* Work toward getting static backends going again, but they're still broken.

[beagle.git] / beagled / LuceneQueryable.cs

//
// LuceneQueryable.cs
//
// Copyright (C) 2004-2005 Novell, Inc.
//

//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//

using System;
using System.Collections;
using System.IO;

using Beagle.Util;

namespace Beagle.Daemon {

        public abstract class LuceneQueryable : BackendBase {
                
                static public bool OptimizeRightAway = false;

                public delegate IIndexer IndexerCreator (string source_name, int source_version);

                static private IndexerCreator indexer_hook = null;

                static public IndexerCreator IndexerHook {
                        set { indexer_hook = value; }
                }
                
                //////////////////////////////////////////////////////////

                public delegate void OptimizeAllHandler ();

                static private OptimizeAllHandler OptimizeAllEvent;

                static public void OptimizeAll ()
                {
                        if (OptimizeAllEvent != null)
                                OptimizeAllEvent ();
                }

                //////////////////////////////////////////////////////////
                
                private Scheduler scheduler = Scheduler.Global;
                private FileAttributesStore fa_store = null;

                private string source_name;

                private LuceneQueryingDriver driver;
                private IIndexer indexer = null;

                private Scheduler.Task our_final_flush_task = null;
                private Scheduler.Task our_optimize_task = null;

                private object request_lock = new object ();
                private IndexerRequest pending_request = new IndexerRequest ();

                //////////////////////////////////////////////////////////

                public LuceneQueryable (string source_name) : this (source_name, -1, false) { }

                public LuceneQueryable (string source_name, bool read_only_mode) : this (source_name, -1, read_only_mode) { }

                public LuceneQueryable (string source_name, int source_version) : this (source_name, source_version, false) { }

                public LuceneQueryable (string source_name, int source_version, bool read_only_mode)
                {
                        this.source_name = source_name;

                        driver = BuildLuceneQueryingDriver (source_name, source_version, read_only_mode);
                        driver.RegisterHitFilter (source_name, this.HitFilter);

                        // If the queryable is in read-only more, don't 
                        // instantiate an indexer for it.
                        if (read_only_mode)
                                return;

                        if (indexer_hook != null)
                                indexer = indexer_hook (source_name, source_version);

                        if (indexer == null)
                                throw new Exception ("No indexer available for source " + source_name);

                        OptimizeAllEvent += OnOptimizeAllEvent;

                        // Schedule an optimize, just in case
                        ScheduleOptimize ();

                        Shutdown.ShutdownEvent += new Shutdown.ShutdownHandler (OnShutdownEvent);
                }

                public override string Name {
                        set {
                                if (value != source_name)
                                        throw new Exception (String.Format ("Backend name (from BackendFlavor) '{0}' does not match source name (from LuceneQueryable ctor) '{1}'", value, source_name));

                                base.Name = value;
                        }
                }

                protected string IndexDirectory {
                        get { return driver.TopDirectory; }
                }

                protected string IndexFingerprint {
                        get { return driver.Fingerprint; }
                }

                protected string SourceDataDir {
                        get { return Path.Combine (IndexDirectory, this.source_name); }
                }

                protected LuceneQueryingDriver Driver {
                        get { return driver; }
                }

                public Scheduler ThisScheduler {
                        get { return scheduler; }
                }

                /////////////////////////////////////////

                public override void Start ()
                {

                }

                public override IQueryable Queryable {
                        get { return driver; }
                }

                /////////////////////////////////////////

                virtual protected void ShutdownHook ()
                {

                }

                private void OnShutdownEvent ()
                {
                        lock (request_lock) 
                                pending_request.Cleanup ();

                        try {
                                ShutdownHook ();
                        } catch (Exception ex) {
                                Logger.Log.Warn (ex, "Caught exception in shutdown hook");
                        }
                }

                /////////////////////////////////////////

                virtual protected bool HitFilter (Hit hit)
                {
                        return true;
                }

                /////////////////////////////////////////

                // DEPRECATED: This does nothing, since everything is now
                // time-based.
                virtual protected double RelevancyMultiplier (Hit hit)
                {
                        return 1.0;
                }

                static protected double HalfLifeMultiplier (DateTime dt, int half_life_days)
                {
                        double days = Math.Abs ((DateTime.Now - dt).TotalDays);
                        if (days < 0)
                                return 1.0f;
                        return Math.Pow (0.5, days / (double) half_life_days);
                }

