[GitHub] [ozone] smengcl commented on a diff in pull request #3658: HDDS-6983. Snapshot Chain - list of snapshots per snapshottable bucket

[GitBox <gi...@apache.org>]

smengcl commented on code in PR #3658:
URL: https://github.com/apache/ozone/pull/3658#discussion_r1000193616


##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainInfo.java:
##########
@@ -0,0 +1,57 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+/**
+ * SnapshotChain supporting SnapshotInfo class.
+ *
+ * Defines and provides the interface to the SnapshotChainInfo
+ * entry that comprises a SnapshotChain.
+ * Getters / setters for current, prev, next element in snapshot chain.
+ */
+public class SnapshotChainInfo {
+  private String snapshotID;
+  private String previousSnapshotID;
+  private String nextSnapshotID;
+
+  public SnapshotChainInfo(String snapshotID, String prev, String next) {
+    this.snapshotID = snapshotID;
+    previousSnapshotID = prev;
+    nextSnapshotID = next;
+  }
+
+  public void setNextSnapshotID(String snapsID) {
+    nextSnapshotID = snapsID;
+  }
+
+  public void setPreviousSnapshotID(String snapsID) {
+    previousSnapshotID = snapsID;
+  }

Review Comment:
   nit
   ```suggestion
     public void setNextSnapshotID(String snapshotID) {
       nextSnapshotID = snapshotID;
     }
   
     public void setPreviousSnapshotID(String snapshotID) {
       previousSnapshotID = snapshotID;
     }
   ```



##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
##########
@@ -0,0 +1,440 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.hdds.utils.db.Table;
+import org.apache.hadoop.hdds.utils.db.TableIterator;
+import org.apache.hadoop.ozone.om.helpers.SnapshotInfo;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class is used for creating and accessing Snapshot Chains.
+ *
+ * The snapshot chain maintains the in-memory sequence of snapshots
+ * created in chronological order.  There are two such snapshots maintained
+ * i.) Path based snapshot chain, sequence of snapshots created for a
+ * given /volume/bucket
+ * ii.) Global snapshot chain, sequence of all snapshots created in order
+ *
+ * On start, the snapshot chains are initialized from the on disk
+ * SnapshotInfoTable from the om RocksDB.
+ */
+public class SnapshotChainManager {
+  private LinkedHashMap<String, SnapshotChainInfo>  snapshotChainGlobal;
+  private Map<String, LinkedHashMap<String, SnapshotChainInfo>>
+      snapshotChainPath;
+  private Map<String, String> latestPathSnapshotID;
+  private String latestGlobalSnapshotID;
+  private static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotChainManager.class);
+
+  public SnapshotChainManager(OMMetadataManager metadataManager)
+          throws IOException {
+    snapshotChainGlobal = new LinkedHashMap<>();
+    snapshotChainPath = new HashMap<>();
+    latestPathSnapshotID = new HashMap<>();
+    latestGlobalSnapshotID = null;
+    loadFromSnapshotInfoTable(metadataManager);
+  }
+
+  private void addSnapshotGlobal(String snapshotID,
+                            String prevGlobalID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevGlobalID != null && prevGlobalID.equals("")) {
+      prevGlobalID = null;
+    }
+    // on add snapshot; set previous snapshot entry nextSnapshotID =
+    // snapshotID
+    if (prevGlobalID != null &&
+        snapshotChainGlobal.containsKey(prevGlobalID)) {
+      snapshotChainGlobal
+          .get(prevGlobalID)
+          .setNextSnapshotID(snapshotID);
+    }
+    if (prevGlobalID != null &&
+        !snapshotChainGlobal.containsKey(prevGlobalID)) {
+      throw new IOException("Snapshot Chain corruption: "
+          + " previous snapshotID given but no associated snapshot "
+      + "found in snapshot chain: SnapshotID "
+      + prevGlobalID);
+    }
+    snapshotChainGlobal.put(snapshotID,
+            new SnapshotChainInfo(snapshotID, prevGlobalID, null));
+
+    // set state variable latestGlobal snapshot entry to this snapshotID
+    latestGlobalSnapshotID = snapshotID;
+  };
+
+  private void addSnapshotPath(String snapshotPath,
+                               String snapshotID,
+                                String prevPathID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevPathID != null && prevPathID.equals("")) {

