/**
    *
    * 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.hadoop.hbase.executor;
  
  import java.io.IOException;
  import java.util.concurrent.atomic.AtomicLong;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.Server;
  import org.cloudera.htrace.Span;
  import org.cloudera.htrace.Trace;
  import org.cloudera.htrace.TraceScope;
  
  /**
   * Abstract base class for all HBase event handlers. Subclasses should
   * implement the {@link #process()} and {@link #prepare()} methods.  Subclasses
   * should also do all necessary checks up in their prepare() if possible -- check
   * table exists, is disabled, etc. -- so they fail fast rather than later when process
   * is running.  Do it this way because process be invoked directly but event
   * handlers are also
   * run in an executor context -- i.e. asynchronously -- and in this case,
   * exceptions thrown at process time will not be seen by the invoker, not till
   * we implement a call-back mechanism so the client can pick them up later.
   * <p>
   * Event handlers have an {@link EventType}.
   * {@link EventType} is a list of ALL handler event types.  We need to keep
   * a full list in one place -- and as enums is a good shorthand for an
   * implemenations -- because event handlers can be passed to executors when
   * they are to be run asynchronously. The
   * hbase executor, see ExecutorService, has a switch for passing
   * event type to executor.
   * <p>
   * Event listeners can be installed and will be called pre- and post- process if
   * this EventHandler is run in a Thread (its a Runnable so if its {@link #run()}
   * method gets called).  Implement
   * {@link EventHandlerListener}s, and registering using
   * {@link #setListener(EventHandlerListener)}.
   * @see ExecutorService
   */
  @InterfaceAudience.Private
  public abstract class EventHandler implements Runnable, Comparable<Runnable> {
    private static final Log LOG = LogFactory.getLog(EventHandler.class);
  
    // type of event this object represents
    protected EventType eventType;
  
    protected Server server;
  
    // sequence id generator for default FIFO ordering of events
    protected static final AtomicLong seqids = new AtomicLong(0);
  
    // sequence id for this event
    private final long seqid;
  
    // Listener to call pre- and post- processing.  May be null.
    private EventHandlerListener listener;
  
    // Time to wait for events to happen, should be kept short
    protected int waitingTimeForEvents;
  
    private final Span parent;
  
    /**
     * This interface provides pre- and post-process hooks for events.
     */
    public interface EventHandlerListener {
      /**
       * Called before any event is processed
       * @param event The event handler whose process method is about to be called.
       */
      void beforeProcess(EventHandler event);
      /**
       * Called after any event is processed
       * @param event The event handler whose process method is about to be called.
       */
      void afterProcess(EventHandler event);
    }
  
    /**
     * Default base class constructor.
     */
   public EventHandler(Server server, EventType eventType) {
     this.parent = Trace.currentSpan();
     this.server = server;
     this.eventType = eventType;
     seqid = seqids.incrementAndGet();
     if (server != null) {
       this.waitingTimeForEvents = server.getConfiguration().
           getInt("hbase.master.event.waiting.time", 1000);
     }
   }
 
   /**
    * Event handlers should do all the necessary checks in this method (rather than
    * in the constructor, or in process()) so that the caller, which is mostly executed
    * in the ipc context can fail fast. Process is executed async from the client ipc,
    * so this method gives a quick chance to do some basic checks.
    * Should be called after constructing the EventHandler, and before process().
    * @return the instance of this class
    * @throws Exception when something goes wrong
    */
   public EventHandler prepare() throws Exception {
     return this;
   }
 
   public void run() {
     TraceScope chunk = Trace.startSpan(this.getClass().getSimpleName(), parent);
     try {
       if (getListener() != null) getListener().beforeProcess(this);
       process();
       if (getListener() != null) getListener().afterProcess(this);
     } catch(Throwable t) {
       handleException(t);
     } finally {
       chunk.close();
     }
   }
 
   /**
    * This method is the main processing loop to be implemented by the various
    * subclasses.
    * @throws IOException
    */
   public abstract void process() throws IOException;
 
   /**
    * Return the event type
    * @return The event type.
    */
   public EventType getEventType() {
     return this.eventType;
   }
 
   /**
    * Get the priority level for this handler instance.  This uses natural
    * ordering so lower numbers are higher priority.
    * <p>
    * Lowest priority is Integer.MAX_VALUE.  Highest priority is 0.
    * <p>
    * Subclasses should override this method to allow prioritizing handlers.
    * <p>
    * Handlers with the same priority are handled in FIFO order.
    * <p>
    * @return Integer.MAX_VALUE by default, override to set higher priorities
    */
   public int getPriority() {
     return Integer.MAX_VALUE;
   }
 
   /**
    * @return This events' sequence id.
    */
   public long getSeqid() {
     return this.seqid;
   }
 
   /**
    * Default prioritized runnable comparator which implements a FIFO ordering.
    * <p>
    * Subclasses should not override this.  Instead, if they want to implement
    * priority beyond FIFO, they should override {@link #getPriority()}.
    */
   @Override
   public int compareTo(Runnable o) {
     EventHandler eh = (EventHandler)o;
     if(getPriority() != eh.getPriority()) {
       return (getPriority() < eh.getPriority()) ? -1 : 1;
     }
     return (this.seqid < eh.seqid) ? -1 : 1;
   }
 
   /**
    * @return Current listener or null if none set.
    */
   public synchronized EventHandlerListener getListener() {
     return listener;
   }
 
   /**
    * @param listener Listener to call pre- and post- {@link #process()}.
    */
   public synchronized void setListener(EventHandlerListener listener) {
     this.listener = listener;
   }
 
   @Override
   public String toString() {
     return "Event #" + getSeqid() +
       " of type " + eventType +
       " (" + getInformativeName() + ")";
   }
 
   /**
    * Event implementations should override thie class to provide an
    * informative name about what event they are handling. For example,
    * event-specific information such as which region or server is
    * being processed should be included if possible.
    */
   public String getInformativeName() {
     return this.getClass().toString();
   }
 
   /**
    * Event exception handler, may be overridden
    * @param t Throwable object
    */
   protected void handleException(Throwable t) {
     LOG.error("Caught throwable while processing event " + eventType, t);
   }
 }