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Posted to common-issues@hadoop.apache.org by GitBox <gi...@apache.org> on 2019/03/19 00:00:34 UTC
[GitHub] [hadoop] arp7 commented on a change in pull request #620: HDDS-1205. Refactor ReplicationManager to handle QUASI_CLOSED contain…
arp7 commented on a change in pull request #620: HDDS-1205. Refactor ReplicationManager to handle QUASI_CLOSED contain…
URL: https://github.com/apache/hadoop/pull/620#discussion_r266627466
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File path: hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/ReplicationManager.java
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@@ -0,0 +1,686 @@
+/**
+ * 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.hdds.scm.container;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdds.protocol.DatanodeDetails;
+import org.apache.hadoop.hdds.protocol.proto.HddsProtos.LifeCycleState;
+import org.apache.hadoop.hdds.protocol.proto
+ .StorageContainerDatanodeProtocolProtos.ContainerReplicaProto.State;
+import org.apache.hadoop.hdds.scm.ScmConfigKeys;
+import org.apache.hadoop.hdds.scm.container.placement.algorithms
+ .ContainerPlacementPolicy;
+import org.apache.hadoop.hdds.scm.events.SCMEvents;
+import org.apache.hadoop.hdds.server.events.EventPublisher;
+import org.apache.hadoop.ozone.lock.LockManager;
+import org.apache.hadoop.ozone.protocol.commands.CloseContainerCommand;
+import org.apache.hadoop.ozone.protocol.commands.CommandForDatanode;
+import org.apache.hadoop.ozone.protocol.commands.DeleteContainerCommand;
+import org.apache.hadoop.ozone.protocol.commands.ReplicateContainerCommand;
+import org.apache.hadoop.util.ExitUtil;
+import org.apache.hadoop.util.Time;
+import org.apache.ratis.util.Preconditions;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.TimeUnit;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+/**
+ * Replication Manager (RM) is the one which is responsible for making sure
+ * that the containers are properly replicated. Replication Manager deals only
+ * with Quasi Closed / Closed container.
+ */
+public class ReplicationManager {
+
+ private static final Logger LOG =
+ LoggerFactory.getLogger(ReplicationManager.class);
+
+ /**
+ * Reference to the ContainerManager.
+ */
+ private final ContainerManager containerManager;
+
+ /**
+ * PlacementPolicy which is used to identify where a container
+ * should be copied.
+ */
+ private final ContainerPlacementPolicy containerPlacement;
+
+ /**
+ * EventPublisher to fire Replicate and Delete container commands.
+ */
+ private final EventPublisher eventPublisher;
+
+ /**
+ * Used for locking a container with its ID while processing it.
+ */
+ private final LockManager<ContainerID> lockManager;
+
+ /**
+ * This is used to track container replication commands which are issued
+ * by ReplicationManager and not yet complete.
+ */
+ private final Map<ContainerID, List<InflightAction>> inflightReplication;
+
+ /**
+ * This is used to track container deletion commands which are issued
+ * by ReplicationManager and not yet complete.
+ */
+ private final Map<ContainerID, List<InflightAction>> inflightDeletion;
+
+ /**
+ * ReplicationMonitor thread is the one which wakes up at configured
+ * interval and processes all the containers.
+ */
+ private final Thread replicationMonitor;
+
+ /**
+ * The frequency in which ReplicationMonitor thread should run.
+ */
+ private final long interval;
+
+ /**
+ * Timeout for container replication & deletion command issued by
+ * ReplicationManager.
+ */
+ private final long eventTimeout;
+
+ /**
+ * Flag used to check if ReplicationMonitor thread is running or not.
+ */
+ private volatile boolean running;
+
+ /**
+ * Constructs ReplicationManager instance with the given configuration.
+ *
+ * @param conf OzoneConfiguration
+ * @param containerManager ContainerManager
+ * @param containerPlacement ContainerPlacementPolicy
+ * @param eventPublisher EventPublisher
+ */
+ public ReplicationManager(final Configuration conf,
+ final ContainerManager containerManager,
+ final ContainerPlacementPolicy containerPlacement,
+ final EventPublisher eventPublisher) {
+ this.containerManager = containerManager;
+ this.containerPlacement = containerPlacement;
+ this.eventPublisher = eventPublisher;
+ this.lockManager = new LockManager<>(conf);
+ this.inflightReplication = new HashMap<>();
+ this.inflightDeletion = new HashMap<>();
+ this.replicationMonitor = new Thread(this::run);
+ this.replicationMonitor.setName("ReplicationMonitor");
+ this.interval = conf.getTimeDuration(
+ ScmConfigKeys.HDDS_SCM_REPLICATION_THREAD_INTERVAL,
+ ScmConfigKeys.HDDS_SCM_REPLICATION_THREAD_INTERVAL_DEFAULT,
+ TimeUnit.MILLISECONDS);
+ this.eventTimeout = conf.getTimeDuration(
+ ScmConfigKeys.HDDS_SCM_REPLICATION_EVENT_TIMEOUT,
+ ScmConfigKeys.HDDS_SCM_REPLICATION_EVENT_TIMEOUT_DEFAULT,
+ TimeUnit.MILLISECONDS);
+ this.running = false;
+ }
+
+ /**
+ * Starts Replication Monitor thread.
