yacy_search_server/source/de/anomic/crawler/Balancer.java

// plasmaCrawlBalancer.java 
// -----------------------
// part of YaCy
// (C) by Michael Peter Christen; mc@yacy.net
// first published on http://www.anomic.de
// Frankfurt, Germany, 2005
// created: 24.09.2005
//
// 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 de.anomic.crawler;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentHashMap;

import de.anomic.kelondro.index.Row;
import de.anomic.kelondro.index.ObjectIndex;
import de.anomic.kelondro.order.Base64Order;
import de.anomic.kelondro.table.EcoTable;
import de.anomic.kelondro.table.Stack;
import de.anomic.kelondro.util.Log;
import de.anomic.yacy.yacySeedDB;

public class Balancer {
    
    private static final String stackSuffix = "9.stack";
    private static final String indexSuffix = "9.db";
    private static final int EcoFSBufferSize = 200;

    // definition of payload for fileStack
    private static final Row stackrow = new Row("byte[] urlhash-" + yacySeedDB.commonHashLength, Base64Order.enhancedCoder, 0);
    
    // class variables
    private final ConcurrentHashMap<String, LinkedList<String>>
                                     domainStacks;    // a map from domain name part to Lists with url hashs
    private final ArrayList<String>  urlRAMStack;     // a list that is flushed first
    private Stack            urlFileStack;    // a file with url hashes
    private ObjectIndex            urlFileIndex;
    private final File               cacheStacksPath;
    private final String             stackname;
    private boolean                  top;             // to alternate between top and bottom of the file stack
    private final boolean            fullram;
    private long                     minimumLocalDelta;
    private long                     minimumGlobalDelta;
    
    public Balancer(final File cachePath, final String stackname, final boolean fullram,
                    final long minimumLocalDelta, final long minimumGlobalDelta) {
        this.cacheStacksPath = cachePath;
        this.stackname = stackname;
        final File stackFile = new File(cachePath, stackname + stackSuffix);
        this.urlFileStack   = Stack.open(stackFile, stackrow);
        this.domainStacks   = new ConcurrentHashMap<String, LinkedList<String>>();
        this.urlRAMStack    = new ArrayList<String>();
        this.top            = true;
        this.fullram        = fullram;
        this.minimumLocalDelta = minimumLocalDelta;
        this.minimumGlobalDelta = minimumGlobalDelta;
        
        // create a stack for newly entered entries
        if (!(cachePath.exists())) cachePath.mkdir(); // make the path
        openFileIndex();
        if (urlFileStack.size() != urlFileIndex.size() || (urlFileIndex.size() < 10000 && urlFileIndex.size() > 0)) {
            // fix the file stack
            Log.logInfo("Balancer", "re-creating the " + stackname + " balancer stack, size = " + urlFileIndex.size() + ((urlFileStack.size() == urlFileIndex.size()) ? "" : " (the old stack size was wrong)" ));
            urlFileStack.clear();
            try {
                final Iterator<byte[]> i = urlFileIndex.keys(true, null);
                byte[] hash;
                while (i != null && i.hasNext()) {
                    hash = i.next();
                    pushHashToDomainStacks(new String(hash), true);
                }
            } catch (final IOException e) {
                e.printStackTrace();
            }
        }
    }

    public long getMinimumLocalDelta() {
        return this.minimumLocalDelta;
    }
    
    public long getMinimumGlobalDelta() {
        return this.minimumGlobalDelta;
    }
    
    public void setMinimumDelta(final long minimumLocalDelta, final long minimumGlobalDelta) {
        this.minimumLocalDelta = minimumLocalDelta;
        this.minimumGlobalDelta = minimumGlobalDelta;
    }
    
    public synchronized void close() {
        while (domainStacksNotEmpty()) flushOnceDomStacks(true, false, Integer.MAX_VALUE); // flush to ram, because the ram flush is optimized
        size();
        try { flushAllRamStack(); } catch (final IOException e) {}
        if (urlFileIndex != null) {
            urlFileIndex.close();
            urlFileIndex = null;
        }
        if (urlFileStack != null) {
            urlFileStack.close();
            urlFileStack = null;
        }
    }
    
    protected void finalize() {
        if (urlFileStack != null) {
            Log.logWarning("Balancer", "crawl stack " + stackname + " closed by finalizer");
            close();
        }
    }
    
