content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF 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.apache.ambari.server.view;
  
  import java.io.IOException;
  import java.util.concurrent.Semaphore;
  import java.util.concurrent.TimeUnit;
  
  import jakarta.servlet.Filter;
  import jakarta.servlet.FilterChain;
  import jakarta.servlet.FilterConfig;
  import jakarta.servlet.ServletException;
  import jakarta.servlet.ServletRequest;
  import jakarta.servlet.ServletResponse;
  import jakarta.servlet.http.HttpServletRequest;
  import jakarta.servlet.http.HttpServletResponse;
  
  import org.apache.ambari.server.configuration.Configuration;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import com.google.inject.Inject;
  import com.google.inject.Singleton;
  
  /**
   * The {@link ViewThrottleFilter} is used to ensure that views which misbehave
   * do not cause a loss of service for Ambari. The underlying problem is that
   * views are accessed off of the REST endpoint (/api/v1/views). This means that
   * the Ambari REST API connector is going to handle the request from its own
   * threadpool. There is no way to configure Jetty to use a different threadpool
   * for the same connector. As a result, if a request to load a view holds the
   * Jetty thread hostage, eventually we will see thread starvation and loss of
   * service.
   * <p/>
   * An example of this situation is a view which makes an innocent request to a
   * remote resource. If the view's request has a timeout of 60 seconds, then the
   * Jetty thread is going to be held for that amount of time. With concurrent
   * users and multiple instances of that view deployed, the Jetty threadpool can
   * becomes exhausted quickly.
   * <p/>
   * Although there are more graceful ways of handling this situation, they mostly
   * involve substantial re-architecture and design.
   * <ul>
   * <li>The use of a new connector and threadpool would require binding to
   * another port for view requests. This will cause problems with "local" views
   * and their assumption that if they run on the Ambari server they can share the
   * same session.
   * <li>The use of a {@link Continuation} in Jetty which can suspend the incoming
   * request. We would need the ability for views to signal that they have
   * completed their work in order to proceed with the suspended request.
   * </ul>
   */
  @Singleton
  public class ViewThrottleFilter implements Filter {
  
    /**
     * Logger.
     */
    private static final Logger LOG = LoggerFactory.getLogger(ViewThrottleFilter.class);
  
    /**
     * Used to determine the correct number of threads to allocate to view
     * requests.
     */
    @Inject
    private Configuration m_configuration;
  
    /**
     * Used to restrict how many REST API threads can be utilizied concurrently by
     * view requests.
     */
    private Semaphore m_semaphore;
  
    /**
     * A timeout that a blocked view request should wait for an available thread
     * before returning an error.
     */
    private int m_timeout;
  
    /**
     * {@inheritDoc}
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
      m_timeout = m_configuration.getViewRequestThreadPoolTimeout();
  
      int clientThreadPoolSize = m_configuration.getClientThreadPoolSize();
      int viewThreadPoolSize = m_configuration.getViewRequestThreadPoolMaxSize();
  
      // start out using 1/2 of the available REST API request threads
      int viewSemaphoreCount = clientThreadPoolSize / 2;
  
      // if the size is specified, see if it's valid
      if (viewThreadPoolSize > 0) {
        viewSemaphoreCount = viewThreadPoolSize;
  
        if (viewThreadPoolSize > clientThreadPoolSize) {
          LOG.warn(
              "The number of view processing threads ({}) cannot be greater than the REST API client threads {{})",
              viewThreadPoolSize, clientThreadPoolSize);
  
          viewSemaphoreCount = clientThreadPoolSize;
        }
      }
  
      // log that we are restricting it
      LOG.info("Ambari Views will be able to utilize {} concurrent REST API threads",
          viewSemaphoreCount);
  
      m_semaphore = new Semaphore(viewSemaphoreCount);
    }
  
    /**
     * {@inheritDoc}
     */
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
        throws IOException, ServletException {
  
      // do nothing if this is not an http request
      if (!(request instanceof HttpServletRequest)) {
        chain.doFilter(request, response);
        return;
      }
  
      HttpServletResponse httpResponse = (HttpServletResponse) response;
      boolean acquired = false;
  
      try {
        acquired = m_semaphore.tryAcquire(m_timeout, TimeUnit.MILLISECONDS);
      } catch (InterruptedException interruptedException) {
        LOG.warn("While waiting for an available thread, the view request was interrupted");
      }
  
      if (!acquired) {
        httpResponse.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
            "There are no available threads to handle view requests");
  
        // return to prevent the view's request from making it down any farther
        return;
      }
  
      // let the request go through
      try {
        chain.doFilter(request, response);
      } finally {
        m_semaphore.release();
      }
    }
  
    /**
     * {@inheritDoc}
     */
    @Override
    public void destroy() {
    }
  }
  