Review Comment:
   or use isEmpty:
   ```suggestion
       if (prevPathID != null && prevPathID.isEmpty()) {
   ```



##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
##########
@@ -0,0 +1,440 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.hdds.utils.db.Table;
+import org.apache.hadoop.hdds.utils.db.TableIterator;
+import org.apache.hadoop.ozone.om.helpers.SnapshotInfo;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class is used for creating and accessing Snapshot Chains.
+ *
+ * The snapshot chain maintains the in-memory sequence of snapshots
+ * created in chronological order.  There are two such snapshots maintained
+ * i.) Path based snapshot chain, sequence of snapshots created for a
+ * given /volume/bucket
+ * ii.) Global snapshot chain, sequence of all snapshots created in order
+ *
+ * On start, the snapshot chains are initialized from the on disk
+ * SnapshotInfoTable from the om RocksDB.
+ */
+public class SnapshotChainManager {
+  private LinkedHashMap<String, SnapshotChainInfo>  snapshotChainGlobal;
+  private Map<String, LinkedHashMap<String, SnapshotChainInfo>>
+      snapshotChainPath;
+  private Map<String, String> latestPathSnapshotID;
+  private String latestGlobalSnapshotID;
+  private static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotChainManager.class);
+
+  public SnapshotChainManager(OMMetadataManager metadataManager)
+          throws IOException {
+    snapshotChainGlobal = new LinkedHashMap<>();
+    snapshotChainPath = new HashMap<>();
+    latestPathSnapshotID = new HashMap<>();
+    latestGlobalSnapshotID = null;
+    loadFromSnapshotInfoTable(metadataManager);
+  }
+
+  private void addSnapshotGlobal(String snapshotID,
+                            String prevGlobalID) throws IOException {

Review Comment:
   nit: indentation
   ```suggestion
     private void addSnapshotGlobal(String snapshotID,
                                    String prevGlobalID) throws IOException {
   ```



##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
##########
@@ -0,0 +1,440 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.hdds.utils.db.Table;
+import org.apache.hadoop.hdds.utils.db.TableIterator;
+import org.apache.hadoop.ozone.om.helpers.SnapshotInfo;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class is used for creating and accessing Snapshot Chains.
+ *
+ * The snapshot chain maintains the in-memory sequence of snapshots
+ * created in chronological order.  There are two such snapshots maintained
+ * i.) Path based snapshot chain, sequence of snapshots created for a
+ * given /volume/bucket
+ * ii.) Global snapshot chain, sequence of all snapshots created in order
+ *
+ * On start, the snapshot chains are initialized from the on disk
+ * SnapshotInfoTable from the om RocksDB.
+ */
+public class SnapshotChainManager {
+  private LinkedHashMap<String, SnapshotChainInfo>  snapshotChainGlobal;
+  private Map<String, LinkedHashMap<String, SnapshotChainInfo>>
+      snapshotChainPath;
+  private Map<String, String> latestPathSnapshotID;
+  private String latestGlobalSnapshotID;
+  private static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotChainManager.class);
+
+  public SnapshotChainManager(OMMetadataManager metadataManager)
+          throws IOException {
+    snapshotChainGlobal = new LinkedHashMap<>();
+    snapshotChainPath = new HashMap<>();
+    latestPathSnapshotID = new HashMap<>();
+    latestGlobalSnapshotID = null;
+    loadFromSnapshotInfoTable(metadataManager);
+  }
+
+  private void addSnapshotGlobal(String snapshotID,
+                            String prevGlobalID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevGlobalID != null && prevGlobalID.equals("")) {
+      prevGlobalID = null;
+    }
+    // on add snapshot; set previous snapshot entry nextSnapshotID =
+    // snapshotID
+    if (prevGlobalID != null &&
+        snapshotChainGlobal.containsKey(prevGlobalID)) {
+      snapshotChainGlobal
+          .get(prevGlobalID)
+          .setNextSnapshotID(snapshotID);
+    }
+    if (prevGlobalID != null &&
+        !snapshotChainGlobal.containsKey(prevGlobalID)) {
+      throw new IOException("Snapshot Chain corruption: "
+          + " previous snapshotID given but no associated snapshot "
+      + "found in snapshot chain: SnapshotID "
+      + prevGlobalID);
+    }
+    snapshotChainGlobal.put(snapshotID,
+            new SnapshotChainInfo(snapshotID, prevGlobalID, null));
+
+    // set state variable latestGlobal snapshot entry to this snapshotID
+    latestGlobalSnapshotID = snapshotID;
+  };
+
+  private void addSnapshotPath(String snapshotPath,
+                               String snapshotID,
+                                String prevPathID) throws IOException {