    public synchronized void clear() {
        urlFileStack.clear();
        domainStacks.clear();
        urlRAMStack.clear();
        resetFileIndex();
    }
    
    private void openFileIndex() {
        cacheStacksPath.mkdirs();
        urlFileIndex = new EcoTable(new File(cacheStacksPath, stackname + indexSuffix), CrawlEntry.rowdef, (fullram) ? EcoTable.tailCacheUsageAuto : EcoTable.tailCacheDenyUsage, EcoFSBufferSize, 0);
    }
    
    private void resetFileIndex() {
        if (urlFileIndex != null) {
            urlFileIndex.close();
            urlFileIndex = null;
            new File(cacheStacksPath, stackname + indexSuffix).delete();
        }
        openFileIndex();
    }
    
    public synchronized CrawlEntry get(final String urlhash) throws IOException {
        assert urlhash != null;
        if (urlFileIndex == null) return null; // case occurs during shutdown
        final Row.Entry entry = urlFileIndex.get(urlhash.getBytes());
        if (entry == null) return null;
        return new CrawlEntry(entry);
    }
    
    public synchronized int removeAllByProfileHandle(final String profileHandle, final long timeout) throws IOException {
        // removes all entries with a specific profile hash.
        // this may last some time
        // returns number of deletions
        
        // first find a list of url hashes that shall be deleted
        final Iterator<Row.Entry> i = urlFileIndex.rows();
        final HashSet<String> urlHashes = new HashSet<String>();
        Row.Entry rowEntry;
        CrawlEntry crawlEntry;
        final long terminate = (timeout > 0) ? System.currentTimeMillis() + timeout : Long.MAX_VALUE;
        while (i.hasNext() && (System.currentTimeMillis() < terminate)) {
            rowEntry = i.next();
            crawlEntry = new CrawlEntry(rowEntry);
            if (crawlEntry.profileHandle().equals(profileHandle)) {
                urlHashes.add(crawlEntry.url().hash());
            }
        }
        
        // then delete all these urls from the queues and the file index
        return this.remove(urlHashes);
    }
    
    /**
     * this method is only here, because so many import/export methods need it
       and it was implemented in the previous architecture
       however, usage is not recommended
     * @param urlHashes, a list of hashes that shall be removed
     * @return number of entries that had been removed
     * @throws IOException
     */
    public synchronized int remove(final HashSet<String> urlHashes) throws IOException {
        final int s = urlFileIndex.size();
        int removedCounter = 0;
        for (final String urlhash: urlHashes) {
            final Row.Entry entry = urlFileIndex.remove(urlhash.getBytes());
            if (entry != null) removedCounter++;
        }
        if (removedCounter == 0) return 0;
        assert urlFileIndex.size() + removedCounter == s : "urlFileIndex.size() = " + urlFileIndex.size() + ", s = " + s;
       
        // now delete these hashes also from the queues

        // iterate through the RAM stack
        Iterator<String> i = urlRAMStack.iterator();
        String h;
        while (i.hasNext()) {
           h = i.next();
           if (urlHashes.contains(h)) i.remove();
       }
       
       // iterate through the file stack
       // in general this is a bad idea. But this can only be avoided by avoidance of this method
       final Iterator<Row.Entry> j = urlFileStack.stackIterator(true);
       while (j.hasNext()) {
           h = new String(j.next().getColBytes(0));
           if (urlHashes.contains(h)) j.remove();
       }
       
       // iterate through the domain stacks
       final Iterator<Map.Entry<String, LinkedList<String>>> k = domainStacks.entrySet().iterator();
       Map.Entry<String, LinkedList<String>> se;
       LinkedList<String> stack;
       while (k.hasNext()) {
           se = k.next();
           stack = se.getValue();
           i = stack.iterator();
           while (i.hasNext()) {
               if (urlHashes.contains(i.next())) i.remove();
           }
           if (stack.size() == 0) k.remove();
       }
       
       return removedCounter;
    }
    
    public synchronized boolean has(final String urlhash) {
        return urlFileIndex.has(urlhash.getBytes());
    }
    
    public synchronized boolean notEmpty() {
        // alternative method to the property size() > 0
        // this is better because it may avoid synchronized access to domain stack summarization
        return urlRAMStack.size() > 0 || urlFileStack.size() > 0 || domainStacksNotEmpty();
    }
    
