/**
 * 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
 * <p>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p>
 * 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.hadoop.hdfs.server.namenode.ha;

import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.net.URI;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.ipc.RemoteException;
import org.apache.hadoop.ipc.StandbyException;

import org.apache.hadoop.io.retry.MultiException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A FailoverProxyProvider implementation that technically does not "failover"
 * per-se. It constructs a wrapper proxy that sends the request to ALL
 * underlying proxies simultaneously. It assumes the in an HA setup, there will
 * be only one Active, and the active should respond faster than any configured
 * standbys. Once it receive a response from any one of the configured proxies,
 * outstanding requests to other proxies are immediately cancelled.
 */
public class RequestHedgingProxyProvider<T> extends
        ConfiguredFailoverProxyProvider<T> {

  public static final Logger LOG =
      LoggerFactory.getLogger(RequestHedgingProxyProvider.class);

  class RequestHedgingInvocationHandler implements InvocationHandler {

    final Map<String, ProxyInfo<T>> targetProxies;
    // Proxy of the active nn
    private volatile ProxyInfo<T> currentUsedProxy = null;

    public RequestHedgingInvocationHandler(
            Map<String, ProxyInfo<T>> targetProxies) {
      this.targetProxies = new HashMap<>(targetProxies);
    }

    /**
     * Creates a Executor and invokes all proxies concurrently. This
     * implementation assumes that Clients have configured proper socket
     * timeouts, else the call can block forever.
     *
     * @param proxy
     * @param method
     * @param args
     * @return
     * @throws Throwable
     */
    @Override
    public Object
    invoke(Object proxy, final Method method, final Object[] args)
            throws Throwable {
      // Need double check locking to guarantee thread-safe since
      // currentUsedProxy is lazily initialized.
      if (currentUsedProxy == null) {
        synchronized (this) {
          if (currentUsedProxy == null) {
            Map<Future<Object>, ProxyInfo<T>> proxyMap = new HashMap<>();
            int numAttempts = 0;

            ExecutorService executor = null;
            CompletionService<Object> completionService;
            try {
              // Optimization : if only 2 proxies are configured and one had
              // failed
              // over, then we dont need to create a threadpool etc.
              targetProxies.remove(toIgnore);
              if (targetProxies.size() == 0) {
                LOG.trace("No valid proxies left");
                throw new RemoteException(IOException.class.getName(),
                    "No valid proxies left. "
                        + "All NameNode proxies have failed over.");
              }
              if (targetProxies.size() == 1) {
                ProxyInfo<T> proxyInfo =
                    targetProxies.values().iterator().next();
                try {
                  currentUsedProxy = proxyInfo;
                  Object retVal = method.invoke(proxyInfo.proxy, args);
                  LOG.debug("Invocation successful on [{}]",
                      currentUsedProxy.proxyInfo);
                  return retVal;
                } catch (InvocationTargetException ex) {
                  Exception unwrappedException =
                      unwrapInvocationTargetException(ex);
                  logProxyException(unwrappedException,
                      currentUsedProxy.proxyInfo);
                  LOG.trace("Unsuccessful invocation on [{}]",
                      currentUsedProxy.proxyInfo);
                  throw unwrappedException;
                }
              }
              executor = Executors.newFixedThreadPool(proxies.size());
              completionService = new ExecutorCompletionService<>(executor);
              for (final Map.Entry<String, ProxyInfo<T>> pEntry : targetProxies
                  .entrySet()) {
                Callable<Object> c = new Callable<Object>() {
                  @Override
                  public Object call() throws Exception {
                    LOG.trace("Invoking method {} on proxy {}", method,
                        pEntry.getValue().proxyInfo);
                    return method.invoke(pEntry.getValue().proxy, args);
                  }
                };
                proxyMap.put(completionService.submit(c), pEntry.getValue());
                numAttempts++;
              }

              Map<String, Exception> badResults = new HashMap<>();
              while (numAttempts > 0) {
                Future<Object> callResultFuture = completionService.take();
                Object retVal;
                try {
                  currentUsedProxy = proxyMap.get(callResultFuture);
                  retVal = callResultFuture.get();
                  LOG.debug("Invocation successful on [{}]",
                      currentUsedProxy.proxyInfo);
                  return retVal;
                } catch (ExecutionException ex) {
                  Exception unwrappedException = unwrapExecutionException(ex);
                  ProxyInfo<T> tProxyInfo = proxyMap.get(callResultFuture);
                  logProxyException(unwrappedException, tProxyInfo.proxyInfo);
                  badResults.put(tProxyInfo.proxyInfo, unwrappedException);
                  LOG.trace("Unsuccessful invocation on [{}]",
                      tProxyInfo.proxyInfo);
                  numAttempts--;
                }
              }