                // FIXME: A decaying half-life is a little sketchy, since data
                // will eventually decay beyond the epsilon and be dropped
                // from the results entirely, which is almost never what we
                // want, particularly in searches with a few number of
                // results.  But with a default half-life of 6 months, it'll
                // take over 13 years to fully decay outside the epsilon on
                // this multiplier alone.
                static protected double HalfLifeMultiplier (DateTime time)
                {
                        // Default relevancy half-life is six months.
                        return HalfLifeMultiplier (time, 182);
                }

                static protected double HalfLifeMultiplierFromProperty (Hit hit,
                                                                        double default_multiplier,
                                                                        params object [] properties)
                {
                        double best_m = -1.0;

                        foreach (object obj in properties) {
                                string key = obj as string;
                                string val = hit [key];
                                if (val != null) {
                                        DateTime dt = StringFu.StringToDateTime (val);
                                        double this_m;
                                        this_m = HalfLifeMultiplier (dt, 182);  /* 182 days == six months */
                                        if (this_m > best_m)
                                                best_m = this_m;
                                }
                        }

                        if (best_m < 0)
                                best_m = default_multiplier;
                        return best_m;
                }

                /////////////////////////////////////////

                protected string GetSnippetFromTextCache (string [] query_terms, Uri uri)
                {
                        // Look up the hit in our text cache.  If it is there,
                        // use the cached version to generate a snippet.

                        TextReader reader;
                        reader = TextCache.UserCache.GetReader (uri);
                        if (reader == null)
                                return null;

                        string snippet = SnippetFu.GetSnippet (query_terms, reader);
                        reader.Close ();

                        return snippet;
                }

                // When remapping, override this with
                // return GetSnippetFromTextCache (query_terms, remapping_fn (hit.Uri))
                public override string GetSnippet (string [] query_terms, Hit hit)
                {
                        return GetSnippetFromTextCache (query_terms, hit.Uri);
                }

                /////////////////////////////////////////

                private int progress_percent = -1;
                private QueryableState state = QueryableState.Idle;
                private DateTime last_state_change = DateTime.MinValue;

                public override QueryableStatus GetBackendStatus ()
                {
                        QueryableStatus status = new QueryableStatus ();

                        status.Name = this.Name;
                        status.State = state;
                        status.ProgressPercent = progress_percent;

                        // If we're in read-only mode, query the driver
                        // and not the indexer for the item count.
                        if (indexer == null)
                                status.ItemCount = driver.GetItemCount ();
                        else
                                status.ItemCount = indexer.GetItemCount ();

                        // Frequent state changes are common, and there isn't
                        // a real state machine with continuity when it comes
                        // to the indexing process.  A delayed indexing task,
                        // for example, might not actually run for several
                        // seconds after it is scheduled.  In this case, the
                        // backend might be in an "Idle" state, but the
                        // indexing process clearly isn't done.  To work
                        // around this, we also track the last time the state
                        // changed.  If it's less than some threshold, then
                        // we consider ourselves to still be in the process of
                        // indexing.
                        if (state != QueryableState.NotApplicable
                            && (state != QueryableState.Idle
                                || (DateTime.Now - last_state_change).TotalSeconds <= 30))
                                status.IsIndexing = true;

                        return status;
                }

                public QueryableState State {
                        get { return this.state; }
                        set { 
                                //Logger.Log.Debug ("State {0}: {1} -> {2}", this, this.state, value);

                                this.state = value;
                                this.last_state_change = DateTime.Now;
                        }
                }

                public int ProgressPercent {
                        get { return this.progress_percent; }
                        set { this.progress_percent = value; }
                }

                /////////////////////////////////////////

                public FileStream ReadDataStream (string name)
                {
                        if (! Directory.Exists (SourceDataDir))
                                return null;

                        string path = Path.Combine (SourceDataDir, name);

                        if (!File.Exists (path))
                                return null;

                        return new FileStream (path, System.IO.FileMode.Open, FileAccess.Read);
                }

                public string ReadDataLine (string name)
                {
                        FileStream stream = ReadDataStream (name);

                        if (stream == null)
                                return null;