    public synchronized int size() {
        final int componentsize = urlFileIndex.size();
        /*
        assert componentsize == urlFileStack.size() + urlRAMStack.size() + sizeDomainStacks() :
		        "size wrong in " + stackname +
		        " - urlFileIndex = " + urlFileIndex.size() +
		        ", componentsize = " + urlFileStack.size() + urlRAMStack.size() + sizeDomainStacks() +
                " = (urlFileStack = " + urlFileStack.size() +
		        ", urlRAMStack = " + urlRAMStack.size() +
		        ", sizeDomainStacks = " + sizeDomainStacks() + ")";
		*/
        return componentsize;
    }
    
    private boolean domainStacksNotEmpty() {
        if (domainStacks == null) return false;
        synchronized (domainStacks) {
            final Iterator<LinkedList<String>> i = domainStacks.values().iterator();
            while (i.hasNext()) {
                if (i.next().size() > 0) return true;
            }
        }
        return false;
    }
    
    private int sizeDomainStacks() {
        if (domainStacks == null) return 0;
        int sum = 0;
        //synchronized (domainStacks) {
            final Iterator<LinkedList<String>> i = domainStacks.values().iterator();
            while (i.hasNext()) sum += i.next().size();
        //}
        return sum;
    }
    
    /**
     * removes the head element of all domain stacks and moves the element in either the ram stack or the file stack
     * @param minimumleft
     * @param ram
     * @param onlyReadyForAccess
     */
    private void flushOnceDomStacks(final boolean ram, final boolean onlyReadyForAccess, int max) {
        // takes one entry from every domain stack and puts it on the ram or file stack
        // the minimumleft value is a limit for the number of entries that should be left
        if (domainStacks.size() == 0) return;
        synchronized (domainStacks) {
            final Iterator<Map.Entry<String, LinkedList<String>>> i = domainStacks.entrySet().iterator();
            Map.Entry<String, LinkedList<String>> entry;
            LinkedList<String> list;
            int c = 0;
            while (i.hasNext() && c < max) {
                entry = i.next();
                list = entry.getValue();
                if (onlyReadyForAccess && CrawlEntry.waitingRemainingGuessed(list.getFirst(), minimumLocalDelta, minimumGlobalDelta) > 0) continue;
                if (ram) {
                    urlRAMStack.add(list.removeFirst());
                } else try {
                    urlFileStack.push(urlFileStack.row().newEntry(new byte[][] { (list.removeFirst()).getBytes() }));
                } catch (final IOException e) {
                    e.printStackTrace();
                }
                if (list.size() == 0)  i.remove();
                c++;
            }
        }
    }
    
    private void flushAllRamStack() throws IOException {
        // this flushes only the ramStack to the fileStack, but does not flush the domainStacks
        for (int i = 0; i < urlRAMStack.size() / 2; i++) {
            urlFileStack.push(urlFileStack.row().newEntry(new byte[][]{(urlRAMStack.get(i)).getBytes()}));
            urlFileStack.push(urlFileStack.row().newEntry(new byte[][]{(urlRAMStack.get(urlRAMStack.size() - i - 1)).getBytes()}));
        }
        if (urlRAMStack.size() % 2 != 0) 
            urlFileStack.push(urlFileStack.row().newEntry(new byte[][]{(urlRAMStack.get(urlRAMStack.size() / 2)).getBytes()}));
    }
    
    private void shiftFileToDomStacks(final int wantedsize) {
        int count = sizeDomainStacks() - wantedsize;
        while ((urlFileStack != null) && (count > 0) && (urlFileStack.size() > 0)) {
            // flush some entries from disc to ram stack
            try {
                // one from the top:
                Row.Entry t = urlFileStack.pop();
                if (t == null) break;
                pushHashToDomainStacks(new String(t.getColBytes(0)), false);
                count--;
                if (urlFileStack.size() == 0) break;
                // one from the bottom:
                t = urlFileStack.pot();
                if (t == null) break;
                pushHashToDomainStacks(new String(t.getColBytes(0)), false);
                count--;
            } catch (final IOException e) {
                break;
            }
        }
    }

