GH-5291: Make it possible to disable transactions in LuceneSail

core/sail/lucene-api/src/main/java/org/eclipse/rdf4j/sail/lucene/LuceneSail.java

@@ -289,6 +289,36 @@ public class LuceneSail extends NotifyingSailWrapper {
 	 */
 	public static final String LUCENE_RAMDIR_KEY = "useramdir";
 
+	/**
+	 * Whether the LuceneIndex should support transactions and rollbacks (true by default). Set the key
+	 * "transactional=false" as sail parameter to DISABLE rollback support. This will improve performance if you are
+	 * doing a lot of inserts/updates and don't need rollback support. It will also run fsync only at an interval (see
+	 * {@link #FSYNC_INTERVAL_KEY}), instead of after each transaction commit. Changes in the index will be only visible
+	 * to readers after the periodic fsync is done.
+	 * <p>
+	 * See <a href="https://github.com/eclipse-rdf4j/rdf4j/issues/5291">the original issue</a>.
+	 */
+	public static final String TRANSACTIONAL_KEY = "transactional";
+
+	/**
+	 * Default value for {@link #TRANSACTIONAL_KEY}, true (rollbacks enabled, fsync after each commit).
+	 */
+	public static final boolean DEFAULT_TRANSACTIONAL = true;
+
+	/**
+	 * Set the key "fsyncInterval=&lt;t&gt;" as sail parameter to configure the interval in milliseconds in which fsync
+	 * is called on the Lucene index, default is defined in {@link #DEFAULT_FSYNC_INTERVAL}. Changes in the index will
+	 * become visible to readers at most after the interval is elapsed. Only used when {@link #TRANSACTIONAL_KEY} is set
+	 * to false (rollbacks disabled).
+	 */
+	public static final String FSYNC_INTERVAL_KEY = "fsyncInterval";
+
+	/**
+	 * Default fsync interval in milliseconds, used when {@link #FSYNC_INTERVAL_KEY} is not set and
+	 * {@link #TRANSACTIONAL_KEY} is set to false (rollbacks disabled).
+	 */
+	public static final long DEFAULT_FSYNC_INTERVAL = 10_000L;
+
 	/**
 	 * Set the key "defaultNumDocs=&lt;n&gt;" as sail parameter to limit the maximum number of documents to return from
 	 * a search query. The default is to return all documents. NB: this may involve extra cost for some SearchIndex

core/sail/lucene/src/main/java/org/eclipse/rdf4j/sail/lucene/impl/LuceneIndex.java

@@ -25,9 +25,11 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.Properties;
 import java.util.Set;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Supplier;
 
@@ -150,6 +152,12 @@ public class LuceneIndex extends AbstractLuceneIndex {
 
 	private volatile int fuzzyPrefixLength;
 
+	private boolean transactionsEnabled = LuceneSail.DEFAULT_TRANSACTIONAL;
+
+	private long fsyncIntervalMillis = LuceneSail.DEFAULT_FSYNC_INTERVAL;
+
+	private ScheduledThreadPoolExecutor fsyncScheduler;
+
 	/**
 	 * The IndexWriter that can be used to alter the index' contents. Created lazily.
 	 */
@@ -213,6 +221,16 @@ public synchronized void initialize(Properties parameters) throws Exception {
 			this.fuzzyPrefixLength = NumberUtils.toInt(parameters.getProperty(FUZZY_PREFIX_LENGTH_KEY), 0);
 		}
 
+		if (parameters.containsKey(LuceneSail.TRANSACTIONAL_KEY)) {
+			this.transactionsEnabled = Boolean.parseBoolean(
+					parameters.getProperty(LuceneSail.TRANSACTIONAL_KEY));
+		}
+		if (parameters.containsKey(LuceneSail.FSYNC_INTERVAL_KEY)) {
+			this.fsyncIntervalMillis = NumberUtils.toLong(
+					parameters.getProperty(LuceneSail.FSYNC_INTERVAL_KEY),
+					LuceneSail.DEFAULT_FSYNC_INTERVAL);
+		}
+
 		postInit();
 	}
 
@@ -270,6 +288,43 @@ private void postInit() throws IOException {
 			IndexWriter writer = new IndexWriter(directory, indexWriterConfig);
 			writer.close();
 		}
+
+		setUpFsyncScheduler();
+	}
+
+	private void setUpFsyncScheduler() {
+		// If transactions are disabled, launch a background thread to fsync periodically.
+		if (this.transactionsEnabled || this.fsyncIntervalMillis <= 0) {
+			return;
+		}
+
+		// Use a daemon thread so the scheduler does not prevent JVM shutdown.
+		this.fsyncScheduler = new ScheduledThreadPoolExecutor(1, r -> {
+			Thread t = Executors.defaultThreadFactory().newThread(r);
+			t.setDaemon(true);
+			t.setName("rdf4j-lucene-fsync-" + t.getId());
+			return t;
+		});
+		// Help GC by removing cancelled tasks from the queue.
+		this.fsyncScheduler.setRemoveOnCancelPolicy(true);
+		this.fsyncScheduler.scheduleAtFixedRate(
+				() -> {
+					try {
+						if (this.getIndexWriter().hasUncommittedChanges()) {
+							this.getIndexWriter().commit();
+							invalidateReaders();
+						}
+					} catch (Throwable e) {
+						// We just log errors here, there's not much else we can do.
+						// Rethrowing exceptions on the next commit would be confusing, especially if
+						// the error is transient.
+						logger.error("Exception during Lucene index fsync", e);
+					}
+				},
+				this.fsyncIntervalMillis,
+				this.fsyncIntervalMillis,
+				TimeUnit.MILLISECONDS
+		);
 	}
 
