source: greenstone3/trunk/resources/java/QBRSOAPServer.java.in@ 16522

/**
 *#########################################################################
 * QBRSOAPServer.java.in: a template for a SOAPServer providing
 * basic Query, Browse, Retrieve web services for Greenstone 3.
 * Part of the Greenstone digital library suite from the New Zealand 
 * Digital Library Project at the University of Waikato, New Zealand.
 * <BR><BR>
 * Copyright (C) 2008 New Zealand Digital Library Project
 * <BR><BR>
 * 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.
 * <BR><BR>
 * 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.
 * <BR><BR>
 * @author ak19
 * based on Katherine Don's SOAPServer@sitename@ template file. 
 *########################################################################
 */

package org.greenstone.gsdl3;

import java.io.File;
import java.io.InputStream;

import java.util.Properties;
import java.util.Enumeration;
import java.util.Map;
import java.util.Map.Entry;
import java.util.HashMap;
import java.util.Set;
import java.util.Iterator;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;

import org.greenstone.gsdl3.core.MessageRouter;
import org.greenstone.gsdl3.util.GlobalProperties;
import org.greenstone.gsdl3.util.GSFile;
import org.greenstone.gsdl3.util.GSXML;
import org.greenstone.gsdl3.util.XMLConverter;

import org.apache.log4j.Logger; // Import log4j classes

/* 
 * Add to $GSDLHOME/web/WEB-INF/server-config.wsdd:
 * <service name="GS3WebServices" provider="java:RPC">
 * <parameter name="allowedMethods" value="*"/>
 * <parameter name="className" value="org.greenstone.gs3services.GS3WebServices"/>
 * </service>
*/

/** Class that provides the basic Query, Browse and Retrieve (QBR) web service
 * operations for Greenstone 3.
 * It contains a MessageRouter that carries out all the tasks by calling the 
 * appropriate Greenstone functionality for each request message passed to it, 
 * and returning a response message.
 *  
 * All response messages are returned from the MessageRouter to clients invoking   
 * the web services. All return values are strings that represent XML messages.
 * 
 * Method help() reads from the file QBRWebServicesHelp.properties to list the web 
 * service operations available. Method helpWithMethod(String methodName) 
 * reads from the same file to display a description of the requested operation.
 * (These method descriptions are mostly the same as those in the Javadoc
 * comments.)
 * 
 * NOTE: The folder containing this web service class' properties helpFile 
 * should be on the classpath. (QBRWebServicesHelp.properties)
 * @author ak19
*/
public class QBRSOAPServer@sitename@ {
    /** site_name the MessageRouter works with, here set to "localsite" */
    protected String site_name = "@sitename@";

    /** Message Router object to pass requests messages to and which 
     * will process them.*/
    protected MessageRouter mr = null;
    
    /** Container Document to create XML Nodes */
    protected Document doc=null;
    /** A converter class to parse XML and create Docs */
    protected XMLConverter converter=null;

    /** The Logger for this class */
    private static Logger LOG = Logger.getLogger(QBRSOAPServer@[email protected]);
    
    /** Error message loading helpFile. Remains at "" if everything is fine */
    protected static String helpErrormessage = "";
    /** Properties map with mappings from methodname to help 
     * description string. */ 
    protected static Properties properties;
    /** The help properties file describing the web service operations */
    protected static String helpFile = "QBRWebServicesHelp.properties";
    
    // static code block to initialise the help Properties from the helpFile
    static {
        properties = new Properties();
        // load the properties file from a location with respect to the 
        // the Web Service .class file
        InputStream input = null;
        try {   
            // load the properties file from a location with respect to the 
            // the Web Service .class file
            input 
            = QBRSOAPServer@[email protected]().getResourceAsStream(
                    helpFile);
            if(input == null) {
                helpErrormessage = "Cannot find file " + helpFile + " to load.";
                LOG.warn(helpErrormessage);
            } else {
                properties.load(input);
                input.close();
            }
        } catch(Exception e) {
            helpErrormessage = "Exception loading properties from help file " 
                + helpFile + "\n" + e.getMessage();
            LOG.warn("Exception loading properties from help file " 
                + helpFile + "\n" + e.getMessage());
        }
    }
    