    private void shiftFileToRAM(final int wantedsize) {
        while ((urlFileStack != null) && (urlRAMStack.size() <= wantedsize) && (urlFileStack.size() > 0)) {
            // flush some entries from disc to ram stack
            try {
                // one from the top:
                Row.Entry t = urlFileStack.pop();
                if (t == null) break;
                urlRAMStack.add(new String(t.getColBytes(0)));
                if (urlFileStack.size() == 0) break;
                // one from the bottom:
                t = urlFileStack.pot();
                if (t == null) break;
                urlRAMStack.add(new String(t.getColBytes(0)));
            } catch (final IOException e) {
                break;
            }
        }
    }
    
    public synchronized void push(final CrawlEntry entry) throws IOException {
        assert entry != null;
        if (urlFileIndex.has(entry.url().hash().getBytes())) {
            //serverLog.logWarning("BALANCER", "double-check has failed for urlhash " + entry.url().hash()  + " in " + stackname + " - fixed");
            return;
        }
        
        // add to index
        urlFileIndex.put(entry.toRow());
        
        // add the hash to a queue
        pushHashToDomainStacks(entry.url().hash(), true);
    }
    
    private void pushHashToDomainStacks(final String hash, boolean flush) {
        // extend domain stack
        final String dom = hash.substring(6);
        LinkedList<String> domainList = domainStacks.get(dom);
        if (domainList == null) {
            // create new list
            domainList = new LinkedList<String>();
            synchronized (domainStacks) {
                domainList.add(hash);
                domainStacks.put(dom, domainList);
            }
        } else {
            // extend existent domain list
            domainList.addLast(hash);
        }
        
        // check size of domainStacks and flush
        if (flush && (domainStacks.size() > 100) || (sizeDomainStacks() > 1000)) {
            flushOnceDomStacks(urlRAMStack.size() < 100, true, 100); // when the ram stack is small, flush it there
        }
    }
    
    /**
     * get the next entry in this crawl queue in such a way that the domain access time delta is maximized
     * and always above the given minimum delay time. An additional delay time is computed using the robots.txt
     * crawl-delay time which is always respected. In case the minimum time cannot ensured, this method pauses
     * the necessary time until the url is released and returned as CrawlEntry object. In case that a profile
     * for the computed Entry does not exist, null is returned
     * @param delay
     * @param profile
     * @return a url in a CrawlEntry object
     * @throws IOException
     */
    public synchronized CrawlEntry pop(boolean delay, CrawlProfile profile) throws IOException {
        // returns a crawl entry from the stack and ensures minimum delta times
        // we have 3 sources to choose from: the ramStack, the domainStacks and the fileStack
        
        String result = null; // the result
        
        // 1st: check ramStack
        if (urlRAMStack.size() > 0) {
            //result = urlRAMStack.remove(0);
            Iterator<String> i = urlRAMStack.iterator();
            String urlhash;
            long waitingtime, min = Long.MAX_VALUE;
            String besthash = null;
            while (i.hasNext()) {
                urlhash = i.next();
                waitingtime = CrawlEntry.waitingRemainingGuessed(urlhash, minimumLocalDelta, minimumGlobalDelta);
                if (waitingtime == 0) {
                    // zero waiting is a good one
                    result = urlhash;
                    i.remove();
                    min = Long.MAX_VALUE; // that causes that the if at the end of this loop is not used
                    besthash = null;
                    break;
                }
                if (waitingtime < min) {
                    min = waitingtime;
                    besthash = urlhash;
                }
            }
            if (min <= 500 && besthash != null) {
                // find that entry that was best end remove it
                i = urlRAMStack.iterator();
                while (i.hasNext()) {
                    urlhash = i.next();
                    if (urlhash.equals(besthash)) {
                        // zero waiting is a good one
                        result = urlhash;
                        i.remove();
                        break;
                    }
                }
            }
        }
        
        // the next options use the domain stack. If this is not filled enough, they dont work at all
        // so just fill them up with some stuff
        if (result == null) shiftFileToDomStacks(1000);
        