 	protected Function<String, ? extends SpatialStrategy> createSpatialStrategyMapper(Map<String, String> parameters) {
@@ -384,15 +439,42 @@ public void shutDown() throws IOException {
 						oldmonitors.clear();
 					}
 				} finally {
+					// shutdown fsync scheduler
 					try {
-						IndexWriter toCloseIndexWriter = indexWriter;
-						indexWriter = null;
-						if (toCloseIndexWriter != null) {
-							toCloseIndexWriter.close();
+						if (fsyncScheduler != null) {
+							fsyncScheduler.shutdown();
+							if (!fsyncScheduler.awaitTermination(10, TimeUnit.SECONDS)) {
+								logger.error("Failed to shut down Lucene fsync scheduler within 10s");
+							}
 						}
+					} catch (InterruptedException e) {
+						logger.error("Interrupted while trying to shut down Lucene fsync scheduler", e);
+						Thread.currentThread().interrupt();
+					} catch (Throwable e) {
+						exceptions.add(e);
 					} finally {
-						if (!exceptions.isEmpty()) {
-							throw new UndeclaredThrowableException(exceptions.get(0));
+						// Do a final sync of any uncommitted changes
+						IndexWriter toCloseIndexWriter = indexWriter;
+						try {
+							if (!this.transactionsEnabled && toCloseIndexWriter != null && toCloseIndexWriter.isOpen()
+									&& toCloseIndexWriter.hasUncommittedChanges()) {
+								toCloseIndexWriter.commit();
+							}
+						} catch (Throwable e) {
+							logger.error("Failed to commit Lucene IndexWriter while closing", e);
+							exceptions.add(e);
+						} finally {
+							// shutdown the index writer
+							try {
+								indexWriter = null;
+								if (toCloseIndexWriter != null) {
+									toCloseIndexWriter.close();
+								}
+							} finally {
+								if (!exceptions.isEmpty()) {
+									throw new UndeclaredThrowableException(exceptions.get(0));
+								}
+							}
 						}
 					}
 				}
@@ -708,14 +790,21 @@ public synchronized void begin() throws IOException {
 	 */
 	@Override
 	public synchronized void commit() throws IOException {
-		getIndexWriter().commit();
-		// the old IndexReaders/Searchers are not outdated
-		invalidateReaders();
+		if (this.transactionsEnabled) {
+			getIndexWriter().commit();
+			// the old IndexReaders/Searchers are not outdated
+			invalidateReaders();
+		} else {
+			getIndexWriter().flush();
+		}
 	}
 
 	@Override
 	public synchronized void rollback() throws IOException {
-		getIndexWriter().rollback();
+		if (this.transactionsEnabled) {
+			getIndexWriter().rollback();
+		}
+		// If transactions are disabled, we cannot rollback changes, so we just ignore the rollback request
 	}
 
 	// //////////////////////////////// Methods for querying the index

core/sail/lucene/src/test/java/org/eclipse/rdf4j/sail/lucene/impl/AbstractGenericLuceneTest.java

@@ -119,6 +119,26 @@ public abstract class AbstractGenericLuceneTest {
 
 	protected abstract void configure(LuceneSail sail) throws IOException;
 
+	/**
+	 * How long to wait after a commit() to ensure index is updated.
+	 *
+	 * @return milliseconds to wait
+	 */
+	protected long waitAfterCommitMillis() {
+		return 0;
+	}
+
+	protected final void sleepAfterCommitIfNeeded() {
+		long waitMillis = waitAfterCommitMillis();
+		if (waitMillis > 0) {
+			try {
+				Thread.sleep(waitMillis);
+			} catch (InterruptedException e) {
+				Thread.currentThread().interrupt();
+			}
+		}
+	}
+
 	@BeforeEach
 	public void setUp() throws Exception {
 		// set logging, uncomment this to get better logging for debugging
@@ -166,6 +186,7 @@ public void tearDown() throws RepositoryException {
 
 	@Test
 	public void testComplexQueryTwo() throws MalformedQueryException, RepositoryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		// prepare the query
 		StringBuilder buffer = new StringBuilder();
 		buffer.append("SELECT ?Resource ?Matching ?Score ");
@@ -275,6 +296,7 @@ private void evaluate(String[] queries, ArrayList<List<Map<String, String>>> exp
 	@Test
 	public void testPredicateLuceneQueries()
 			throws MalformedQueryException, RepositoryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		// prepare the query
 		String[] queries = new String[] {
 				"SELECT ?Resource ?Score ?Snippet \n"
@@ -337,6 +359,7 @@ public void testPredicateLuceneQueries()
 
