
     propertyValues.put(StringUtils.format("%s.base", CONFIG_PREFIX), base);
     propertyValues.put(StringUtils.format("%s.propertiesPath", CONFIG_PREFIX), ZKPaths.makePath(base, "properties"));
     propertyValues.put(StringUtils.format("%s.announcementsPath", CONFIG_PREFIX), ZKPaths.makePath(base, "announcements"));
-    propertyValues.put(StringUtils.format("%s.servedSegmentsPath", CONFIG_PREFIX), ZKPaths.makePath(base, "servedSegments"));
     propertyValues.put(StringUtils.format("%s.liveSegmentsPath", CONFIG_PREFIX), ZKPaths.makePath(base, "segments"));
     propertyValues.put(StringUtils.format("%s.coordinatorPath", CONFIG_PREFIX), ZKPaths.makePath(base, "coordinator"));
-    propertyValues.put(StringUtils.format("%s.loadQueuePath", CONFIG_PREFIX), ZKPaths.makePath(base, "loadQueue"));
     propertyValues.put(StringUtils.format("%s.connectorPath", CONFIG_PREFIX), ZKPaths.makePath(base, "connector"));
 
     ZkPathsConfig zkPathsConfigObj = zkPathsConfig.get();

server/src/test/java/org/apache/druid/server/coordination/ZkCoordinatorTest.java

@@ -19,34 +19,16 @@
 
 package org.apache.druid.server.coordination;
 
-import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.curator.framework.CuratorFramework;
-import com.google.common.collect.ImmutableMap;
+import org.apache.curator.utils.EnsurePath;
 import org.apache.curator.utils.ZKPaths;
-import org.apache.druid.curator.CuratorTestBase;
+import org.apache.druid.server.initialization.BatchDataSegmentAnnouncerConfig;
-import org.apache.druid.java.util.common.Intervals;
-import org.apache.druid.java.util.emitter.EmittingLogger;
-import org.apache.druid.segment.IndexIO;
-import org.apache.druid.segment.TestHelper;
-import org.apache.druid.segment.loading.SegmentLoaderConfig;
-import org.apache.druid.server.SegmentManager;
 import org.apache.druid.server.initialization.ZkPathsConfig;
-import org.apache.druid.server.metrics.NoopServiceEmitter;
-import org.apache.druid.timeline.DataSegment;
-import org.apache.druid.timeline.partition.NoneShardSpec;
-import org.apache.zookeeper.CreateMode;
 import org.easymock.EasyMock;
-import org.junit.After;
-import org.junit.Before;
 import org.junit.Test;
 
-import java.util.Arrays;
+public class ZkCoordinatorTest
-import java.util.concurrent.CountDownLatch;
-
-/**
- */
-public class ZkCoordinatorTest extends CuratorTestBase
 {
-  private final ObjectMapper jsonMapper = TestHelper.makeJsonMapper();
   private final DruidServerMetadata me = new DruidServerMetadata(
       "dummyServer",
       "dummyHost",
@@ -65,100 +47,55 @@ public class ZkCoordinatorTest extends CuratorTestBase
     }
   };
 
-  @Before
+  @Test(timeout = 60_000L)
-  public void setUp() throws Exception
+  public void testSegmentPathIsCreatedIfZkAnnouncementIsEnabled() throws Exception
   {
-    setupServerAndCurator();
+    testSegmentPathCreated(true);
-    curator.start();
-    curator.blockUntilConnected();
-  }
-
-  @After
-  public void tearDown()
-  {
-    tearDownServerAndCurator();
   }
 
   @Test(timeout = 60_000L)
-  public void testLoadDrop() throws Exception
+  public void testSegmentPathIsNotCreatedIfZkAnnouncementIsDisabled() throws Exception
   {
-    EmittingLogger.registerEmitter(new NoopServiceEmitter());
+    testSegmentPathCreated(false);
-    DataSegment segment = new DataSegment(
+  }
-        "test",
+
-        Intervals.of("P1d/2011-04-02"),
+  private void testSegmentPathCreated(boolean announceSegmentsOnZk) throws Exception
-        "v0",
+  {
-        ImmutableMap.of("version", "v0", "interval", Intervals.of("P1d/2011-04-02"), "cacheDir", "/no"),
+    final String liveSegmentsPath = ZKPaths.makePath(
-        Arrays.asList("dim1", "dim2", "dim3"),
+        zkPaths.getLiveSegmentsPath(),
-        Arrays.asList("metric1", "metric2"),
+        me.getName()
-        NoneShardSpec.instance(),
-        IndexIO.CURRENT_VERSION_ID,
-        123L
     );
 