        // 2nd-b: check domainStacks for best match between stack size and retrieval time
        String maxhash = null;
        if ((result == null) && (domainStacks.size() > 0)) synchronized (domainStacks) {
            // we order all domains by the number of entries per domain
            // then we iterate through these domains in descending entry order
            // and take that one, that has a zero waiting time
            final Iterator<Map.Entry<String, LinkedList<String>>> i = domainStacks.entrySet().iterator();
            Map.Entry<String, LinkedList<String>> entry;
            String domhash;
            LinkedList<String> domlist;
            final TreeMap<Integer, String> hitlist = new TreeMap<Integer, String>();
            int count = 0;
            // first collect information about sizes of the domain lists
            while (i.hasNext()) {
                entry = i.next();
                domhash = entry.getKey();
                domlist = entry.getValue();
                hitlist.put(Integer.valueOf(domlist.size() * 100 + count++), domhash);
            }
            
            // now iterate in descending order and fetch that one,
            // that is acceptable by the minimumDelta constraint
            long waitingtime;
            while (hitlist.size() > 0) {
                domhash = hitlist.remove(hitlist.lastKey());
                if (maxhash == null) maxhash = domhash; // remember first entry
                waitingtime = CrawlEntry.waitingRemainingGuessed(domhash, minimumLocalDelta, minimumGlobalDelta);
                if (waitingtime < 100) {
                    domlist = domainStacks.get(domhash);
                    result = domlist.removeFirst();
                    if (domlist.size() == 0) domainStacks.remove(domhash);
                    break;
                }
            }
            
        }
        
        // 2nd-a: check domainStacks for latest arrivals
        if ((result == null) && (domainStacks.size() > 0)) synchronized (domainStacks) {
            // we select specific domains that have not been used for a long time
            // Latest arrivals that have not yet been crawled fit also in that scheme
            final Iterator<Map.Entry<String, LinkedList<String>>> i = domainStacks.entrySet().iterator();
            Map.Entry<String, LinkedList<String>> entry;
            String domhash;
            long waitingtime, min = Long.MAX_VALUE;
            String besthash = null;
            LinkedList<String> domlist;
            while (i.hasNext()) {
                entry = i.next();
                domhash = entry.getKey();
                waitingtime = CrawlEntry.waitingRemainingGuessed(domhash, minimumLocalDelta, minimumGlobalDelta);
                if (waitingtime == 0) {
                    // zero waiting is a good one
                    domlist = entry.getValue();
                    result = domlist.removeFirst();
                    if (domlist.size() == 0) i.remove();
                    min = Long.MAX_VALUE; // that causes that the if at the end of this loop is not used
                    besthash = null;
                    break;
                }
                if (waitingtime < min) {
                    min = waitingtime;
                    besthash = domhash;
                }
            }
            if (min <= 500 && besthash != null) {
                domlist = domainStacks.get(besthash);
                result = domlist.removeFirst();
                if (domlist.size() == 0) domainStacks.remove(besthash);
            }
        }
        
        // 2nd-c: if we did yet not choose any entry, we simply take that one with the most entries
        if ((result == null) && (maxhash != null)) {
            LinkedList<String> domlist = domainStacks.get(maxhash);
            if (domlist != null) {
                result = domlist.removeFirst();
                if (domlist.size() == 0) domainStacks.remove(maxhash);
            }
        }
        
        // 3rd: take entry from file
        if ((result == null) && (urlFileStack.size() > 0)) {
            final Row.Entry nextentry = (top) ? urlFileStack.top() : urlFileStack.bot();
            if (nextentry == null) {
                // emergency case: this means that something with the stack organization is wrong
                // the file appears to be broken. We kill the file.
                urlFileStack.clear();
                Log.logSevere("BALANCER", "get() failed to fetch entry from file stack. reset stack file.");
            } else {
                final String nexthash = new String(nextentry.getColBytes(0));

                // check if the time after retrieval of last hash from same
                // domain is not shorter than the minimumDelta
                long waitingtime = CrawlEntry.waitingRemainingGuessed(nexthash, minimumLocalDelta, minimumGlobalDelta);
                if (waitingtime == 0) {
                    // the entry is fine
                    result = new String((top) ? urlFileStack.pop().getColBytes(0) : urlFileStack.pot().getColBytes(0));
                } else {
                    // try other entry
                    result = new String((top) ? urlFileStack.pot().getColBytes(0) : urlFileStack.pop().getColBytes(0));
                }
            }
            top = !top; // alternate top/bottom
        }
        