              // At this point we should have All bad results (Exceptions)
              // Or should have returned with successful result.
              if (badResults.size() == 1) {
                throw badResults.values().iterator().next();
              } else {
                throw new MultiException(badResults);
              }
            } finally {
              if (executor != null) {
                LOG.trace("Shutting down threadpool executor");
                executor.shutdownNow();
              }
            }
          }
        }
      }
      // Because the above synchronized block will return or throw an exception,
      // so we don't need to do any check to prevent the first initialized
      // thread from stepping to following codes.
      try {
        Object retVal = method.invoke(currentUsedProxy.proxy, args);
        LOG.debug("Invocation successful on [{}]", currentUsedProxy.proxyInfo);
        return retVal;
      } catch (InvocationTargetException ex) {
        Exception unwrappedException = unwrapInvocationTargetException(ex);
        logProxyException(unwrappedException, currentUsedProxy.proxyInfo);
        LOG.trace("Unsuccessful invocation on [{}]",
            currentUsedProxy.proxyInfo);
        throw unwrappedException;
      }
    }
  }

  /** A proxy wrapping {@link RequestHedgingInvocationHandler}. */
  private ProxyInfo<T> currentUsedHandler = null;
  private volatile String toIgnore = null;

  public RequestHedgingProxyProvider(Configuration conf, URI uri,
      Class<T> xface, HAProxyFactory<T> proxyFactory) {
    super(conf, uri, xface, proxyFactory);
  }

  @SuppressWarnings("unchecked")
  @Override
  public synchronized ProxyInfo<T> getProxy() {
    if (currentUsedHandler != null) {
      return currentUsedHandler;
    }
    Map<String, ProxyInfo<T>> targetProxyInfos = new HashMap<>();
    StringBuilder combinedInfo = new StringBuilder("[");
    for (int i = 0; i < proxies.size(); i++) {
      ProxyInfo<T> pInfo = super.getProxy();
      incrementProxyIndex();
      targetProxyInfos.put(pInfo.proxyInfo, pInfo);
      combinedInfo.append(pInfo.proxyInfo).append(',');
    }
    combinedInfo.append(']');
    T wrappedProxy = (T) Proxy.newProxyInstance(
            RequestHedgingInvocationHandler.class.getClassLoader(),
            new Class<?>[]{xface},
            new RequestHedgingInvocationHandler(targetProxyInfos));
    currentUsedHandler =
        new ProxyInfo<T>(wrappedProxy, combinedInfo.toString());
    return currentUsedHandler;
  }

  @Override
  public synchronized void performFailover(T currentProxy) {
    toIgnore = ((RequestHedgingInvocationHandler) Proxy.getInvocationHandler(
        currentUsedHandler.proxy)).currentUsedProxy.proxyInfo;
    this.currentUsedHandler = null;
  }

  /**
   * Check the exception returned by the proxy log a warning message if it's
   * not a StandbyException (expected exception).
   * @param ex Exception to evaluate.
   * @param proxyInfo Information of the proxy reporting the exception.
   */
  private void logProxyException(Exception ex, String proxyInfo) {
    if (isStandbyException(ex)) {
      LOG.debug("Invocation returned standby exception on [{}]", proxyInfo, ex);
    } else {
      LOG.warn("Invocation returned exception on [{}]", proxyInfo, ex);
    }
  }

  /**
   * Check if the returned exception is caused by an standby namenode.
   * @param exception Exception to check.
   * @return If the exception is caused by an standby namenode.
   */
  private boolean isStandbyException(Exception exception) {
    if (exception instanceof RemoteException) {
      return ((RemoteException) exception).unwrapRemoteException()
          instanceof StandbyException;
    }
    return false;
  }

  /**
   * Unwraps the ExecutionException. <p>
   * Example:
   * <blockquote><pre>
   * if ex is
   * ExecutionException(InvocationTargetException(SomeException))
   * returns SomeException
   * </pre></blockquote>
   *
   * @return unwrapped exception
   */
  private Exception unwrapExecutionException(ExecutionException ex) {
    if (ex != null) {
      Throwable cause = ex.getCause();
      if (cause instanceof InvocationTargetException) {
        return
            unwrapInvocationTargetException((InvocationTargetException)cause);
      }
    }
    return ex;

  }

  /**
   * Unwraps the InvocationTargetException. <p>
   * Example:
   * <blockquote><pre>
   * if ex is InvocationTargetException(SomeException)
   * returns SomeException
   * </pre></blockquote>
   *
   * @return unwrapped exception
   */
  private Exception unwrapInvocationTargetException(
      InvocationTargetException ex) {
    if (ex != null) {
      Throwable cause = ex.getCause();
      if (cause instanceof Exception) {
        return (Exception) cause;
      }
    }
    return ex;
  }
}