Review Comment:
   nit
   ```suggestion
                                  String prevPathID) throws IOException {
   ```



##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
##########
@@ -0,0 +1,440 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.hdds.utils.db.Table;
+import org.apache.hadoop.hdds.utils.db.TableIterator;
+import org.apache.hadoop.ozone.om.helpers.SnapshotInfo;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class is used for creating and accessing Snapshot Chains.
+ *
+ * The snapshot chain maintains the in-memory sequence of snapshots
+ * created in chronological order.  There are two such snapshots maintained
+ * i.) Path based snapshot chain, sequence of snapshots created for a
+ * given /volume/bucket
+ * ii.) Global snapshot chain, sequence of all snapshots created in order
+ *
+ * On start, the snapshot chains are initialized from the on disk
+ * SnapshotInfoTable from the om RocksDB.
+ */
+public class SnapshotChainManager {
+  private LinkedHashMap<String, SnapshotChainInfo>  snapshotChainGlobal;
+  private Map<String, LinkedHashMap<String, SnapshotChainInfo>>
+      snapshotChainPath;
+  private Map<String, String> latestPathSnapshotID;
+  private String latestGlobalSnapshotID;
+  private static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotChainManager.class);
+
+  public SnapshotChainManager(OMMetadataManager metadataManager)
+          throws IOException {
+    snapshotChainGlobal = new LinkedHashMap<>();
+    snapshotChainPath = new HashMap<>();
+    latestPathSnapshotID = new HashMap<>();
+    latestGlobalSnapshotID = null;
+    loadFromSnapshotInfoTable(metadataManager);
+  }
+
+  private void addSnapshotGlobal(String snapshotID,
+                            String prevGlobalID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevGlobalID != null && prevGlobalID.equals("")) {
+      prevGlobalID = null;
+    }
+    // on add snapshot; set previous snapshot entry nextSnapshotID =
+    // snapshotID
+    if (prevGlobalID != null &&
+        snapshotChainGlobal.containsKey(prevGlobalID)) {
+      snapshotChainGlobal
+          .get(prevGlobalID)
+          .setNextSnapshotID(snapshotID);
+    }
+    if (prevGlobalID != null &&
+        !snapshotChainGlobal.containsKey(prevGlobalID)) {
+      throw new IOException("Snapshot Chain corruption: "
+          + " previous snapshotID given but no associated snapshot "
+      + "found in snapshot chain: SnapshotID "
+      + prevGlobalID);
+    }
+    snapshotChainGlobal.put(snapshotID,
+            new SnapshotChainInfo(snapshotID, prevGlobalID, null));
+
+    // set state variable latestGlobal snapshot entry to this snapshotID
+    latestGlobalSnapshotID = snapshotID;
+  };
+
+  private void addSnapshotPath(String snapshotPath,
+                               String snapshotID,
+                                String prevPathID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevPathID != null && prevPathID.equals("")) {
+      prevPathID = null;
+    }
+
+    // on add snapshot; set previous snapshot entry nextSnapshotID =
+    // snapshotID
+    if (prevPathID != null &&
+        ((!snapshotChainPath
+            .containsKey(snapshotPath)) ||
+        (!snapshotChainPath
+            .get(snapshotPath)
+            .containsKey(prevPathID)))) {
+      throw new IOException("Snapshot Chain corruption: "
+          + "previous snapshotID given but no associated snapshot "
+          + "found in snapshot chain: SnapshotID "
+          + prevPathID);
+    }
+
+    if (prevPathID != null &&
+            snapshotChainPath.containsKey(snapshotPath)) {
+      snapshotChainPath
+              .get(snapshotPath)
+              .get(prevPathID)
+              .setNextSnapshotID(snapshotID);
+    }
+
+    if (!snapshotChainPath.containsKey(snapshotPath)) {
+      snapshotChainPath.put(snapshotPath, new LinkedHashMap<>());
+    }
+
+    snapshotChainPath
+        .get(snapshotPath)
+        .put(snapshotID,
+                new SnapshotChainInfo(snapshotID, prevPathID, null));
+
+    // set state variable latestPath snapshot entry to this snapshotID
+    latestPathSnapshotID.put(snapshotPath, snapshotID);
+  };
+
+  private boolean deleteSnapshotGlobal(String snapshotID) throws IOException {
+    boolean status = true;
+    if (snapshotChainGlobal.containsKey(snapshotID)) {
+      // reset prev and next snapshot entries in chain ordered list
+      // for node removal
+      String next = snapshotChainGlobal.get(snapshotID).getNextSnapshotID();
+      String prev = snapshotChainGlobal.get(snapshotID).getPreviousSnapshotID();
+