                        StreamReader reader = new StreamReader (stream);
                        string line = reader.ReadLine ();
                        reader.Close ();

                        return line;
                }

                public FileStream WriteDataStream (string name)
                {
                        if (! Directory.Exists (SourceDataDir))
                                Directory.CreateDirectory (SourceDataDir);

                        string path = Path.Combine (SourceDataDir, name);
                        
                        return new FileStream (path, System.IO.FileMode.Create, FileAccess.Write, FileShare.ReadWrite);
                }

                public void WriteDataLine (string name, string line)
                {
                        if (line == null) {
                                if (! Directory.Exists (SourceDataDir))
                                        return;

                                string path = Path.Combine (SourceDataDir, name);

                                if (File.Exists (path))
                                        File.Delete (path);

                                return;
                        }

                        FileStream stream = WriteDataStream (name);
                        StreamWriter writer = new StreamWriter (stream);
                        writer.WriteLine (line);
                        writer.Close ();
                }


                //////////////////////////////////////////////////////////////////////////////////

                // More hooks.  These are mostly here for the file system backend.

                virtual protected bool PreAddIndexableHook (Indexable indexable)
                {
                        // By default, we like everything.
                        return true;
                }

                // If we are remapping Uris, indexables should be added to the
                // index with the internal Uri attached.  This the receipt
                // will come back w/ an internal Uri.  In order for change
                // notification to work correctly, we have to map it to
                // an external Uri.
                virtual protected void PostAddHook (Indexable indexable, IndexerAddedReceipt receipt)
                {
                        // Does nothing by default
                }

                virtual protected void PostRemoveHook (Indexable indexable, IndexerRemovedReceipt receipt)
                {
                        // Does nothing by default
                }

                //////////////////////////////////////////////////////////////////////////////////

                // Adding a single indexable
                
                private class AddTask : Scheduler.Task {
                        LuceneQueryable    queryable;
                        Indexable          indexable;

                        public AddTask (LuceneQueryable    queryable,
                                        Indexable          indexable)
                        {
                                this.queryable = queryable;
                                this.indexable = indexable;
                                this.Tag = indexable.DisplayUri.ToString ();
                                this.Weight = 1;
                        }

                        override protected void DoTaskReal ()
                        {
                                QueryableState old_state = queryable.State;
                                queryable.State = QueryableState.Indexing;

                                if (queryable.PreAddIndexableHook (indexable)) {
                                        queryable.AddIndexable (indexable);

                                        if (Priority == Scheduler.Priority.Immediate)
                                                queryable.Flush ();
                                        else
                                                queryable.ConditionalFlush ();
                                }

                                queryable.State = old_state;
                        }

                        override protected void DoCleanup ()
                        {
                                indexable.Cleanup ();
                        }
                }

                public Scheduler.Task NewAddTask (Indexable indexable)
                {
                        AddTask task;
                        task = new AddTask (this, indexable);
                        task.Source = this;
                        return task;
                }

                //////////////////////////////////////////////////////////////////////////////////

                // Adding an indexable generator

                private class AddGeneratorTask : Scheduler.Task {
                        LuceneQueryable queryable;
                        IIndexableGenerator generator;

                        public AddGeneratorTask (LuceneQueryable     queryable,
                                                 IIndexableGenerator generator)
                        {
                                this.queryable = queryable;
                                this.generator = generator;
                                this.Tag = generator.StatusName;
                        }

                        override protected void DoTaskReal ()
                        {
                                // Since this is a generator, we want the task to
                                // get re-scheduled after it is run.
                                Reschedule = true;

                                QueryableState old_state = queryable.State;
                                queryable.State = QueryableState.Indexing;

                                // Number of times a null indexable was returned.  We don't want
                                // to spin tightly in a loop here if we're not actually indexing
                                // things.
                                int misfires = 0;

                                do {
                                        if (! generator.HasNextIndexable ()) {
                                                // Of course, don't reschedule if there is no more work to do.
                                                Reschedule = false;
                                                break;
                                        }

                                        Indexable generated;
                                        generated = generator.GetNextIndexable ();

                                        // Note that the indexable generator can return null.
                                        // This means that the generator didn't have an indexable
                                        // to return this time through, but it does not mean that
                                        // its processing queue is empty.
                                        if (generated == null) {
                                                misfires++;