    
    /* Describe subset options for the various Greenstone3 modules */
    protected static final String mrSubsetOptions = // messageRouter 
        "collectionList serviceClusterList serviceList siteList"; 
    protected static final String csSubsetOptions = // collections and serviceClusters 
        "metadataList serviceList displayItemList";
    protected static final String serviceSubsetOptions = // services
        "paramList displayItemList";
    protected static final String docStructureOptions =
        "entire ancestors parent siblings children descendants"; // note the spelling
    protected static final String docStructureInfo =
        "numSiblings siblingPosition numChildren";
    protected static final String browseStructureOptions = 
        "ancestors parent siblings children descendants"; // note the spelling
    
        
    /** Constructor that initializes the web services' MessageRouter object 
     * Reads from GlobalProperties to get gsdl3_home and set the sitename. */
    public QBRSOAPServer@sitename@() {
        String gsdl3_home = GlobalProperties.getGSDL3Home();
        if (gsdl3_home == null || gsdl3_home.equals("")) {
            LOG.error(
                "Couldn't access GSDL3Home from GlobalProperties.getGSDL3HOME,"
                + "can't initialize the SOAP Server.");
            return;
        }
        
        String site_home = GSFile.siteHome(gsdl3_home, this.site_name);
        
        File site_file = new File(site_home);
        if (!site_file.isDirectory()) {
            LOG.error("The site directory "+site_file.getPath()
                +" doesn't exist. Can't initialize the SOAP Server.");
            return;
        } 
        this.converter = new XMLConverter();
        this.doc = this.converter.newDOM();
        
        mr = new MessageRouter();
        mr.setSiteName(this.site_name);
        mr.configure();
    }
    
    /* (1) DESCRIBE MESSAGES, manual pages 35-41 */
    /** Sends a describe message to the MessageRouter.
     * @param lang is the language of the display content in the response.
     * @param subsetOption are the requested list of items to return in the 
     * response. For the Message Router this can be collectionList, 
     * serviceClusterList, serviceList, siteList 
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
    */
    public String describe(String lang, String subsetOption) 
    {
        return describe("", lang, subsetOption, mrSubsetOptions);
    }
    
    /** For sending Describe messages to ServiceClusters.
     * @param serviceCluster is the name of the Service Cluster that this describe 
     * request is sent to. 
     * @param lang is the language of the display content in the response
     * @param subsetOption is the requested list of items to return in the response
     * For Service Clusters this can be metadataList, serviceList, displayItemList.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
     */
    public String describeServiceCluster(
            String serviceCluster, String lang, String subsetOption)
    {
        return describe(serviceCluster, lang, subsetOption, csSubsetOptions);
    }
    
    /** For sending Describe messages to Collections.
     * @param collection is the name of the Collection that this describe request 
     * is sent to. 
     * @param lang is the language of the display content in the response
     * @param subsetOption is the requested list of items to return in the response
     * For Collections this can be metadataList, serviceList and displayItemList.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
     */
    public String describeCollection(
            String collection, String lang, String subsetOption)
    {
        return describe(collection, lang, subsetOption, csSubsetOptions);
    }
    
    /**
     * For sending a describe message to a Collection's Service.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
     * @param collection is the name of the Collection whose service 
     * this describe request is sent to.
     * @param service is the name of the Service (of that collection) to
     * which this describe request is sent.
     * @param lang is the language of the display content in the response
     * @param subsetOption is the requested list of items to return in the response
     * For Services this can be paramList, displayItemList */
    public String describeCollectionService(String collection, String service, 
            String lang, String subsetOption) 
    {
        return describe(collection + "/" + service, 
                lang, subsetOption, serviceSubsetOptions);
    }
    
    /** 
     * For sending a describe message to a Service hosted by the Message Router 
     * (no collection).
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
     * @param service is the name of the MessageRouter's Service to which this 
     * describe request is sent.
     * @param lang is the language of the display content in the response
     * @param subsetOption is the requested list of items to return in the response
     * For Services this can be paramList, displayItemList 
    */
    public String describeService(
            String service, String lang, String subsetOption) 
    {
        return describe(service, lang, subsetOption, serviceSubsetOptions);
    }
    
