Copilot commented on code in PR #3081:
URL: https://github.com/apache/hugegraph/pull/3081#discussion_r3663686279


##########
hugegraph-store/hg-store-rocksdb/src/main/java/org/apache/hugegraph/rocksdb/access/SessionOperatorImpl.java:
##########
@@ -208,7 +208,14 @@ public void deleteRange(byte[] keyFrom, byte[] keyTo) 
throws DBStoreException {
     @Override
     public byte[] get(String table, byte[] key) throws DBStoreException {
         try (CFHandleLock cf = this.getLock(table)) {
-            return rocksdb().get(cf.get(), key);
+            byte[] value = rocksdb().get(cf.get(), key);
+            if (value != null) {
+                return value;
+            }
+            if (RocksDBFactory.getInstance().onReadMiss(this.session, table, 
key)) {

Review Comment:
   onReadMiss() can perform cloud hydration (network + filesystem I/O via 
CloudStorageEventListener.restoreMissingLiveFiles()), but it is currently 
invoked while holding CFHandleLock from getLock(table). This can block other 
threads needing the same CF handle lock for the duration of hydration. Release 
the CFHandleLock before calling onReadMiss(), then reacquire it to retry the 
get().



##########
hugegraph-store/hg-store-rocksdb/src/main/java/org/apache/hugegraph/rocksdb/access/RocksDBSession.java:
##########
@@ -693,6 +764,70 @@ public void saveSnapshot(String snapshotPath) throws 
DBStoreException {
                  System.currentTimeMillis() - startTime);
     }
 
+    /**
+     * Captures a consistent copy of this DB's metadata (CURRENT / MANIFEST-* 
/ OPTIONS-* / WAL) plus
+     * hard-links to the live SST set into a temporary directory, using a 
RocksDB
+     * {@link Checkpoint}. See {@link 
RocksDBFactory#captureMetadataSnapshot(String)} for the metadata
+     * rationale.
+     *
+     * <p>The temporary directory is a sibling of the DB directory (same 
filesystem), so SST files
+     * are hard-linked rather than copied. The caller owns cleanup via
+     * {@link RocksDBFactory.MetadataSnapshot#cleanup()}.
+     *
+     * @return the captured snapshot; never {@code null}
+     * @throws DBStoreException if the checkpoint cannot be created
+     */
+    RocksDBFactory.MetadataSnapshot captureMetadataCheckpoint() throws 
DBStoreException {
+        String tempDir = this.dbPath + "_cloudmeta_" + System.nanoTime();
+        cfHandleLock.readLock().lock();
+        try (final Checkpoint checkpoint = Checkpoint.create(this.rocksDB)) {
+            final File tempFile = new File(tempDir);
+            // Clean any stale temp checkpoint dir from a previous 
failed/interrupted attempt.
+            // If it exists, RocksDB checkpoint creation can fail on 
pre-existing files.
+            FileUtils.deleteDirectory(tempFile);
+            checkpoint.createCheckpoint(tempDir);
+        } catch (final Exception e) {
+            try {
+                FileUtils.deleteDirectory(new File(tempDir));
+            } catch (IOException ignore) {
+                // best-effort cleanup of a partial checkpoint
+            }
+            log.error("Fail to create metadata checkpoint at {}", tempDir, e);
+            throw new DBStoreException(
+                    String.format("Fail to create metadata checkpoint at %s", 
tempDir));

Review Comment:
   captureMetadataCheckpoint() logs the original exception but drops it when 
throwing DBStoreException, which makes diagnosing checkpoint failures harder 
upstream. DBStoreException supports a (message, cause, args...) constructor, so 
the cause can be preserved.