                                                if (misfires > 179) // Another totally arbitrary number
                                                        break;
                                                else
                                                        continue;
                                        }

                                        if (queryable.PreAddIndexableHook (generated))
                                                queryable.AddIndexable (generated);
                                        else
                                                generated.Cleanup ();
                                        
                                        // We keep adding indexables until a flush goes through.
                                } while (! queryable.ConditionalFlush ());

                                generator.PostFlushHook ();

                                queryable.State = old_state;
                        }

                        override protected void DoCleanup ()
                        {
                        }
                }

                public Scheduler.Task NewAddTask (IIndexableGenerator generator)
                {
                        AddGeneratorTask task;
                        task = new AddGeneratorTask (this, generator);
                        task.Source = this;
                        return task;
                }

                //////////////////////////////////////////////////////////////////////////////////

                // There used to be a separate type of task for doing removes.
                // This is all that remains of that old code.
                public Scheduler.Task NewRemoveTask (Uri uri)
                {
                        Indexable indexable;
                        indexable = new Indexable (IndexableType.Remove, uri);
                        
                        return NewAddTask (indexable);
                }

                //////////////////////////////////////////////////////////////////////////////////

                public Scheduler.Task NewRemoveByPropertyTask (Property prop)
                {
                        PropertyRemovalGenerator prg = new PropertyRemovalGenerator (driver, prop);

                        return NewAddTask (prg);
                }

                ///////////////////////////////////////////////////////////////////////////////////

                //
                // An IIndexableGenerator that returns remove Indexables for
                // all items which match a certain property
                //

                private class PropertyRemovalGenerator : IIndexableGenerator {

                        private LuceneQueryingDriver driver;
                        private Property prop_to_match;
                        private Uri[] uris_to_remove;
                        private int idx;

                        public PropertyRemovalGenerator (LuceneQueryingDriver driver, Property prop)
                        {
                                this.driver = driver;
                                this.prop_to_match = prop;
                        }

                        public Indexable GetNextIndexable ()
                        {
                                Indexable indexable;

                                indexable = new Indexable (IndexableType.Remove, uris_to_remove [idx]);
                                idx++;

                                return indexable;
                        }

                        public bool HasNextIndexable ()
                        {
                                if (uris_to_remove == null)
                                        uris_to_remove = this.driver.PropertyQuery (this.prop_to_match);

                                if (idx < uris_to_remove.Length)
                                        return true;
                                else 
                                        return false;
                        }

                        public string StatusName {
                                get {
                                        return String.Format ("Removing {0}={1}", prop_to_match.Key, prop_to_match.Value);
                                }
                        }

                        public void PostFlushHook () { }
                }


                //////////////////////////////////////////////////////////////////////////////////

                // When all other tasks are complete, we need to do a final flush.
                // We schedule that as a maintenance task.

                private class FinalFlushTask : Scheduler.Task {
                        LuceneQueryable queryable;

                        public FinalFlushTask (LuceneQueryable queryable)
                        {
                                this.queryable = queryable;

                        }

                        override protected void DoTaskReal ()
                        {
                                queryable.Flush ();
                        }
                }

                private void ScheduleFinalFlush ()
                {
                        if (our_final_flush_task == null) {
                                our_final_flush_task = new FinalFlushTask (this);

                                our_final_flush_task.Tag = "Final Flush for " + Name;
                                our_final_flush_task.Priority = Scheduler.Priority.Maintenance;
                                our_final_flush_task.SubPriority = 100; // do this first when starting maintenance
                                our_final_flush_task.Source = this;
                        }
                        
                        ThisScheduler.Add (our_final_flush_task);
                }


                //////////////////////////////////////////////////////////////////////////////////

                // Optimize the index

                private DateTime last_optimize_time = DateTime.MinValue;

                public DateTime LastOptimizeTime {
                        get { return last_optimize_time; }
                        set { last_optimize_time = value; }
                }
                
                private class OptimizeTask : Scheduler.Task {
                        LuceneQueryable queryable;

                        public OptimizeTask (LuceneQueryable queryable)
                        {
                                this.queryable = queryable;
                        }

                        override protected void DoTaskReal ()
                        {
                                queryable.Optimize ();
                                queryable.LastOptimizeTime = DateTime.Now;
                        }
                }

                public Scheduler.Task NewOptimizeTask ()
                {
                        Scheduler.Task task;
                        task = new OptimizeTask (this);
                        task.Tag = "Optimize " + Name;
                        task.Priority = Scheduler.Priority.Maintenance;
                        task.Source = this;