 	@Test
 	public void testSnippetQueries() throws MalformedQueryException, RepositoryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		// prepare the query
 		// search for the term "one", but only in predicate 1
 		StringBuilder buffer = new StringBuilder();
@@ -396,6 +419,7 @@ public void testSnippetLimitedToPredicate()
 			localConnection.add(SUBJECT_1, PREDICATE_1, vf.createLiteral("but the unicorn charly said to goaway"));
 			localConnection.add(SUBJECT_1, PREDICATE_2, vf.createLiteral("there was poor charly without a kidney"));
 			localConnection.commit();
+			sleepAfterCommitIfNeeded();
 		}
 
 		// prepare the query
@@ -471,6 +495,7 @@ public void testCharlyTerm() {
 			localConnection.add(SUBJECT_1, PREDICATE_1, vf.createLiteral("but the unicorn charly said to goaway"));
 			localConnection.add(SUBJECT_1, PREDICATE_2, vf.createLiteral("there was poor charly without a kidney"));
 			localConnection.commit();
+			sleepAfterCommitIfNeeded();
 		}
 		// search for the term "charly" in all predicates
 		StringBuilder buffer = new StringBuilder();
@@ -529,6 +554,7 @@ public void testCharlyTerm() {
 
 	@Test
 	public void testGraphQuery() throws QueryEvaluationException, MalformedQueryException, RepositoryException {
+		sleepAfterCommitIfNeeded();
 		IRI score = vf.createIRI(LuceneSailSchema.NAMESPACE + "score");
 		StringBuilder query = new StringBuilder();
 
@@ -577,6 +603,7 @@ public void testGraphQuery() throws QueryEvaluationException, MalformedQueryExce
 	@Test
 	public void testQueryWithSpecifiedSubject()
 			throws RepositoryException, MalformedQueryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		// fire a query with the subject pre-specified
 		TupleQuery query = connection.prepareTupleQuery(QUERY_STRING);
 		query.setBinding("Subject", SUBJECT_1);
@@ -594,6 +621,7 @@ public void testQueryWithSpecifiedSubject()
 
 	@Test
 	public void testUnionQuery() throws RepositoryException, MalformedQueryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		String queryStr = "";
 		queryStr += "PREFIX search: <http://www.openrdf.org/contrib/lucenesail#> ";
 		queryStr += "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> ";
@@ -626,6 +654,7 @@ public void testContextHandling() {
 		connection.add(SUBJECT_5, PREDICATE_1, vf.createLiteral("sfiveponectwo"), CONTEXT_2);
 		connection.add(SUBJECT_5, PREDICATE_2, vf.createLiteral("sfiveptwoctwo"), CONTEXT_2);
 		connection.commit();
+		sleepAfterCommitIfNeeded();
 		// connection.close();
 		// connection = repository.getConnection();
 		// connection.setAutoCommit(false);
@@ -640,6 +669,7 @@ public void testContextHandling() {
 		// remove a context
 		connection.clear(CONTEXT_1);
 		connection.commit();
+		sleepAfterCommitIfNeeded();
 		assertNoQueryResult("sfourponecone");
 		assertNoQueryResult("sfourptwocone");
 		assertNoQueryResult("sfiveponecone");
@@ -659,6 +689,7 @@ public void testNullContextHandling() {
 		connection.add(SUBJECT_5, PREDICATE_1, vf.createLiteral("sfiveponectwo"), CONTEXT_2);
 		connection.add(SUBJECT_5, PREDICATE_2, vf.createLiteral("sfiveptwoctwo"), CONTEXT_2);
 		connection.commit();
+		sleepAfterCommitIfNeeded();
 		// connection.close();
 		// connection = repository.getConnection();
 		// connection.setAutoCommit(false);
@@ -673,6 +704,7 @@ public void testNullContextHandling() {
 		// remove a context
 		connection.clear((Resource) null);
 		connection.commit();
+		sleepAfterCommitIfNeeded();
 		assertNoQueryResult("sfourponecone");
 		assertNoQueryResult("sfourptwocone");
 		assertNoQueryResult("sfiveponecone");
@@ -682,6 +714,7 @@ public void testNullContextHandling() {
 
 	@Test
 	public void testFuzzyQuery() throws MalformedQueryException, RepositoryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		// prepare the query
 		// search for the term "one" with 80% fuzzyness
 		StringBuilder buffer = new StringBuilder();
@@ -729,11 +762,13 @@ public void testFuzzyQuery() throws MalformedQueryException, RepositoryException
 	@Test
 	public void testReindexing() {
 		sail.reindex();
+		sleepAfterCommitIfNeeded();
 		testComplexQueryTwo();
 	}
 
 	@Test
 	public void testPropertyVar() throws MalformedQueryException, RepositoryException, QueryEvaluationException {
+		sleepAfterCommitIfNeeded();
 		StringBuilder buffer = new StringBuilder();
 		buffer.append("SELECT ?Resource ?Property \n");
 		buffer.append("WHERE { \n");