http accessible while loading + status endpoint

server/src/main/java/io/druid/server/coordination/BaseZkCoordinator.java

@@ -37,7 +37,6 @@ import org.apache.curator.framework.recipes.cache.PathChildrenCacheListener;
 import org.apache.curator.utils.ZKPaths;
 
 import java.io.IOException;
-import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
 /**
@@ -212,6 +211,11 @@ public abstract class BaseZkCoordinator implements DataSegmentChangeHandler
     }
   }
 
+  public boolean isStarted()
+  {
+    return started;
+  }
+
   public abstract void loadLocalCache();
 
   public abstract DataSegmentChangeHandler getDataSegmentChangeHandler();

server/src/main/java/io/druid/server/http/HistoricalResource.java

@@ -0,0 +1,32 @@
+package io.druid.server.http;
+
+import com.google.common.collect.ImmutableMap;
+import io.druid.server.coordination.ZkCoordinator;
+
+import javax.inject.Inject;
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+import javax.ws.rs.Produces;
+import javax.ws.rs.core.Response;
+
+@Path("/druid/historical/v1")
+public class HistoricalResource
+{
+  private final ZkCoordinator coordinator;
+
+  @Inject
+  public HistoricalResource(
+      ZkCoordinator coordinator
+  )
+  {
+    this.coordinator = coordinator;
+  }
+
+  @GET
+  @Path("/loadstatus")
+  @Produces("application/json")
+  public Response getLoadStatus()
+  {
+    return Response.ok(ImmutableMap.of("cacheInitialized", coordinator.isStarted())).build();
+  }
+}

services/src/main/java/io/druid/cli/CliHistorical.java

@@ -38,6 +38,7 @@ import io.druid.query.QuerySegmentWalker;
 import io.druid.server.QueryResource;
 import io.druid.server.coordination.ServerManager;
 import io.druid.server.coordination.ZkCoordinator;
+import io.druid.server.http.HistoricalResource;
 import io.druid.server.initialization.JettyServerInitializer;
 import io.druid.server.metrics.MetricsModule;
 import org.eclipse.jetty.server.Server;
@@ -68,6 +69,8 @@ public class CliHistorical extends ServerRunnable
           @Override
           public void configure(Binder binder)
           {
+            // register Server before binding ZkCoordinator to ensure HTTP endpoints are available immediately
+            LifecycleModule.register(binder, Server.class);
             binder.bind(ServerManager.class).in(LazySingleton.class);
             binder.bind(ZkCoordinator.class).in(ManageLifecycle.class);
             binder.bind(QuerySegmentWalker.class).to(ServerManager.class).in(LazySingleton.class);
@@ -75,10 +78,10 @@ public class CliHistorical extends ServerRunnable
             binder.bind(NodeTypeConfig.class).toInstance(new NodeTypeConfig("historical"));
             binder.bind(JettyServerInitializer.class).to(QueryJettyServerInitializer.class).in(LazySingleton.class);
             Jerseys.addResource(binder, QueryResource.class);
+            Jerseys.addResource(binder, HistoricalResource.class);
             LifecycleModule.register(binder, QueryResource.class);
-
+            LifecycleModule.register(binder, HistoricalResource.class);
             LifecycleModule.register(binder, ZkCoordinator.class);
-            LifecycleModule.register(binder, Server.class);
 
             binder.bind(Cache.class).toProvider(CacheProvider.class).in(ManageLifecycle.class);
             JsonConfigProvider.bind(binder, "druid.historical.cache", CacheProvider.class);