    /** For sending a describe message.
     * If public, this method would give full access: a describe message that 
     * lets the user specify all the details of who the receiver is, and what 
     * details are requested. 
     * @param to - the Greenstone module (MessageRouter, Collection, 
     * ServiceCluster or (Collection-)Service to send this describe message to.
     * (The module asked to describe itself.) 
     * @param lang - the language of the display content in the response.
     * @param subsetOption - the set of elements of the describe response that
     * are requested. These vary depending on the GS3 module asked to describe
     * itself.
     * @param validSubsetOptions - the list of subsetOptions that are allowed
     * for the module this describe message is sent to. Parameter subsetOption
     * has to be among the list of validSubsetOptions.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 35-41</a>
    */ 
    protected String describe(String to, String lang, 
            String subsetOption, String validSubsetOptions)
    {
        // Create message element: <message></message>
        Element message = this.doc.createElement(GSXML.MESSAGE_ELEM);
        // <message><request lang="en" to="" type="describe" uid="" /></message>
        Element request = GSXML.createBasicRequest(
                this.doc, GSXML.REQUEST_TYPE_DESCRIBE, to, lang, "");
        
        // Check if only a subset of this Module Interface's data is asked
        // to be described
        if(!subsetOption.equals("")) {
            // Now deal with the value for subset param:
            // only deal with valid params for subset of to-ModuleInterface
            if(validSubsetOptions.indexOf(subsetOption) == -1)
                return error("Invalid List to be described. Choose one of:\n"
                    + validSubsetOptions);
            
            // else, append <paramList>
            // <param name="subset" value="subsetOption" /></paramList>
            Element paramList = this.doc.createElement(
                    GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
            // append <param name="subset" value=paramValue />
            // createParam(Document, name, value); 
            // Name needs to be "subset", but this can be either GSXML.SUBSET_PARAM
            // or GSXML.SYSTEM_SUBSET_ATT. It's the first one probably.
            paramList.appendChild(GSXML.createParameter(
                            this.doc, GSXML.SUBSET_PARAM, subsetOption));
            request.appendChild(paramList);
        }
        message.appendChild(request);
        
        // Send it off to the Message Router and return the response
        return this.processInternal(message);
    }

    /* (2) Process-type message, QUERY-TYPE SERVICES - p.45 */
    /** For executing a (process-type message) query-type service.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 45</a>
     * @param collection is the name of the Collection whose query service this  
     * query-process request is sent to. If "", then the Message Router is assumed.
     * @param service is the name of the Query Service (of that collection) to  
     * which  this request is sent.
     * @param lang is the language of the display content in the response
     * @param nameToValsMap is a Map of the (fieldname, value) pairs for the 
     * parameters of the query. The field names should be those recognised by 
     * Greenstone 3. That is, the names must exist for the (Collection-)Service Query that this 
     * message is sent To (as given in 'to' argument). 
     * For names of Greenstone-accepted arguments,
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Actions_and_Arguments">Greenstone wiki - Actions and Arguments</a>
    */
    public String query(String collection, String service, 
            String lang, Map nameToValsMap) 
    {   
        // <paramList></paramList>
        Element paramList = this.doc.createElement(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        // <param>s: creating parameters of (name, value) pairs
        Set entrySet = nameToValsMap.entrySet();
        Iterator i = entrySet.iterator();
        while(i.hasNext()) {
            Entry entry = (Entry)i.next();
            String name = (String)entry.getKey();
            String value = (String)entry.getValue();
            paramList.appendChild(GSXML.createParameter(
                    this.doc, name, value));
        }

        Element message = this.doc.createElement(GSXML.MESSAGE_ELEM);
        Element request = GSXML.createBasicRequest(
                this.doc, GSXML.REQUEST_TYPE_PROCESS, 
                collection+"/"+service, lang, "");
        
        request.appendChild(paramList);
        message.appendChild(request);
        
        // Send it off to the Message Router and return the response
        return this.processInternal(message);   
    }