+      if (prev != null && !snapshotChainGlobal.containsKey(prev)) {
+        throw new IOException("Snapshot chain corruption: snapshot node to be "
+                + "deleted has prev node element not found in snapshot chain: "
+                + "SnapshotID " + prev);
+      }
+      if (next != null && !snapshotChainGlobal.containsKey(next)) {
+        throw new IOException("Snapshot chain corruption: snapshot node to be "
+                + "deleted has next node element not found in snapshot chain: "
+                + "SnapshotID " + next);
+      }
+      snapshotChainGlobal.remove(snapshotID);
+      if (next != null) {
+        snapshotChainGlobal.get(next).setPreviousSnapshotID(prev);
+      }
+      if (prev != null) {
+        snapshotChainGlobal.get(prev).setNextSnapshotID(next);
+      }
+      // remove from latest list if necessary
+      if (latestGlobalSnapshotID.equals(snapshotID)) {
+        latestGlobalSnapshotID = prev;
+      }
+    } else {
+      // snapshotID not found in snapshot chain, log warning and return
+      LOG.warn("Snapshot chain: snapshotID not found: SnapshotID {}",
+          snapshotID);
+      status = false;
+    }
+
+    return status;
+  }
+
+  private boolean deleteSnapshotPath(String snapshotPath,
+                                     String snapshotID) throws IOException {
+    boolean status = true;
+    if (snapshotChainPath.containsKey(snapshotPath) &&
+            snapshotChainPath
+                    .get(snapshotPath)
+                    .containsKey(snapshotID)) {
+      // reset prev and next snapshot entries in chain ordered list
+      // for node removal
+      String next = snapshotChainPath
+          .get(snapshotPath)
+          .get(snapshotID)
+          .getNextSnapshotID();
+      String prev = snapshotChainPath
+          .get(snapshotPath)
+          .get(snapshotID)
+          .getPreviousSnapshotID();
+
+      if (prev != null &&
+          !snapshotChainPath
+              .get(snapshotPath)
+              .containsKey(prev)) {
+        throw new IOException("Snapshot chain corruption: snapshot node to "
+                + "be deleted has prev node element not found in snapshot "
+                + "chain: Snapshot path " + snapshotPath + ", SnapshotID "
+                + prev);
+      }
+      if (next != null && !snapshotChainPath
+          .get(snapshotPath)
+          .containsKey(next)) {
+        throw new IOException("Snapshot chain corruption: snapshot node to "
+                + "be deleted has next node element not found in snapshot "
+                + "chain:  Snapshot path " + snapshotPath + ", SnapshotID "
+                + next);
+      }
+      snapshotChainPath
+          .get(snapshotPath)
+          .remove(snapshotID);
+      if (next != null) {
+        snapshotChainPath
+            .get(snapshotPath)
+            .get(next)
+            .setPreviousSnapshotID(prev);
+      }
+      if (prev != null) {
+        snapshotChainPath
+            .get(snapshotPath)
+            .get(prev)
+            .setNextSnapshotID(next);
+      }
+      // remove path if no entries
+      if (snapshotChainPath.get(snapshotPath).isEmpty()) {
+        snapshotChainPath.remove(snapshotPath);
+      }
+      // remove from latest list if necessary
+      if (latestPathSnapshotID.get(snapshotPath).equals(snapshotID)) {
+        latestPathSnapshotID.remove(snapshotPath);
+        if (prev != null) {
+          latestPathSnapshotID.put(snapshotPath, prev);
+        }
+      }
+
+    } else {
+      // snapshotID not found in snapshot chain, log warning and return
+      LOG.warn("Snapshot chain: snapshotID not found: Snapshot path {}," +
+              " SnapshotID {}",
+          snapshotPath, snapshotID);
+      status = false;
+    }
+
+    return status;
+  }
+
+  private void loadFromSnapshotInfoTable(OMMetadataManager metadataManager)
+          throws IOException {
+    // read from snapshotInfo table to populate
+    // snapshot chains - both global and local path
+    TableIterator<String, ? extends Table.KeyValue<String, SnapshotInfo>>
+            keyIter = metadataManager.getSnapshotInfoTable().iterator();
+    Map<Long, SnapshotInfo> snaps = new TreeMap<>();
+    Table.KeyValue< String, SnapshotInfo > kv;
+    snapshotChainGlobal.clear();
+    snapshotChainPath.clear();
+
+    while (keyIter.hasNext()) {
+      kv = keyIter.next();
+      snaps.put(kv.getValue().getCreationTime(), kv.getValue());
+    }
+    for (SnapshotInfo sinfo : snaps.values()) {
+      addSnapshot(sinfo);
+    }
+  }
+
+  public void addSnapshot(SnapshotInfo sinfo) throws IOException {