                        return task;
                }

                private void OnOptimizeAllEvent ()
                {
                        Scheduler.Task task;
                        task = NewOptimizeTask (); // construct an optimizer task
                        task.Priority = Scheduler.Priority.Delayed; // but boost the priority
                        ThisScheduler.Add (task);
                }

                private void ScheduleOptimize ()
                {
                        double optimize_delay;

                        // Really we only want to optimize at most once a day, even if we have
                        // indexed a ton of dat
                        TimeSpan span = DateTime.Now - last_optimize_time;
                        if (span.TotalDays > 1.0)
                                optimize_delay = 10.0; // minutes;
                        else
                                optimize_delay = (new TimeSpan (TimeSpan.TicksPerDay) - span).TotalMinutes;

                        if (our_optimize_task == null)
                                our_optimize_task = NewOptimizeTask ();

                        if (OptimizeRightAway || Environment.GetEnvironmentVariable ("BEAGLE_UNDER_BLUDGEON") != null)
                                optimize_delay = 1/120.0; // half a second

                        // Changing the trigger time of an already-scheduled process
                        // does what you would expect.
                        our_optimize_task.TriggerTime = DateTime.Now.AddMinutes (optimize_delay);

                        // Adding the same task more than once is a harmless no-op.
                        ThisScheduler.Add (our_optimize_task);
                }

                //////////////////////////////////////////////////////////////////////////////////

                // Other hooks

                // If this returns true, a task will automatically be created to
                // add the child.
                virtual protected bool PreChildAddHook (Indexable child)
                {
                        return true;
                }

                virtual protected void PreFlushHook (IndexerRequest flushed_request)
                { }

                virtual protected void PostFlushHook (IndexerRequest    flushed_request,
                                                      IndexerReceipt [] receipts)
                { }

                //////////////////////////////////////////////////////////////////////////////////

                protected void AddIndexable (Indexable indexable)
                {
                        indexable.Source = this.Name;

                        lock (request_lock)
                                pending_request.Add (indexable);

                        // Schedule a final flush every time we add anything.
                        // Better safe than sorry.
                        ScheduleFinalFlush ();
                }

                protected void Optimize ()
                {
                        lock (request_lock) {
                                pending_request.OptimizeIndex = true;
                                Flush ();
                        }
                }

                // Returns true if we actually did flush, false otherwise.
                protected bool ConditionalFlush ()
                {
                        QueryableState old_state = State;
                        State = QueryableState.Flushing;

                        try {
                                lock (request_lock) {
                                        if (pending_request.Count > 37) { // a total arbitrary magic number
                                                Flush ();
                                                return true;
                                        }
                                }
                                return false;
                        } finally {
                                State = old_state;
                        }
                }

                protected void Flush ()
                {
                        QueryableState old_state = State;
                        State = QueryableState.Flushing;

                        try {
                                DoFlush ();
                        } finally {
                                State = old_state;
                        }
                }

                private void DoFlush ()
                {
                        IndexerRequest flushed_request;

                        lock (request_lock) {
                                if (pending_request.IsEmpty)
                                        return;

                                flushed_request = pending_request;
                                pending_request = new IndexerRequest ();

                                // We hold the request_lock when calling PreFlushHook, so
                                // that no other requests can come in until it exits.
                                PreFlushHook (flushed_request);
                        }

                        IndexerReceipt [] receipts;
                        receipts = indexer.Flush (flushed_request);

                        PostFlushHook (flushed_request, receipts);

                        // Silently return if we get a null back.  This is probably
                        // a bad thing to do.
                        if (receipts == null)
                                return;

                        // Nothing happened (except maybe an optimize, which does not
                        // generate a receipt).  Also do nothing.
                        if (receipts.Length == 0)
                                return;

                        // Update the cached count of items in the driver
                        driver.SetItemCount (indexer.GetItemCount ());

                        // Something happened, so schedule an optimize just in case.
                        ScheduleOptimize ();
                        
                        if (fa_store != null)
                                fa_store.BeginTransaction ();

                        ArrayList added_uris = new ArrayList ();
                        ArrayList removed_uris  = new ArrayList ();