        /** 
     * This method is used to perform the most basic query:
     * it assumes defaults for all other parameters and provides only
     * the query string. It is built on top of a TextQuery.
     * @param collection is the Greenstone collection to be searched
     * @param lang is the preferred language of the display content in 
     * the response to be returned.
     * @param query is the string to be sought in the Greenstone collection
     * @return a Greenstone 3 XML response message for the query specifying 
     * the search results.
     */
    public String basicQuery(String collection, String lang, String query) {
        // The basicQuery is built on top of the TextQuery service
        final String queryService = "TextQuery";
        
        // (1) describe request on the TextQuery
        String queryDescription = describeCollectionService(
                collection, queryService, "en", "paramList"); // just get paramList
        //System.out.println(queryDescription);
        
        Document doc = this.converter.getDOM(queryDescription);
        NodeList nl = doc.getElementsByTagName(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        
        Element paramList = null;
        if(nl.getLength() <= 0) { // no paramList in textQuery description means
                // no query field either: that means we can't continue
            return this.error("BasicQuery is not available for this collection" 
                + " as it provides no TextQuery service.");
        } //else
        
        paramList = (Element)nl.item(0);
        nl = paramList.getElementsByTagName(GSXML.PARAM_ELEM);
        if(nl.getLength() <= 0) { // no params, means no query field, so return
            return this.error("BasicQuery is not available for this collection.");
        }
        
        // (2) get the defaults for each parameter and use that to set
        // the defaults
        Map params = new HashMap(nl.getLength()); // field name to value map
        for(int i = 0; i < nl.getLength(); i++) {
            Element param = (Element)nl.item(i);
            String paramName = param.getAttribute(GSXML.NAME_ATT);
            String def = param.getAttribute(GSXML.DEFAULT_ATT);
            if(def.equals("")) { 
                // if there's no default, the field must want the query String
                params.put(paramName, query);
            } else { // there is a default, use the default for this param
                params.put(paramName, def);
            }
        }
        
        // (3) Perform the query using defaults and return the response
        return this.query(collection, queryService, lang, params);
    }

        
    /* (3) RETRIEVE PROCESS METHODS - Manual, pp.47-49 */
    /** DocumentContentRetrieve request sent to a collection's 
     * DocumentContentRetrieve service (see manual, p.48)
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 48</a>
     * @param collection is the name of the Collection whose 
     * DocumentContentRetrieve is requested 
     * @param lang is the language of the display content in the response 
     * @param docNodeIDs is the list of documentNodeIDs for which the
     * content ought to be retrieved. */
    public String retrieveDocumentContent(
            String collection, String lang, String[] docNodeIDs) 
    {
        // creating <documentNodeList></documentNodeList>
        Element docNodeList = this.doc.createElement(
                GSXML.DOC_NODE_ELEM+GSXML.LIST_MODIFIER);
        
        // creating subelements: <documentNode nodeID="..." />
        for(int i = 0; i < docNodeIDs.length; i++) {
            Element docNode = this.doc.createElement(GSXML.DOC_NODE_ELEM);
            docNode.setAttribute(GSXML.NODE_ID_ATT, docNodeIDs[i]);
            docNodeList.appendChild(docNode);
        }

        Element message = doc.createElement(GSXML.MESSAGE_ELEM);
        Element request = GSXML.createBasicRequest(
                doc, GSXML.REQUEST_TYPE_PROCESS, 
                collection+"/DocumentContentRetrieve", lang, "");
        
        // create an empty <paramlist /> element (as example in manual)
        Element paramlist = doc.createElement(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        
        request.appendChild(paramlist);
        request.appendChild(docNodeList);
        message.appendChild(request);

        // Send it off to the Message Router and return the response
        return this.processInternal(message);
    }
    
    /** DocumentStructureRetrieve request sent to a collection's 
     * DocumentStructureRetrieve service (manual pp.48, 49) to retrieve
     * the entire document structure.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 48, 49</a>
     * @param collection is the name of the Collection whose 
     * DocumentStructureRetrieve is requested 
     * @param lang is the language of the display content in the response 
     * @param docNodeIDs is the list of documentNodeIDs for which the
     * entire structure ought to be retrieved. */
    public String retrieveEntireDocumentStructure(String collection, 
            String lang, String[] docNodeIDs) 
    {
        return retrieveDocumentStructure(collection, lang, docNodeIDs, 
                new String[] { "entire" }, null);
    }
    
