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About Brian Du Preez

Brian Du Preez

Mini Search Engine – Just the basics, using Neo4j, Crawler4j, Graphstream and Encog

Continuing to chapter 4 of Programming Collection Intelligence  (PCI) which is implementing a search engine.

I may have bitten off a little more than I should of in 1 exercise. Instead of using the normal relational database construct as used in the book, I figured, I always wanted to have a look at Neo4J so now was the time. Just to say, this isn’t necessarily the ideal use case for a graph db, but how hard could to be to kill 3 birds with 1 stone.

Working through the tutorials trying to reset my SQL Server, Oracle mindset took a little longer than expected, but thankfully there are some great resources around Neo4j.

Just a couple:

Since I just wanted to run this as a little exercise, I decided to go for a in memory implementation and not run it as a service on my machine. In hindsight this was probably a mistake and the tools and web interface would have helped me visualise my data graph quicker in the beginning.

As you can only have 1 writable instance of the in memory implementation, I made a little double lock singleton factory to create and clear the DB.

package net.briandupreez.pci.chapter4;

import org.neo4j.graphdb.GraphDatabaseService;
import org.neo4j.graphdb.factory.GraphDatabaseFactory;
import org.neo4j.kernel.impl.util.FileUtils;

import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class CreateDBFactory {

    private static GraphDatabaseService graphDb = null;
    public static final String RESOURCES_CRAWL_DB = "resources/crawl/db";

    public static GraphDatabaseService createInMemoryDB() {
        if (null == graphDb) {
            synchronized (GraphDatabaseService.class) {
                if (null == graphDb) {
                    final Map<String, String> config = new HashMap<>();
                    config.put("neostore.nodestore.db.mapped_memory", "50M");
                    config.put("string_block_size", "60");
                    config.put("array_block_size", "300");
                    graphDb = new GraphDatabaseFactory()

        return graphDb;

    private static void registerShutdownHook(final GraphDatabaseService graphDb) {
        Runtime.getRuntime().addShutdownHook(new Thread() {
            public void run() {

    public static void clearDb() {
        try {
            if(graphDb != null){
                graphDb = null;
            FileUtils.deleteRecursively(new File(RESOURCES_CRAWL_DB));
        } catch (final IOException e) {
            throw new RuntimeException(e);


Then using Crawler4j created a graph of all the URLs starting with my blog, their relationships to other URLs and all the words and indexes of the words that those URLs contain.

package net.briandupreez.pci.chapter4;

import edu.uci.ics.crawler4j.crawler.Page;
import edu.uci.ics.crawler4j.crawler.WebCrawler;
import edu.uci.ics.crawler4j.parser.HtmlParseData;
import edu.uci.ics.crawler4j.url.WebURL;
import org.neo4j.graphdb.GraphDatabaseService;
import org.neo4j.graphdb.Node;
import org.neo4j.graphdb.Relationship;
import org.neo4j.graphdb.Transaction;
import org.neo4j.graphdb.index.Index;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Neo4JWebCrawler extends WebCrawler {

    private final GraphDatabaseService graphDb;

     * Constructor.
    public Neo4JWebCrawler() {
        this.graphDb = CreateDBFactory.createInMemoryDB();

    public boolean shouldVisit(final WebURL url) {
        final String href = url.getURL().toLowerCase();
        return !NodeConstants.FILTERS.matcher(href).matches();

     * This function is called when a page is fetched and ready
     * to be processed by your program.
    public void visit(final Page page) {

        final String url = page.getWebURL().getURL();
        System.out.println("URL: " + url);

        final Index<Node> nodeIndex = graphDb.index().forNodes(NodeConstants.PAGE_INDEX);

        if (page.getParseData() instanceof HtmlParseData) {
            HtmlParseData htmlParseData = (HtmlParseData) page.getParseData();
            String text = htmlParseData.getText();
            //String html = htmlParseData.getHtml();
            List<WebURL> links = htmlParseData.getOutgoingUrls();
            Transaction tx = graphDb.beginTx();
            try {

                final Node pageNode = graphDb.createNode();
                pageNode.setProperty(NodeConstants.URL, url);
                nodeIndex.add(pageNode, NodeConstants.URL, url);

                //get all the words
                final List<String> words = cleanAndSplitString(text);
                int index = 0;
                for (final String word : words) {
                    final Node wordNode = graphDb.createNode();
                    wordNode.setProperty(NodeConstants.WORD, word);
                    wordNode.setProperty(NodeConstants.INDEX, index++);
                    final Relationship relationship = pageNode.createRelationshipTo(wordNode, RelationshipTypes.CONTAINS);
                    relationship.setProperty(NodeConstants.SOURCE, url);

                for (final WebURL webURL : links) {
                    System.out.println("Linking to " + webURL);
                    final Node linkNode = graphDb.createNode();
                    linkNode.setProperty(NodeConstants.URL, webURL.getURL());
                    final Relationship relationship = pageNode.createRelationshipTo(linkNode, RelationshipTypes.LINK_TO);
                    relationship.setProperty(NodeConstants.SOURCE, url);
                    relationship.setProperty(NodeConstants.DESTINATION, webURL.getURL());

            } finally {


    private static List<String> cleanAndSplitString(final String input) {
        if (input != null) {
            final String[] dic = input.toLowerCase().replaceAll("\\p{Punct}", "").replaceAll("\\p{Digit}", "").split("\\s+");
            return Arrays.asList(dic);
        return new ArrayList<>();