        // check case where we did not found anything
        if (result == null) {
            Log.logSevere("BALANCER", "get() was not able to find a valid urlhash - total size = " + size() + ", fileStack.size() = " + urlFileStack.size() + ", ramStack.size() = " + urlRAMStack.size() + ", domainStacks.size() = " + domainStacks.size());
            return null;
        }
        
        // finally: check minimumDelta and if necessary force a sleep
        final int s = urlFileIndex.size();
        Row.Entry rowEntry = urlFileIndex.remove(result.getBytes());
        if (rowEntry == null) {
            throw new IOException("get() found a valid urlhash, but failed to fetch the corresponding url entry - total size = " + size() + ", fileStack.size() = " + urlFileStack.size() + ", ramStack.size() = " + urlRAMStack.size() + ", domainStacks.size() = " + domainStacks.size());
        }
        assert urlFileIndex.size() + 1 == s : "urlFileIndex.size() = " + urlFileIndex.size() + ", s = " + s + ", result = " + result;
        final CrawlEntry crawlEntry = new CrawlEntry(rowEntry);
        // at this point we must check if the crawlEntry has relevancy because the crawl profile still exists
        // if not: return null. A calling method must handle the null value and try again
        if (profile != null && !profile.hasEntry(crawlEntry.profileHandle())) return null;
        long sleeptime = crawlEntry.waitingRemaining(minimumLocalDelta, minimumGlobalDelta); // this uses the robots.txt database and may cause a loading of robots.txt from the server
        
        if (delay && sleeptime > 0) {
            // force a busy waiting here
            // in best case, this should never happen if the balancer works propertly
            // this is only to protection against the worst case, where the crawler could
            // behave in a DoS-manner
            Log.logInfo("BALANCER", "forcing crawl-delay of " + sleeptime + " milliseconds for " + crawlEntry.url().getHost() + ((sleeptime > Math.max(minimumLocalDelta, minimumGlobalDelta)) ? " (caused by robots.txt)" : ""));
            try {synchronized(this) { this.wait(sleeptime); }} catch (final InterruptedException e) {}
        }
        
        // update statistical data
        crawlEntry.updateAccess();
        
        return crawlEntry;
    }

    /**
     * return top-elements from the crawl stack
     * we do not produce here more entries than exist on the stack
     * because otherwise the balancing does not work properly
     * @param count
     * @return
     * @throws IOException
     */
    public synchronized ArrayList<CrawlEntry> top(int count) throws IOException {
        // if we need to flush anything, then flush the domain stack first,
        // to avoid that new urls get hidden by old entries from the file stack
        if (urlRAMStack == null) return null;

        // ensure that the domain stacks are filled enough
        shiftFileToDomStacks(count);
        
        // flush from the domain stacks first until they are empty
        if ((domainStacksNotEmpty()) && (urlRAMStack.size() <= count)) {
            flushOnceDomStacks(true, true, 100);
        }
        while ((domainStacksNotEmpty()) && (urlRAMStack.size() <= count)) {
            // flush only that much as we need to display
            flushOnceDomStacks(true, false, 100);
        }
        
        // if the ram is still not full enough, use the file stack
        shiftFileToRAM(count);
        
        // finally, construct a list using the urlRAMStack which was filled with this procedure
        count = Math.min(count, urlRAMStack.size());
        final ArrayList<CrawlEntry> list = new ArrayList<CrawlEntry>();
        for (int i = 0; i < count; i++) {
            final String urlhash = urlRAMStack.get(i);
            final Row.Entry entry = urlFileIndex.get(urlhash.getBytes());
            if (entry == null) break;
            list.add(new CrawlEntry(entry));
        }
        return list;
    }
    
    public synchronized Iterator<CrawlEntry> iterator() throws IOException {
        return new EntryIterator();
    }
    
    private class EntryIterator implements Iterator<CrawlEntry> {

        private Iterator<Row.Entry> rowIterator;
        
        public EntryIterator() throws IOException {
            rowIterator = urlFileIndex.rows();
        }
        
        public boolean hasNext() {
            return (rowIterator == null) ? false : rowIterator.hasNext();
        }

        public CrawlEntry next() {
            final Row.Entry entry = rowIterator.next();
            try {
                return (entry == null) ? null : new CrawlEntry(entry);
            } catch (final IOException e) {
                rowIterator = null;
                return null;
            }
        }

        public void remove() {
            if (rowIterator != null) rowIterator.remove();
        }
        
    }
    
}