    /** DocumentStructureRetrieve request sent to a collection's 
     * DocumentStructureRetrieve service (manual pp.48, 49) to retrieve
     * the specified part of the document's structure.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 48, 49</a>
     * @param collection is the name of the Collection whose 
     * DocumentStructureRetrieve is requested 
     * @param lang is the language of the display content in the response 
     * @param docNodeIDs is the list of documentNodeIDs for which the
     * structure ought to be retrieved. 
     * @param structure specifies what structure information needs to
     * be retrieved. The values can be one or more of ancestors, parent, 
     * siblings, children, descendants (<b>note spelling</b>), entire.
     * @param info - for specifying extra information to be retrieved. 
     * Possible values for info parameters are numSiblings, siblingPosition, 
     * numChildren */
    public String retrieveDocumentStructure(String collection, String lang,
            String[] docNodeIDs, String[] structure, String[] info)
    {
        // creating subelements: <documentNode nodeID="..." />
        Element docNodeList = this.doc.createElement(
                GSXML.DOC_NODE_ELEM+GSXML.LIST_MODIFIER);
        for(int i = 0; i < docNodeIDs.length; i++) {
            Element docNode = this.doc.createElement(GSXML.DOC_NODE_ELEM);
            docNode.setAttribute(GSXML.NODE_ID_ATT, docNodeIDs[i]);
            docNodeList.appendChild(docNode);
        } 
        
        Element message = doc.createElement(GSXML.MESSAGE_ELEM);
        Element request = GSXML.createBasicRequest(
                doc, GSXML.REQUEST_TYPE_PROCESS, 
                collection+"/DocumentStructureRetrieve", lang, "");
        
        // Create the <paramlist></paramlist> element of param elements,
        // if any; and only if values are legal (that is, if they occur in  
        // static Strings docStructureOptions and docStructureInfo):
        // <param name="structure" value = "structure[i]">
        // <param name="info" value = "info[i]">
        Element paramList = doc.createElement(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        
        if(structure != null) {
            for(int i = 0; i < structure.length; i++) {
                if(docStructureOptions.indexOf(structure[i]) != -1) {
                    paramList.appendChild(GSXML.createParameter(
                            this.doc, "structure", structure[i]));                  
                }
            } 
        }
        if(info != null) {
            for(int i = 0; i < info.length; i++) {
                if(docStructureInfo.indexOf(info[i]) != -1) {
                    paramList.appendChild(GSXML.createParameter(
                            this.doc, "info", info[i]));
                }
            }
        }
        
        // paramList is allowed to be empty and may indeed be empty:
        request.appendChild(paramList);
        request.appendChild(docNodeList);
        message.appendChild(request);
        
        // Send it off to the Message Router and return the response
        return this.processInternal(message);
    }
    
    /* Retrieve for Doc Metadata: explained in the manual on page 47 */
    /** DocumentMetadataRetrieve request sent to a collection's 
     * DocumentMetadataRetrieve service to retrieve all of a document's metadata. 
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 47</a>
     * @param collection is the name of the Collection whose 
     * DocumentMetadataRetrieve is requested 
     * @param lang is the language of the display content in the response 
     * @param docNodeIDs is the list of documentNodeIDs for which the
     * structure ought to be retrieved. 
    */
    public String retrieveAllDocumentMetadata(String collection, String lang,
            String[] docNodeIDs) 
    {
        // See bottom of manual p.44 for the fact that "all" is used
        // as the metaName value when retrieving all metadata for a doc
        return retrieveDocumentMetadata(collection, lang, docNodeIDs, 
                new String[]{ "all" });
    }
    
    /** DocumentMetadataRetrieve service to retrieve some specific 
     * metadata values of a document. (Manual on page 47.)
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 47</a>
     * @param collection is the name of the Collection whose 
     * DocumentContentRetrieve is requested 
     * @param lang is the language of the display content in the response 
     * @param docNodeIDs is the list of documentNodeIDs for which the
     * structure ought to be retrieved.
     * @param metaNames is a list of metadata names which are requested  
     * to be fetched for the specified documents */
    public String retrieveDocumentMetadata(String collection, String lang,
            String[] docNodeIDs, String[] metaNames)
    {
        return metadataRetrieve(collection+"/DocumentMetadataRetrieve",
                lang, docNodeIDs, metaNames, GSXML.DOC_NODE_ELEM);
    }