After the data was collected, I could query it and perform the functions of a search engine. For this I decided to use java futures as it was another thing I had only read about and not yet implemented. In my day to day working environment we use Weblogic  / CommonJ work managers within the application server to perform the same task.

  final ExecutorService executorService = Executors.newFixedThreadPool(4);
  final String[] searchTerms = {"java", "spring"};

  List<Callable<TaskResponse>> tasks = new ArrayList<>();
  tasks.add(new WordFrequencyTask(searchTerms));
  tasks.add(new DocumentLocationTask(searchTerms));
  tasks.add(new PageRankTask(searchTerms));
  tasks.add(new NeuralNetworkTask(searchTerms));

  final List<Future<TaskResponse>> results = executorService.invokeAll(tasks);

I then went about creating a task for each of the following counting the word frequency, document location, Page Rank and neural network (with fake input / training data) to rank the pages returned based on the search criteria. All the code is in my public github blog repo.

Disclaimer: The Neural Network task, either didn’t have enough data to be affective, or I implemented the data normalisation incorrectly, so it is currently not very useful, I’ll return to it once I have completed the journey through the while PCI book.

The one task worth sharing was the Page Rank one, I quickly read some of the theory for it, decided I am not that clever and went searching for a library that had it implemented. I discovered Graphstream a wonderful opensource project that does a WHOLE lot more than just PageRank, check out their video.

From that it was then simple to implement my PageRank task of this exercise.

package net.briandupreez.pci.chapter4.tasks;

import net.briandupreez.pci.chapter4.NodeConstants;
import net.briandupreez.pci.chapter4.NormalizationFunctions;
import org.graphstream.algorithm.PageRank;
import org.graphstream.graph.Graph;
import org.graphstream.graph.implementations.SingleGraph;
import org.neo4j.cypher.javacompat.ExecutionEngine;
import org.neo4j.cypher.javacompat.ExecutionResult;
import org.neo4j.graphdb.Node;
import org.neo4j.graphdb.Relationship;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.Callable;

public class PageRankTask extends SearchTask implements Callable<TaskResponse> {

    public PageRankTask(final String... terms) {

    protected ExecutionResult executeQuery(final String... words) {
        final ExecutionEngine engine = new ExecutionEngine(graphDb);
        final StringBuilder bob = new StringBuilder("START page=node(*) MATCH (page)-[:CONTAINS]->words ");
        bob.append(", (page)-[:LINK_TO]->related ");
        bob.append("WHERE words.word in [");
        bob.append("] ");
        bob.append("RETURN DISTINCT page, related");

        return engine.execute(bob.toString());

    public TaskResponse call() {
        final ExecutionResult result = executeQuery(searchTerms);
        final Map<String, Double> returnMap = convertToUrlTotalWords(result);

        final TaskResponse response = new TaskResponse();
        response.taskClazz = this.getClass();
        response.resultMap = NormalizationFunctions.normalizeMap(returnMap, true);
        return response;

    private Map<String, Double> convertToUrlTotalWords(final ExecutionResult result) {
        final Map<String, Double> uniqueUrls = new HashMap<>();

        final Graph g = new SingleGraph("rank", false, true);
        final Iterator<Node> pageIterator = result.columnAs("related");

        while (pageIterator.hasNext()) {
            final Node node = pageIterator.next();
            final Iterator<Relationship> relationshipIterator = node.getRelationships().iterator();
            while (relationshipIterator.hasNext()) {

                final Relationship relationship = relationshipIterator.next();
                final String source = relationship.getProperty(NodeConstants.SOURCE).toString();
                uniqueUrls.put(source, 0.0);
                final String destination = relationship.getProperty(NodeConstants.DESTINATION).toString();
                g.addEdge(String.valueOf(node.getId()), source, destination, true);


        computeAndSetPageRankScores(uniqueUrls, g);
        return uniqueUrls;

     * Compute score
     * @param uniqueUrls urls
     * @param graph      the graph of all links
    private void computeAndSetPageRankScores(final Map<String, Double> uniqueUrls, final Graph graph) {
        final PageRank pr = new PageRank();

        for (final Map.Entry<String, Double> entry : uniqueUrls.entrySet()) {
            final double score = 100 * pr.getRank(graph.getNode(entry.getKey()));


In between all of this I found a great implementation of sorting a map by values on Stackoverflow.

package net.briandupreez.pci.chapter4;

import java.util.*;

public class MapUtil {

     * Sort a map based on values.
     * The values must be Comparable.
     * @param map       the map to be sorted
     * @param ascending in ascending order, or descending if false
     * @param <K>       key generic
     * @param <V>       value generic
     * @return sorted list
    public static <K, V extends Comparable<? super V>> List<Map.Entry<K, V>> entriesSortedByValues(final Map<K, V> map, final boolean ascending) {

        final List<Map.Entry<K, V>> sortedEntries = new ArrayList<>(map.entrySet());
                new Comparator<Map.Entry<K, V>>() {
                    public int compare(final Map.Entry<K, V> e1, final Map.Entry<K, V> e2) {
                        if (ascending) {
                            return e1.getValue().compareTo(e2.getValue());
                        } else {
                            return e2.getValue().compareTo(e1.getValue());

        return sortedEntries;


The Maven dependencies used to implement all of this


Now to chapter 5 on PCI… Optimisation.

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2 Comments on "Mini Search Engine – Just the basics, using Neo4j, Crawler4j, Graphstream and Encog"

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Arun Shankar

It’s very nice


Is it free to use NEO4j