code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.grid;
  
  /**
   * The Selenium Grid is composed of a number of moving pieces, all of which are designed to be used
   * either locally or across an HTTP boundary.
   *
   * <p>Sitting at the front of the system is the {@link org.openqa.selenium.grid.router.Router}. This
   * is responsible for looking at incoming requests and routing them to the correct location. In the
   * case of a <a href="https://www.w3.org/TR/webdriver/#new-session"></a>New Session</a> command, it
   * will be sent to the {@link org.openqa.selenium.grid.distributor.Distributor}.
   *
   * <p>The {@code Distributor} has a number of {@link org.openqa.selenium.grid.node.Node}s associated
   * with it, each of which is capable of running one or more {@link
   * org.openqa.selenium.grid.data.Session}s which may or may not be running in memory. Each {@code
   * Node} should register with a single {@code Distributor}.
   *
   * <p>When a new session is started, the {@code Node} tells the {@link
   * org.openqa.selenium.grid.sessionmap.SessionMap} where the {@code Session} (identified by its
   * {@link org.openqa.selenium.remote.SessionId} can be located (typically the URL of the server on
   * which the {@code Node} is running. Conversely, when the session comes to an end, the {@code Node}
   * is responsible for ensuring that the session is removed from the {@code SessionMap}.
   */
  