    /** Retrieve all classification Metadata for browsing (sent to the 
     * ClassifierBrowseMetadataRetrieve service).
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 47, 48</a>
     * @param collection is the name of the Collection whose 
     * ClassifierBrowseMetadataRetrieve service is called 
     * @param categoryName - name of the browsing category, usually
     * ClassifierBrowse. (If left as "", then it defaults to ClassifierBrowse) 
     * @param lang is the language of the display content in the response 
     * @param nodeIDs is the list of document or classifier NodeIDs 
     * for which the metadata ought to be retrieved.*/
    public String retrieveAllBrowseMetadata(String collection, 
            String categoryName, String lang, String[] nodeIDs) 
    {
        if(categoryName.equals(""))
            categoryName = "ClassifierBrowse";
        // See bottom of manual p.47 for the fact that "all" is used as 
        // the metaName value when retrieving all metadata for a classifier
        return metadataRetrieve(collection+"/"+categoryName+"MetadataRetrieve", 
                lang, nodeIDs, new String[]{ "all" }, GSXML.CLASS_NODE_ELEM);
    }
    
    /** ClassifierBrowseMetadataRetrieve service to retrieve some specific 
     * metadata values of a document. 
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - pages 47, 48</a>
     * @param collection is the name of the Collection whose 
     * ClassifierBrowseMetadataRetrieve service is called
     * @param categoryName - name of the browsing category, usually
     * ClassifierBrowse. (If left as "", then it defaults to ClassifierBrowse) 
     * @param lang is the language of the display content in the response 
     * @param nodeIDs is the list of document or classifier NodeIDs 
     * for which the metadata ought to be retrieved.
     * @param metaNames is a list of metadata names which are requested  
     * to be fetched for the specified documents or classifiers */
    public String retrieveBrowseMetadata(String collection, String categoryName,  
            String lang, String[] nodeIDs, String[] metaNames) 
    {
        if(categoryName.equals(""))
            categoryName = "ClassifierBrowse";
        return metadataRetrieve(collection+"/"+categoryName+"MetadataRetrieve", 
                lang, nodeIDs, metaNames, GSXML.CLASS_NODE_ELEM);
    }
    
    /** Performs a metadata retrieve for documents and (browse) classification 
     * hierarchies. Sends a Document- or ClassifierBrowse- MetadataRetrieve message 
     * to the Document- or ClassifierBrowse- MetadataRetrieve service.
     * @param to - the Document- or ClassifierBrowse- MetadataRetrieve service to
     * send this metadata retrieve message to.
     * @param lang - the language of the display content in the response
     * @param nodeIDs - the list of (document or classifier) nodeIDs for which
     * to retrieve the metadata for
     * @param metaNames - a list specifiying the names of the metadata items 
     * to be retrieved for each nodeID. E.g. "Title", but a list is allowed.
     * @param NODE_ELEM - either of GSXML's names for the &lt;documentNode&gt; or 
     * &lt;classifierNode&gt; elements.
    */
    protected String metadataRetrieve(String to, String lang, 
            String[] nodeIDs, String[] metaNames, final String NODE_ELEM)
    {
        // create the <paramlist></paramlist> element of param elements:  
        // <param name="metadata" value = "metaName[i]">
        Element metadataParamList = this.doc.createElement(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        for(int i = 0; i < metaNames.length; i++) {
            metadataParamList.appendChild(GSXML.createParameter(
                    this.doc, GSXML.METADATA_ELEM, metaNames[i]));
        }
        
        // creating subelements: <documentNode nodeID="..." />
        // or <classifierNode nodeID="..." />
        Element nodeList = this.doc.createElement(
                NODE_ELEM+GSXML.LIST_MODIFIER);
        for(int i = 0; i < nodeIDs.length; i++) {
            Element docNode = this.doc.createElement(NODE_ELEM);
            docNode.setAttribute(GSXML.NODE_ID_ATT, nodeIDs[i]);
            nodeList.appendChild(docNode);
        } 
        
