[ZEPPELIN-1480] rework websocket sending to prevent partial frontend …

…hangup (apache#4800)

* [ZEPPELIN-1480] synchronize websocket sync to prevent issues with blocked websockets due to multithreading

* [ZEPPELIN-1480] use async call to websocket for sending

also add debug logging to the class

* [ZEPPELIN-1480] improve error logging

* [ZEPPELIN-1480] re-add license text

* [ZEPPELIN-1480] remove debug logging from getters

* [ZEPPELIN-1480] fix indentation

zeppelin-server/src/main/java/org/apache/zeppelin/socket/NotebookSocket.java

@@ -18,6 +18,8 @@
 
 import org.apache.commons.lang3.StringUtils;
 import org.apache.zeppelin.utils.ServerUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
 import java.util.Map;
@@ -28,6 +30,8 @@
  * Notebook websocket.
  */
 public class NotebookSocket {
+  private static final Logger LOGGER = LoggerFactory.getLogger(NotebookSocket.class);
+
   private Session session;
   private Map<String, Object> headers;
   private String user;
@@ -36,21 +40,27 @@ public NotebookSocket(Session session, Map<String, Object> headers) {
     this.session = session;
     this.headers = headers;
     this.user = StringUtils.EMPTY;
+    LOGGER.debug("NotebookSocket created for session: {}", session.getId());
   }
 
   public String getHeader(String key) {
     return String.valueOf(headers.get(key));
   }
 
   public void send(String serializeMessage) throws IOException {
-    session.getBasicRemote().sendText(serializeMessage);
+    session.getAsyncRemote().sendText(serializeMessage, result -> {
+      if (result.getException() != null) {
+        LOGGER.error("Failed to send async message for User {} in Session {}: {}", this.user, this.session.getId(), result.getException());
+      }
+    });
   }
 
   public String getUser() {
     return user;
   }
 
   public void setUser(String user) {
+    LOGGER.debug("Setting user: {}", user);
     this.user = user;
   }