                        for (int i = 0; i < receipts.Length; ++i) {

                                if (receipts [i] is IndexerAddedReceipt) {
                                        
                                        IndexerAddedReceipt r;
                                        r = (IndexerAddedReceipt) receipts [i];

                                        // Add the Uri to the list for our change data
                                        // before doing any post-processing.
                                        // This ensures that we have internal uris when
                                        // we are remapping.
                                        added_uris.Add (r.Uri);
                                        
                                        // Call the appropriate hook
                                        try {
                                                // Map from internal->external Uris in the PostAddHook
                                                PostAddHook (flushed_request.GetByUri (r.Uri), r);
                                        } catch (Exception ex) {
                                                Logger.Log.Warn (ex, "Caught exception in PostAddHook '{0}' '{1}' '{2}'",
                                                                 r.Uri, r.FilterName, r.FilterVersion);
                                        }

                                        // Every added Uri also needs to be listed as removed,
                                        // to avoid duplicate hits in the query.  Since the
                                        // removed Uris need to be external Uris, we add them
                                        // to the list *after* post-processing.
                                        removed_uris.Add (r.Uri);


                                } else if (receipts [i] is IndexerRemovedReceipt) {

                                        IndexerRemovedReceipt r;
                                        r = (IndexerRemovedReceipt) receipts [i];
                                        
                                        // Drop the removed item from the text cache
                                        TextCache.UserCache.Delete (r.Uri);

                                        
                                        // Call the appropriate hook
                                        try {
                                                PostRemoveHook (flushed_request.GetByUri (r.Uri), r);
                                        } catch (Exception ex) {
                                                Logger.Log.Warn (ex, "Caught exception in PostRemoveHook '{0}'",
                                                                 r.Uri);
                                        }

                                        // Add the removed Uri to the list for our
                                        // change data.  This will be an external Uri
                                        // when we are remapping.
                                        removed_uris.Add (r.Uri);
                                        
                                } else if (receipts [i] is IndexerChildIndexablesReceipt) {
                                        
                                        IndexerChildIndexablesReceipt r;
                                        r = (IndexerChildIndexablesReceipt) receipts [i];

                                        foreach (Indexable child in r.Children) {
                                                bool please_add_a_new_task = false;

                                                try {
                                                        please_add_a_new_task = PreChildAddHook (child);
                                                } catch (InvalidOperationException ex) {
                                                        // Queryable does not support adding children
                                                } catch (Exception ex) {
                                                        Logger.Log.Warn (ex, "Caught exception in PreChildAddHook '{0}'", child.DisplayUri);
                                                }

                                                if (please_add_a_new_task) {
                                                        //Logger.Log.Debug ("Adding child {0}", child.Uri);
                                                        Scheduler.Task task = NewAddTask (child);
                                                        task.SubPriority = 1;
                                                        ThisScheduler.Add (task);
                                                } else
                                                        child.Cleanup ();
                                        }
                                }
                        }

                        if (fa_store != null)
                                fa_store.CommitTransaction ();

                        // Propagate the change notification to any open queries.
                        if (added_uris.Count > 0 || removed_uris.Count > 0)
                                driver.QueryableChanged (added_uris, removed_uris);
                }

                //////////////////////////////////////////////////////////////////////////////////

                //
                // It is often convenient to have easy access to a FileAttributeStore
                //

                virtual protected IFileAttributesStore BuildFileAttributesStore ()
                {
                        if (ExtendedAttribute.Supported)
                                return new FileAttributesStore_ExtendedAttribute (IndexFingerprint);
                        else
                                return new FileAttributesStore_Sqlite (IndexDirectory, IndexFingerprint);

                }

                public FileAttributesStore FileAttributesStore {
                        get { 
                                if (fa_store == null)
                                        fa_store = new FileAttributesStore (BuildFileAttributesStore ());
                                return fa_store;
                        }
                }

                //////////////////////////////////////////////////////////////////////////////////

                virtual protected LuceneQueryingDriver BuildLuceneQueryingDriver (string source_name,
                                                                                  int    source_version,
                                                                                  bool   read_only_mode)
                {
                        //return new LuceneQueryingDriver (source_name, source_version, read_only_mode);
                        return LuceneQueryingDriver.Singleton;
                }
        }
}