        Element message = doc.createElement(GSXML.MESSAGE_ELEM);
        Element request = GSXML.createBasicRequest(doc, 
                GSXML.REQUEST_TYPE_PROCESS, to, lang, "");
        
        request.appendChild(metadataParamList);
        request.appendChild(nodeList);
        message.appendChild(request);
        
        // Send it off to the Message Router and return the response
        return this.processInternal(message);
    }
    
    /* (4) Classifier BROWSE PROCESS METHODS - p.46 */
    /** To send a browse request for all the descendants of a classifier node.
     * Useful for getting the entire structure of a top-level &lt;classificationNode&gt;
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 46</a>
     * @param collection is the name of the Collection whose browse Classifier
     * Browse Service is called
     * @param browseService is the name of the (Classifier) Browse Service (of 
     * the given collection) to which this request message is sent. 
     * @param lang is the language of the display content in the response 
     * @param classifierNodeIDs is an array of classifierNodeIDs for which the
     * structures ought to be retrieved.
    */
    public String browseDescendants(String collection, String browseService, 
            String lang, String[] classifierNodeIDs) 
    {
        // We are at the top level, we want all the descendants:
        // <param name="structure" value = "descendants">
        // <classifierNodeList><classifier nodeID="CLx" /></classifierNodeList>
        return browse(collection, browseService, lang,   
                classifierNodeIDs, 
                new String[] {"descendants"}); // note the spelling
    }
    
    /** To send a browse request for specific parts of a classifier node 
     * (children, ancestors, descendants). Useful for getting specific parts
     * of the structure of a top-level &lt;classificationNode&gt;.
     * @see <a href="http://wiki.greenstone.org/wiki/index.php/Greenstone3">The Greenstone 3 Developer's Manual - page 46</a> 
     * @param collection is the name of the Collection whose browse Classifier
     * Browse Service is called
     * @param browseService is the name of the (Classifier) Browse Service (of 
     * the given collection) to which this request message is sent. 
     * @param lang is the language of the display content in the response 
     * @param classifierNodeIDs is the list of classifierNodeIDs for which the
     * structure ought to be retrieved.
     * @param structureParams the list of parameters indicating what structure 
     * information is requested. Accepted values are ancestors, parent, siblings,
     * children, descendants.
    */
    public String browse(String collection, String browseService, String lang, 
            String[] classifierNodeIDs, String[] structureParams) 
    {
        if(browseService.equals(""))
            browseService = "ClassifierBrowse";
        
        // Create message element: <message></message>
        Element message = this.doc.createElement(GSXML.MESSAGE_ELEM);
        // <message><request lang="en" to="" type="process" uid="" /></message>
        Element request = GSXML.createBasicRequest(this.doc,
            GSXML.REQUEST_TYPE_PROCESS, collection+"/"+browseService, lang, "");
        
        // <param name="structure" value = "structureParams[i]">
        Element paramList = this.doc.createElement(
                GSXML.PARAM_ELEM+GSXML.LIST_MODIFIER);
        for(int i = 0; i < structureParams.length; i++) {
            // check it is a valid structure parameter
            if(browseStructureOptions.indexOf(structureParams[i]) != -1) {
                paramList.appendChild(GSXML.createParameter(
                        this.doc, "structure", structureParams[i]));
            }
        }
        
        // <classifierNodeList><classifier nodeID="CLx" />
        //      <classifier nodeID="CLy" /></classifierNodeList> 
        // where CLx and CLy are given in the parameter classifierNodeIDs
        Element classifierNodeList = this.doc.createElement(
                GSXML.CLASS_NODE_ELEM+GSXML.LIST_MODIFIER);
        for(int i = 0; i < classifierNodeIDs.length; i++) {
            Element classifier = this.doc.createElement(GSXML.CLASS_NODE_ELEM);
            classifier.setAttribute(GSXML.NODE_ID_ATT, classifierNodeIDs[i]);
            classifierNodeList.appendChild(classifier);
        }
        
        // now finish constructing the request message:
        request.appendChild(paramList);
        request.appendChild(classifierNodeList);
        message.appendChild(request);
        
