Remove SegmentLoaderConfig.numLoadingThreads config (#4829)

docs/content/configuration/historical.md

@@ -36,8 +36,7 @@ The historical node uses several of the global configs in [Configuration](../con
 |`druid.segmentCache.dropSegmentDelayMillis`|How long a node delays before completely dropping segment.|30000 (30 seconds)|
 |`druid.segmentCache.infoDir`|Historical nodes keep track of the segments they are serving so that when the process is restarted they can reload the same segments without waiting for the Coordinator to reassign. This path defines where this metadata is kept. Directory will be created if needed.|${first_location}/info_dir|
 |`druid.segmentCache.announceIntervalMillis`|How frequently to announce segments while segments are loading from cache. Set this value to zero to wait for all segments to be loaded before announcing.|5000 (5 seconds)|
-|`druid.segmentCache.numLoadingThreads`|How many segments to load concurrently from from deep storage.|1|
-|`druid.segmentCache.numBootstrapThreads`|How many segments to load concurrently from local storage at startup.|Same as numLoadingThreads|
+|`druid.segmentCache.numBootstrapThreads`|How many segments to load concurrently from local storage at startup.|1|
 
 ### Query Configs
 

server/src/main/java/io/druid/segment/loading/SegmentLoaderConfig.java

@@ -31,6 +31,8 @@ import java.util.List;
  */
 public class SegmentLoaderConfig
 {
+  private static final int DEFAULT_NUM_BOOTSTRAP_THREADS = 1;
+
   @JsonProperty
   @NotEmpty
   private List<StorageLocationConfig> locations = null;
@@ -44,9 +46,6 @@ public class SegmentLoaderConfig
   @JsonProperty("announceIntervalMillis")
   private int announceIntervalMillis = 0; // do not background announce
 
-  @JsonProperty("numLoadingThreads")
-  private int numLoadingThreads = 1;
-
   @JsonProperty("numBootstrapThreads")
   private Integer numBootstrapThreads = null;
 
@@ -73,14 +72,9 @@ public class SegmentLoaderConfig
     return announceIntervalMillis;
   }
 
-  public int getNumLoadingThreads()
-  {
-    return numLoadingThreads;
-  }
-
   public int getNumBootstrapThreads()
   {
-    return numBootstrapThreads == null ? numLoadingThreads : numBootstrapThreads;
+    return numBootstrapThreads == null ? DEFAULT_NUM_BOOTSTRAP_THREADS : numBootstrapThreads;
   }
 
   public File getInfoDir()

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

@@ -127,7 +127,7 @@ public class ZkCoordinator implements DataSegmentChangeHandler
           loadQueueLocation,
           true,
           true,
-          Execs.multiThreaded(config.getNumLoadingThreads(), "ZkCoordinator-%s")
+          Execs.singleThreaded("ZkCoordinator-%s")
       );
 
       try {

server/src/test/java/io/druid/server/coordination/ZkCoordinatorTest.java

@@ -202,12 +202,6 @@ public class ZkCoordinatorTest extends CuratorTestBase
             return infoDir;
           }
 
-          @Override
-          public int getNumLoadingThreads()
-          {
-            return 5;
-          }
-
           @Override
           public int getAnnounceIntervalMillis()
           {
@@ -493,12 +487,6 @@ public class ZkCoordinatorTest extends CuratorTestBase
                     return infoDir;
                   }
 
-                  @Override
-                  public int getNumLoadingThreads()
-                  {
-                    return 5;
-                  }
-
                   @Override
                   public int getAnnounceIntervalMillis()
                   {