package org.apache.turbine.pipeline;

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2001 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Apache" and "Apache Software Foundation" and
 *    "Apache Turbine" must not be used to endorse or promote products
 *    derived from this software without prior written permission. For
 *    written permission, please contact apache@apache.org.
 *
 * 5. Products derived from this software may not be called "Apache",
 *    "Apache Turbine", nor may "Apache" appear in their name, without
 *    prior written permission of the Apache Software Foundation.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 */

import java.io.IOException;

import org.apache.turbine.util.RunData;
import org.apache.turbine.util.TurbineException;

/**
 * <p>A <b>Valve</b> is a request processing component.  A series of
 * Valves are generally associated with each other into a Pipeline.
 * The detailed contract for a Valve is included in the description of
 * the <code>invoke()</code> method below.</p>
 *
 * <b>HISTORICAL NOTE</b>:  The "Valve" name was assigned to this concept
 * because a valve is what you use in a real world pipeline to control and/or
 * modify flows through it.
 *
 * @author Craig R. McClanahan
 * @author Gunnar Rjnning
 * @author Peter Donald
 * @author <a href="mailto:dlr@finemaltcoding.com">Daniel Rall</a>
 *
 * @see #invoke(RunData, ValveContext)
 */
public interface Valve
{
    /**
     * <p>Perform request processing as required by this Valve.</p>
     *
     * <p>An individual Valve <b>MAY</b> perform the following actions, in
     * the specified order:</p>
     * <ul>
     * <li>Examine and/or modify the properties of the specified Request and
     *     Response.
     * <li>Examine the properties of the specified Request, completely generate
     *     the corresponding Response, and return control to the caller.
     * <li>Examine the properties of the specified Request and Response, wrap
     *     either or both of these objects to supplement their functionality,
     *     and pass them on.
     * <li>If the corresponding Response was not generated (and control was not
     *     returned, call the next Valve in the pipeline (if there is one) by
     *     executing <code>context.invokeNext()</code>.
     * <li>Examine, but not modify, the properties of the resulting Response
     *     (which was created by a subsequently invoked Valve via a
     *     call to <code>context.invokeNext()</code>).
     * </ul>
     *
     * <p>A Valve <b>MUST NOT</b> do any of the following things:</p>
     * <ul>
     * <li>Change request properties that have already been used to direct
     *     the flow of processing control for this request.
     * <li>Create a completed Response <strong>AND</strong> pass this
     *     Request and Response on to the next Valve in the pipeline.
     * <li>Consume bytes from the input stream associated with the Request,
     *     unless it is completely generating the response, or wrapping the
     *     request before passing it on.
     * <li>Modify the HTTP headers included with the Response after the
     *     <code>invokeNext()</code> method has returned.
     * <li>Perform any actions on the output stream associated with the
     *     specified Response after the <code>invokeNext()</code> method has
     *     returned.
     * </ul>
     *
     * @param data The run-time information, including the servlet
     * request and response we are processing.
     * @param context The valve context used to invoke the next valve
     *  in the current processing pipeline
     *
     * @exception IOException Thrown by a subsequent Valve.
     * @exception TurbineException Thrown by a subsequent Valve.
     */
    public void invoke(PipelineData data, ValveContext context)
        throws IOException, TurbineException;

    /**
     * Initialize the valve before using in a pipeline.
     */
    public void initialize()
        throws Exception;
}