        // Send it off to the Message Router and return the response
        return this.processInternal(message);
    }
    
    /** Called by most other methods in order to send the constructed message
     * to the Greenstone's MessageRouter, intercept the response and return it.
     * @param message is the XML message Element to send to GS3's MessageRouter.
     * @return the XML response in String format. */
    protected String processInternal(Element message) {
            if(LOG.isDebugEnabled()) { // or LOG.isEnabledFor(Level.DEBUG). 
                                   // Testing for this improves efficiency
            LOG.debug(this.converter.getPrettyString(message));
        }
        
        // Let the messagerouter process the request message and get the response
        Element response = mr.process(message); 
              // won't be null, MR always returns some response

        // Return it as a String formatted for display
        String responseMsg = this.converter.getPrettyString(response); 
                          // this.converter.getString(response);

        // In order to avoid "Content is not allowed in prolog" exception on the 
        // web services' client end (problem encountered in GS mailing list), need 
        // to make sure no characters (incl spaces) preceed the  XML sent back 
        // from here. It may also require the <?xml?> tag at the very start.
        if(responseMsg.charAt(0) == ' ') {
            responseMsg = responseMsg.trim();
        }
        
        return "<?xml version=\"1.0\" encoding=\"utf-8\"?>"+responseMsg;
    }
    
    /** Creates a String response message to represent an XML error response  
     * message using the error specified in the message parameter. A String is 
     * created because this method ensures that a response message is reliably 
     * constructed (no exceptions are thrown) that can be sent to clients.
     * @param errorMessage - the errormessage to be conveyed
     * @return an XML response message containing an GS3 error element. */
    protected String error(String errorMessage) {
        StringBuffer buf = new StringBuffer("<" + GSXML.MESSAGE_ELEM + ">");
        buf.append("<" + GSXML.RESPONSE_ELEM + " " 
                + GSXML.FROM_ATT + "=\"" + "Greenstone 3 Web Services\"" + ">");
        buf.append("<" + GSXML.ERROR_ELEM + " " 
            + GSXML.ERROR_TYPE_ATT + "=\""+ GSXML.ERROR_TYPE_OTHER + "\"" + ">");
        buf.append(errorMessage+"\n");
        buf.append("</" + GSXML.ERROR_ELEM + ">");
        buf.append("</" + GSXML.RESPONSE_ELEM + ">");
        buf.append("</" + GSXML.MESSAGE_ELEM + ">");
        return buf.toString();
    }

    /*
    Look in file QBRWebServicesHelp.properties
    - Have a properties file that maps methodname to help string specific
    to the method.
    - Read it all in statically at the start of the class, into a Properties Map.
    - When this method is called, display the usage: "help methodname"
    and list all the available methods by going over the keys in the Map.
    - When the helpWithMethod(String methodname) method is called, return the 
    value of the Map for the methodname key. This value would be the help
    description for that method.        
    */
    /** @return a help string for listing all the web service methods. */
    public static String help() {
        if(!helpErrormessage.equals("")) {
            return helpErrormessage;
        }
        
        StringBuffer helpString = new StringBuffer(
                "USAGE: helpWithMethod(String <method name>)\n");
        helpString.append(
                "\nNearly all the web service operations return a String\n"); 
        helpString.append(
                "representing a Greenstone 3 XML response message.\n");
        helpString.append("\nA list of all the method names: \n");
        
        Enumeration props = properties.keys();
        while(props.hasMoreElements()){
            String methodName = (String)props.nextElement();
            helpString.append("\t");
            helpString.append(methodName);
            helpString.append("\n");
        }
        
        return helpString.toString();
    }
    
    /** @param methodname is the name of the method to be described.  
     * @return a help string for the given method, explaining what the method
     * does, what parameters it expects and their types and what it returns.
    */
    public static String helpWithMethod(String methodname) {
        if(!helpErrormessage.equals("")) {
            return helpErrormessage;
        }
        // else we can get the method's description from the properties
        // map loaded from the QBRWebServicesHelp.properties file:
        String helpString = properties.getProperty(methodname,
            "No description for " + methodname); // if the method does not exist
        
        return helpString;
    }
} // end web service class