/*
 * $Id: CallFilter.java,v 1.1.1.1 2002/09/30 15:08:51 smartine Exp $
 * Copyright (C) 1999-2000 David Brownell
 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package xml.pipeline;

import java.io.*;
import java.net.*;

import org.xml.sax.*;
import org.xml.sax.ext.*;

import xml.EchoHandler;


// $Id: CallFilter.java,v 1.1.1.1 2002/09/30 15:08:51 smartine Exp $

/**
 * Input is sent as an XML request to given URI, and the output of this
 * filter is the parsed response to that request.
 * A connection is opened to the remote URI when the startDocument call is
 * issued through this filter, and the request is finished when the
 * endDocument call is issued.  Events should be written quickly enough to
 * prevent the remote HTTP server from aborting the connection due to
 * inactivity.  If your application requires validity checking of such
 * outputs, have the output pipeline include a validation stage.
 *
 * <p>In effect, this makes a remote procedure call to the URI, with the
 * request and response document syntax as chosen by the application.
 * <em>Note that all the input events must be seen, and sent to the URI,
 * before the first output event can be seen. </em>  Clients are delayed
 * at least by waiting for the server to respond, constraining concurrency.
 * Services can thus be used to synchronize concurrent activities, and
 * even to prioritize service among different clients.
 *
 * <p> You are advised to avoid restricting yourself to an "RPC" model
 * for distributed computation.  With a World Wide Web, network latencies
 * and failures (e.g. non-availability)
 * are significant; adopting a "procedure" model, rather than a workflow
 * model where bulk requests are sent and worked on asynchronously, is not
 * generally an optimal system-wide architecture.  When the messages may
 * need authentication, such as with an OpenPGP signature, or when server
 * loads don't argue in favor of immediate responses, non-RPC models can
 * be advantageous.  (So-called "peer to peer" computing models are one
 * additional type of model, though too often that term is applied to
 * systems that still have a centralized control structure.)
 *
 * <p> <em>Be strict in what you send, liberal in what you accept,</em> as
 * the Internet tradition goes.  Strictly conformant data should never cause
 * problems to its receiver; make your request pipeline be very strict, and
 * don't compromise on that.  Make your response pipeline strict as well,
 * but be ready to tolerate specific mild, temporary, and well-documented
 * variations from specific communications peers.
 *
 * @see XmlServlet
 *
 * @author David Brownell
 * @version $Date: 2002/09/30 15:08:51 $
 */
final public class CallFilter implements EventConsumer
{
    private Requestor			req;
    private EventConsumer		next;
    private URL				target;
    private URLConnection		conn;
    private ErrorHandler		errHandler;


    /**
     * Initializes a call filter so that its inputs are sent to the
     * specified URI, and its outputs are sent to the next consumer
     * provided.
     *
     * @exception IOException if the URI isn't accepted as a URL
     */
	// constructor used by PipelineFactory
    public CallFilter (String uri, EventConsumer next)
    throws IOException
    {
	this.next = next;
	req = new Requestor ();
	setCallTarget (uri);
    }

    /**
     * Assigns the URI of the call target to be used.
     * Does not affect calls currently being made.
     */
    final public void setCallTarget (String uri)
    throws IOException
    {
	target = new URL (uri);
    }

    /**
     * Assigns the error handler to be used to present most fatal
     * errors.
     */
    public void setErrorHandler (ErrorHandler handler)
    {
	req.setErrorHandler (handler);
    }


    /**
     * Returns the call target's URI.
     */
    final public String getCallTarget ()
    {
	return target.toString ();
    }

    /** Returns the content handler currently in use. */
    final public org.xml.sax.ContentHandler getContentHandler ()
    {
	return req;
    }

    /** Returns the DTD handler currently in use. */
    final public DTDHandler getDTDHandler ()
    {
	return req;
    }


    /**
     * Returns the declaration or lexical handler currently in
     * use, or throws an exception for other properties.
     */
    final public Object getProperty (String id)
    throws SAXNotRecognizedException
    {
	if ((EventFilter.HANDLER_URI + "declaration-handler").equals (id))
	    return req;
	if ((EventFilter.HANDLER_URI + "lexical-handler").equals (id))
	    return req;
	throw new SAXNotRecognizedException (id);
    }


    //
    // Takes input and echoes to server as POST input.
    // Then sends the POST reply to the next pipeline element.
    //
    final class Requestor extends EchoHandler
    {
	Requestor ()
	{
	    super ((Writer)null);
	}

	public synchronized void startDocument () throws SAXException
	{
	    // Connect to remote object and set up to send it XML text
	    try {
		if (conn != null)
		    throw new IllegalStateException ("call is being made");

		conn = target.openConnection ();
		conn.setDoOutput (true);
		conn.setRequestProperty ("Content-Type",
			    "application/xml;charset=UTF-8");

		setWriter (new OutputStreamWriter (
			conn.getOutputStream (),
			"UTF8"));

	    } catch (IOException e) {
		fatal ("can't write (POST) to URI: " + target, e);
	    }

	    // NOW base class can safely write that text!
	    super.startDocument ();
	}

	public void endDocument () throws SAXException
	{
	    //
	    // Finish writing the request (for HTTP, a POST);
	    // this closes the output stream.
	    //
	    super.endDocument ();

	    //
	    // Receive the response.
	    // Produce events for the next stage.
	    //
	    InputSource		source;
	    EventProducer	producer;

	    try {

		source = new InputSource (conn.getInputStream ());

// XXX if it's anything but success, report it!!  It'd be good to
// save the request data just in case we need to deal with a forward.

// XXX "should" set charset as provided in content-type ...

		producer = new EventProducer (source);
		producer.setErrorHandler (errHandler);
		producer.produce (next);
		conn = null;

	    } catch (IOException e) {
		fatal ("I/O Exception reading response, " + e.getMessage (), e);
	    }
	}
    }
}