Github user uce commented on a diff in the pull request:
https://github.com/apache/flink/pull/1153#discussion_r41527930
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/ZooKeeperCompletedCheckpointStore.java
---
@@ -0,0 +1,310 @@
+/*
+ * 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.flink.runtime.checkpoint;
+
+import org.apache.curator.framework.CuratorFramework;
+import org.apache.curator.framework.api.BackgroundCallback;
+import org.apache.curator.framework.api.CuratorEvent;
+import org.apache.curator.framework.api.CuratorEventType;
+import org.apache.curator.utils.ZKPaths;
+import org.apache.flink.runtime.jobmanager.RecoveryMode;
+import org.apache.flink.runtime.state.StateHandleProvider;
+import org.apache.flink.runtime.zookeeper.ZooKeeperStateHandleStore;
+import
org.apache.flink.runtime.zookeeper.ZooKeeperStateHandleStore.ZooKeeperStateHandle;
+import org.apache.zookeeper.data.Stat;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.ConcurrentModificationException;
+import java.util.List;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * {@link CompletedCheckpointStore} for JobManagers running in {@link
RecoveryMode#ZOOKEEPER}.
+ *
+ * <p>Checkpoints are added under a ZNode per job:
+ * <pre>
+ * +----O /flink/checkpoints/<job-id> [persistent]
+ * . |
+ * . +----O /flink/checkpoints/<job-id>/1 [persistent_sequential]
+ * . . .
+ * . . .
+ * . . .
+ * . +----O /flink/checkpoints/<job-id>/N [persistent_sequential]
+ * </pre>
+ *
+ * <p>During recovery, the latest checkpoint is read from ZooKeeper. If
there is more than one,
+ * only the latest one is used and older ones are discarded (even if the
maximum number
+ * of retained checkpoints is greater than one). The sequential; cversion
determines, which
+ * checkpoint is the latest one.
+ *
+ * <p>If there is a network partition and multiple JobManagers run
concurrent checkpoints for the
+ * same program, it is OK to take any valid successful checkpoint as long
as the "history" of
+ * checkpoints is consistent. Currently, after recovery we start out with
only a single
+ * checkpoint in oder to guarantee this.
+ */
+public class ZooKeeperCompletedCheckpointStore implements
CompletedCheckpointStore {
+
+ private static final Logger LOG =
LoggerFactory.getLogger(ZooKeeperCompletedCheckpointStore.class);
+
+ /** Curator ZooKeeper client */
+ private final CuratorFramework client;
+
+ /** Completed checkpoints in ZooKeeper */
+ private final ZooKeeperStateHandleStore<CompletedCheckpoint>
checkpointsInZooKeeper;
+
+ /** The maximum number of checkpoints to retain (at least 1). */
+ private final int maxNumberOfCheckpointsToRetain;
+
+ /** User class loader for discarding {@link CompletedCheckpoint}
instances. */
+ private final ClassLoader userClassLoader;
+
+ /** Local completed checkpoints. */
+ private final ArrayDeque<ZooKeeperStateHandle<CompletedCheckpoint>>
checkpointStateHandles;
+
+ /**
+ * Creates a {@link ZooKeeperCompletedCheckpointStore} instance.
+ *
+ * @param maxNumberOfCheckpointsToRetain The maximum number of
checkpoints to retain (at
+ * least 1). Adding more
checkpoints than this results
+ * in older checkpoints being
discarded. On recovery,
+ * we will only start with a
single checkpoint.
+ * @param userClassLoader The user class loader used to
discard checkpoints
+ * @param client The Curator ZooKeeper client
+ * @param checkpointsPath The ZooKeeper path for the
checkpoints (needs to
+ * start with a '/')
+ * @param stateHandleProvider The state handle provider for
checkpoints
+ * @throws Exception
+ */
+ public ZooKeeperCompletedCheckpointStore(
+ int maxNumberOfCheckpointsToRetain,
+ ClassLoader userClassLoader,
+ CuratorFramework client,
+ String checkpointsPath,
+ StateHandleProvider<CompletedCheckpoint>
stateHandleProvider) throws Exception {
+
+ checkArgument(maxNumberOfCheckpointsToRetain >= 1, "Must retain
at least one checkpoint.");
+
+ this.maxNumberOfCheckpointsToRetain =
maxNumberOfCheckpointsToRetain;
+ this.userClassLoader = checkNotNull(userClassLoader, "User
class loader");
+
+ checkNotNull(client, "Curator client");
+ checkNotNull(checkpointsPath, "Checkpoints path");
+ checkNotNull(stateHandleProvider, "State handle provider");
+
+ // Ensure that the checkpoints path exists
+ client.newNamespaceAwareEnsurePath(checkpointsPath)
+ .ensure(client.getZookeeperClient());
+
+ // All operations will have the path as root
+ this.client = client.usingNamespace(client.getNamespace() +
checkpointsPath);
+
+ this.checkpointsInZooKeeper = new ZooKeeperStateHandleStore<>(
+ this.client, stateHandleProvider);
+
+ this.checkpointStateHandles = new
ArrayDeque<>(maxNumberOfCheckpointsToRetain + 1);
+
+ LOG.info("Initialized in '{}'.", checkpointsPath);
+ }
+
+ /**
+ * Gets the latest checkpoint from ZooKeeper and removes all others.
+ *
+ * <p><strong>Important</strong>: Even if there are more than one
checkpoint in ZooKeeper,
+ * this will only recover the latest and discard the others. Otherwise,
there is no guarantee
+ * that the history of checkpoints is consistent.
+ */
+ @Override
+ public void recover() throws Exception {
+ LOG.info("Recovering checkpoints from ZooKeeper.");
+
+ // Get all there is first
+ List<ZooKeeperStateHandle<CompletedCheckpoint>>
initialCheckpoints;
+ while (true) {
+ try {
+ initialCheckpoints =
checkpointsInZooKeeper.getAllSortedByName();
+ break;
+ }
+ catch (ConcurrentModificationException e) {
+ LOG.warn("Concurrent modification while reading
from ZooKeeper. Retrying.");
+ }
+ }
+
+ int numberOfInitialCheckpoints = initialCheckpoints.size();
+
+ LOG.info("Found {} checkpoints in ZooKeeper.",
numberOfInitialCheckpoints);
+
+ if (numberOfInitialCheckpoints > 0) {
+ // Take the last one. This is the latest checkpoints,
because path names are strictly
+ // increasing (checkpoint ID).
+ ZooKeeperStateHandle<CompletedCheckpoint> latest =
initialCheckpoints
+ .get(numberOfInitialCheckpoints - 1);
+
+ CompletedCheckpoint latestCheckpoint =
latest.getState(userClassLoader);
+
+ checkpointStateHandles.add(latest);
+
+ LOG.info("Initialized with {}. Removing all older
checkpoints.", latestCheckpoint);
+
+ for (int i = 0; i < numberOfInitialCheckpoints - 1;
i++) {
+ try {
+
removeFromZooKeeperAndDiscardCheckpoint(initialCheckpoints.get(i));
+ }
+ catch (Exception e) {
+ LOG.error("Failed to discard
checkpoint", e);
+ }
+ }
+ }
+ }
+
+ @Override
+ public int getNextCheckpointID() throws Exception {
--- End diff --
Yes, the name/missing comment is confusing I think.
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