Review Comment:
   It would be nice to have some javadocs on the public methods.



##########
hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/SnapshotChainManager.java:
##########
@@ -0,0 +1,440 @@
+/*
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hadoop.ozone.om;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.hdds.utils.db.Table;
+import org.apache.hadoop.hdds.utils.db.TableIterator;
+import org.apache.hadoop.ozone.om.helpers.SnapshotInfo;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.LinkedHashMap;
+import java.util.TreeMap;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This class is used for creating and accessing Snapshot Chains.
+ *
+ * The snapshot chain maintains the in-memory sequence of snapshots
+ * created in chronological order.  There are two such snapshots maintained
+ * i.) Path based snapshot chain, sequence of snapshots created for a
+ * given /volume/bucket
+ * ii.) Global snapshot chain, sequence of all snapshots created in order
+ *
+ * On start, the snapshot chains are initialized from the on disk
+ * SnapshotInfoTable from the om RocksDB.
+ */
+public class SnapshotChainManager {
+  private LinkedHashMap<String, SnapshotChainInfo>  snapshotChainGlobal;
+  private Map<String, LinkedHashMap<String, SnapshotChainInfo>>
+      snapshotChainPath;
+  private Map<String, String> latestPathSnapshotID;
+  private String latestGlobalSnapshotID;
+  private static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotChainManager.class);
+
+  public SnapshotChainManager(OMMetadataManager metadataManager)
+          throws IOException {
+    snapshotChainGlobal = new LinkedHashMap<>();
+    snapshotChainPath = new HashMap<>();
+    latestPathSnapshotID = new HashMap<>();
+    latestGlobalSnapshotID = null;
+    loadFromSnapshotInfoTable(metadataManager);
+  }
+
+  private void addSnapshotGlobal(String snapshotID,
+                            String prevGlobalID) throws IOException {
+    // set previous snapshotID to null if it is "" for
+    // internal in-mem structure
+    if (prevGlobalID != null && prevGlobalID.equals("")) {

Review Comment:
   could use `isEmpty()`



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