Github user tillrohrmann commented on a diff in the pull request: https://github.com/apache/flink/pull/1153#discussion_r41527783 --- 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 -- I think we cannot be sure that the cversion increases at the same speed as the `CheckpointCoordinator's` checkpoint ID counter does. Assume that we trigger multiple checkpoints which all fail. Thus, they increase the internal counter but not the `cversion`. And now a checkpoint succeeds. Then the checkpoint ID can be greater than the `cversion`. That's what I think at least.
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