/*
 * Copyright 2009 Red Hat, Inc.
 *
 * Red Hat 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.jboss.netty.channel;

import java.util.concurrent.Executor;

import org.jboss.netty.handler.execution.ExecutionHandler;
import org.jboss.netty.handler.execution.OrderedMemoryAwareThreadPoolExecutor;


/**
 * Handles or intercepts an upstream {@link ChannelEvent}, and sends a
 * {@link ChannelEvent} to the next handler in a {@link ChannelPipeline}.
 * <p>
 * The most common use case of this interface is to intercept an I/O event
 * generated by I/O workers to transform the received messages or execute
 * the relevant business logic.
 *
 * <h3>{@link SimpleChannelUpstreamHandler}</h3>
 * <p>
 * In most cases, you will get to use a {@link SimpleChannelUpstreamHandler} to
 * implement an upstream handler because it provides an individual handler
 * method for each event type.  You might want to implement this interface
 * directly though if you want to handle various types of events in more
 * generic way.
 *
 * <h3>Firing an event to the next handler</h3>
 * <p>
 * You can forward the received event upstream or downstream.  In most cases,
 * {@link ChannelUpstreamHandler} will send the event upstream (i.e. inbound)
 * although it is legal to send the event downstream (i.e. outbound):
 *
 * <pre>
 * // Sending the event upstream (inbound)
 * void handleUpstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendUpstream(e);
 *     ...
 * }
 *
 * // Sending the event downstream (outbound)
 * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendDownstream(new MessageEvent(...));
 *     ...
 * }
 * </pre>
 *
 * <h4>Using the helper class to send an event</h4>
 * <p>
 * You will also find various helper methods in {@link Channels} to be useful
 * to generate and send an artificial or manipulated event.
 *
 * <h3>Thread safety</h3>
 * <p>
 * If there's no {@link ExecutionHandler} in the {@link ChannelPipeline},
 * {@link #handleUpstream(ChannelHandlerContext, ChannelEvent) handleUpstream}
 * will be invoked sequentially by the same thread (i.e. an I/O thread).
 * Please note that this does not necessarily mean that there's a dedicated
 * thread per {@link Channel}; the I/O thread of some transport can serve more
 * than one {@link Channel} (e.g. NIO transport), while the I/O thread of
 * other transports can serve only one (e.g. OIO transport).
 * <p>
 * If an {@link ExecutionHandler} is added in the {@link ChannelPipeline},
 * {@link #handleUpstream(ChannelHandlerContext, ChannelEvent) handleUpstream}
 * may be invoked by different threads at the same time, depending on what
 * {@link Executor} implementation is used with the {@link ExecutionHandler}.
 * <p>
 * {@link OrderedMemoryAwareThreadPoolExecutor} is provided to guarantee the
 * order of {@link ChannelEvent}s.  It does not guarantee that the invocation
 * will be made by the same thread for the same channel, but it does guarantee
 * that the invocation will be made sequentially for the events of the same
 * channel.  For example, the events can be processed as depicted below:
 *
 * <pre>
 *           -----------------------------------&gt; Timeline -----------------------------------&gt;
 *
 * Thread X: --- Channel A (Event 1) --.   .-- Channel B (Event 2) --- Channel B (Event 3) ---&gt;
 *                                      \ /
 *                                       X
 *                                      / \
 * Thread Y: --- Channel B (Event 1) --'   '-- Channel A (Event 2) --- Channel A (Event 3) ---&gt;
 * </pre>
 * <p>
 * Also, please refer to the {@link ChannelPipelineCoverage} annotation to
 * understand the relationship between a handler and its stateful properties.
 *
 * @author <a href="http://www.jboss.org/netty/">The Netty Project</a>
 * @author <a href="http://gleamynode.net/">Trustin Lee</a>
 *
 * @version $Rev$, $Date$
 *
 * @apiviz.exclude ^org\.jboss\.netty\.handler\..*$
 */
public interface ChannelUpstreamHandler extends ChannelHandler {

    /**
     * Handles the specified upstream event.
     *
     * @param ctx  the context object for this handler
     * @param e    the upstream event to process or intercept
     */
    void handleUpstream(ChannelHandlerContext ctx, ChannelEvent e) throws Exception;
}