-    CountDownLatch loadLatch = new CountDownLatch(1);
+    final EnsurePath mockEnsurePath = EasyMock.mock(EnsurePath.class);
-    CountDownLatch dropLatch = new CountDownLatch(1);
+    final CuratorFramework mockCurator = EasyMock.mock(CuratorFramework.class);
 
-    SegmentLoadDropHandler segmentLoadDropHandler = new SegmentLoadDropHandler(
+    if (announceSegmentsOnZk) {
-        new SegmentLoaderConfig(),
+      EasyMock.expect(mockCurator.newNamespaceAwareEnsurePath(liveSegmentsPath))
-        EasyMock.createNiceMock(DataSegmentAnnouncer.class),
+              .andReturn(mockEnsurePath).once();
-        EasyMock.createNiceMock(SegmentManager.class)
-    )
-    {
-      @Override
-      public void addSegment(DataSegment s, DataSegmentChangeCallback callback)
-      {
-        if (segment.getId().equals(s.getId())) {
-          loadLatch.countDown();
-          callback.execute();
-        }
-      }
 
-      @Override
+      EasyMock.expect(mockCurator.getZookeeperClient())
-      public void removeSegment(DataSegment s, DataSegmentChangeCallback callback)
+              .andReturn(null).once();
-      {
-        if (segment.getId().equals(s.getId())) {
-          dropLatch.countDown();
-          callback.execute();
-        }
-      }
-    };
 
-    ZkCoordinator zkCoordinator = new ZkCoordinator(
+      mockEnsurePath.ensure(EasyMock.anyObject());
-        segmentLoadDropHandler,
+      EasyMock.expectLastCall().once();
-        jsonMapper,
+    }
+
+    EasyMock.replay(mockCurator, mockEnsurePath);
+    final ZkCoordinator zkCoordinator = new ZkCoordinator(
         zkPaths,
         me,
-        curator,
+        mockCurator,
-        new SegmentLoaderConfig()
+        new BatchDataSegmentAnnouncerConfig() {
+          @Override
+          public boolean isSkipSegmentAnnouncementOnZk()
+          {
+            return !announceSegmentsOnZk;
+          }
+        }
     );
+
     zkCoordinator.start();
-
+    EasyMock.verify();
-    String segmentZkPath = ZKPaths.makePath(zkPaths.getLoadQueuePath(), me.getName(), segment.getId().toString());
-
-    curator
-        .create()
-        .creatingParentsIfNeeded()
-        .withMode(CreateMode.EPHEMERAL)
-        .forPath(segmentZkPath, jsonMapper.writeValueAsBytes(new SegmentChangeRequestLoad(segment)));
-
-    loadLatch.await();
-
-    while (curator.checkExists().forPath(segmentZkPath) != null) {
-      Thread.sleep(100);
-    }
-
-    curator
-        .create()
-        .creatingParentsIfNeeded()
-        .withMode(CreateMode.EPHEMERAL)
-        .forPath(segmentZkPath, jsonMapper.writeValueAsBytes(new SegmentChangeRequestDrop(segment)));
-
-    dropLatch.await();
-
-    while (curator.checkExists().forPath(segmentZkPath) != null) {
-      Thread.sleep(100);
-    }
-
     zkCoordinator.stop();
   }
 }

server/src/test/java/org/apache/druid/server/coordinator/simulate/README.md

@@ -63,7 +63,6 @@ of the coordinator in these situations.
    interfaces to communicate with external dependencies have been provided as simple in-memory implementations:
     - communication with metadata store: `SegmentMetadataManager`, `MetadataRuleManager`
     - communication with historicals: `HttpClient`, `ServerInventoryView`
-    - `CuratorFramework`: provided as a mock as simulations of `CuratorLoadQueuePeon` are not supported yet
 4. __Inventory__: The coordinator maintains an inventory view of the cluster state. Simulations can choose from two
    modes of inventory update - auto and manual. In auto update mode, any change made to the cluster is immediately
    reflected in the inventory view. In manual update mode, the inventory must be explicitly synchronized with the