##########
hugegraph-store/hg-store-common/src/main/java/org/apache/hugegraph/store/cloud/CloudStorageProviderFactory.java:
##########
@@ -0,0 +1,212 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hugegraph.store.cloud;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.ServiceLoader;
+import java.util.concurrent.ConcurrentHashMap;
+
+import lombok.Getter;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Factory for {@link CloudStorageProvider} instances.
+ *
+ * <p>Providers are discovered at class-loading time via {@link ServiceLoader}.
+ * Any JAR that includes
+ * {@code 
META-INF/services/org.apache.hugegraph.store.cloud.CloudStorageProvider}
+ * is automatically picked up when it is present on the classpath.
+ *
+ * <p>Usage:
+ * <pre>
+ *   CloudStorageConfig cfg = ...; // populated from application.yml
+ *   CloudStorageProvider provider = 
CloudStorageProviderFactory.initialize(cfg);
+ *   // later:
+ *   CloudStorageProvider active = 
CloudStorageProviderFactory.getActiveProvider();
+ * </pre>
+ */
+public final class CloudStorageProviderFactory {
+
+    private static final Logger log = 
LoggerFactory.getLogger(CloudStorageProviderFactory.class);
+
+    /** All discovered providers keyed by {@link 
CloudStorageProvider#providerName()}. */
+    private static final Map<String, CloudStorageProvider> REGISTRY = new 
ConcurrentHashMap<>();
+
+    /** The currently active (initialized) provider; null when disabled or not 
yet initialized.
+     * -- GETTER --
+     *  Returns the currently active provider, or
+     *  if cloud storage is
+     *  disabled or
+     *  has not yet been called.
+     */
+    @Getter
+    private static volatile CloudStorageProvider activeProvider;
+
+    static {
+        loadProviders();
+    }
+
+    private CloudStorageProviderFactory() {
+    }
+
+    // -----------------------------------------------------------------------
+    // Public API
+    // -----------------------------------------------------------------------
+
+    /**
+     * Initializes and activates the cloud storage provider described by 
{@code config}.
+     *
+     * <p>The method is idempotent: if called multiple times, the existing 
active
+     * provider is closed before a new one is initialized.
+     *
+     * @param config cloud storage configuration
+     * @return the initialized provider, or {@code null} when
+     *         {@link CloudStorageConfig#isEnabled()} is {@code false}
+     * @throws IllegalArgumentException if no provider matching {@code 
config.getProvider()}
+     *                                  was found on the classpath
+     */
+    public static synchronized CloudStorageProvider 
initialize(CloudStorageConfig config) {
+        if (!config.isEnabled()) {
+            // Disabling cloud storage must deactivate any currently active 
provider: otherwise a

Review Comment:
   CloudStorageProviderFactory.initialize() dereferences config without a null 
check, so callers get a NullPointerException on bad input. Since this is a 
public factory API, prefer validating and throwing IllegalArgumentException 
with a clear message.



##########
hugegraph-store/hg-store-core/src/main/java/org/apache/hugegraph/store/business/BusinessHandlerImpl.java:
##########
@@ -1008,17 +1008,27 @@ public void batchGet(String graph, String table, 
Supplier<HgPair<Integer, byte[]
     }
 
     /**
-     * Clear map data
+     * Clear map data.
+     *
+     * <p>Fires {@code notifyTruncateBegin} before the key-range delete so that
+     * registered listeners can prepare for truncate and suppress racing 
callbacks,
+     * and {@code notifyTruncate} afterwards so that listeners can finalize any
+     * post-truncate cleanup — matching the same callback pair fired by
+     * {@link org.apache.hugegraph.rocksdb.access.RocksDBSession#truncate()}.
      */
     @Override
     public void truncate(String graphName, int partId) throws HgStoreException 
{
         // Each partition corresponds to a rocksdb instance, so the rocksdb 
instance name is
         // rocksdb + partId
         try (RocksDBSession dbSession = getSession(graphName, partId)) {
+            String dbName = dbSession.getGraphName();
+            String dbPath = dbSession.getDbPath();
+            factory.notifyTruncateBegin(dbName, dbPath);
             dbSession.sessionOp().deleteRange(keyCreator.getStartKey(partId, 
graphName),
                                               keyCreator.getEndKey(partId, 
graphName));
             // Release map ID
             keyCreator.delGraphId(partId, graphName);
+            factory.notifyTruncate(dbName, dbPath);
         }

Review Comment:
   truncate() fires notifyTruncateBegin(), but if deleteRange() or delGraphId() 
throws, notifyTruncate() is skipped and listeners may remain stuck in a 
“truncating” suppression state indefinitely. Mirror RocksDBSession.truncate() 
by guaranteeing a terminal notification (notifyTruncate on success, 
notifyTruncateAbort on failure) via try/finally.



##########
hugegraph-store/pom.xml:
##########
@@ -302,5 +307,13 @@
                 </plugins>
             </build>
         </profile>
+
+        <!-- Build the S3 cloud-storage plugin:  mvn install -P cloud-s3 
-DskipTests -->
+        <profile>
+            <id>cloud-s3</id>
+            <modules>
+                <module>hg-store-cloud-s3</module>
+            </modules>
+        </profile>

Review Comment:
   The cloud-s3 Maven profile is currently redundant because hg-store-cloud-s3 
is already listed under the top-level <modules>. Activating -P cloud-s3 will 
not change the build, and in some Maven versions duplicate module declarations 
can lead to confusing warnings. Consider removing the profile (or removing the 
module from the default build if it truly should be profile-gated).



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