+ */
+ public void start() {
+ LOG.info("Starting Replication Monitor Thread.");
+ running = true;
+ replicationMonitor.start();
+ }
+
+ /**
+ * Process all the containers immediately.
+ */
+ @VisibleForTesting
+ void processContainersNow() {
+ containerManager.getContainerIDs()
+ .parallelStream().forEach(this::processContainer);
+ }
+
+ /**
+ * Stops Replication Monitor thread.
+ */
+ public void stop() {
+ LOG.info("Stopping Replication Monitor Thread.");
+ running = false;
+ replicationMonitor.interrupt();
+ }
+
+ /**
+ * ReplicationMonitor thread runnable. This wakes up at configured
+ * interval and processes all the containers in the system.
+ */
+ private void run() {
+ try {
+ while (running) {
+ try {
+ final long start = Time.monotonicNow();
+ final List<ContainerID> containerIds =
+ containerManager.getContainerIDs();
+ containerIds.parallelStream().forEach(this::processContainer);
+ LOG.debug("Replication Monitor Thread took {} milliseconds for" +
+ " processing {} containers.", Time.monotonicNow() - start,
+ containerIds.size());
+ Thread.sleep(interval);
+ } catch (InterruptedException ex) {
+ // Wakeup and process the containers.
+ LOG.debug("Replication Monitor Thread got interrupt exception.");
+ }
+ }
+ } catch (Throwable t) {
+ // When we get runtime exception, we should terminate SCM.
+ LOG.error("Exception in Replication Monitor Thread.", t);
+ ExitUtil.terminate(1, t);
+ }
+ }
+
+ /**
+ * Process the given container.
+ *
+ * @param id ContainerID
+ */
+ private void processContainer(ContainerID id) {
+ lockManager.lock(id);
+ try {
+ final ContainerInfo container = containerManager.getContainer(id);
+ final Set<ContainerReplica> replicas = containerManager
+ .getContainerReplicas(container.containerID());
+ final LifeCycleState state = container.getState();
+
+ /*
+ * We don't take any action if the container is in OPEN state.
+ */
+ if (state == LifeCycleState.OPEN) {
+ return;
+ }
+
+ /*
+ * If the container is in CLOSING state, the replicas can either
+ * be in OPEN or in CLOSING state. In both of this cases
+ * we have to resend close container command to the datanodes.
+ */
+ if (state == LifeCycleState.CLOSING) {
+ replicas.forEach(replica -> sendCloseCommand(
+ container, replica.getDatanodeDetails(), false));
+ return;
+ }
+
+ /*
+ * Before processing the container we have to reconcile the
+ * inflightReplication and inflightDeletion actions.
+ *
+ * We remove the entry from inflightReplication and inflightDeletion
+ * list, if the operation is completed or if it has timed out.
+ */
+ updateInflightAction(container, inflightReplication,
+ action -> replicas.stream()
+ .anyMatch(r -> r.getDatanodeDetails().equals(action.datanode)));
+
+ updateInflightAction(container, inflightDeletion,
+ action -> replicas.stream()
+ .noneMatch(r -> r.getDatanodeDetails().equals(action.datanode)));
+
+
+ /*
+ * If the container is in QUASI_CLOSED state, check and close the
+ * container if possible.
+ */
+ if (state == LifeCycleState.QUASI_CLOSED) {
+ forceCloseContainer(container, replicas);
+ }
+
+ /*
+ * We don't have to take any action if the container is healthy.
+ *
+ * According to ReplicationMonitor container is considered healthy if
+ * the container is either in QUASI_CLOSED or in CLOSED state and has
+ * exact number of replicas in the same state.
+ */
+ if (isContainerHealthy(container, replicas)) {
+ return;
+ }
+
+ /*
+ * Check if the container if under replicated and take appropriate
+ * action.
+ */
+ if (isContainerUnderReplicated(container, replicas)) {
+ handleUnderReplicatedContainer(container, replicas);
+ return;
+ }
+
+ /*
+ * Check if the container if over replicated and take appropriate
+ * action.
+ */
+ if (isContainerOverReplicated(container, replicas)) {
+ handleOverReplicatedContainer(container, replicas);
+ return;
+ }
+
+ /*
+ * The container is neither under nor over replicated and the container
+ * is not healthy. This means that the container has unhealthy/corrupted
+ * replica.
+ */
+ handleInconsistentContainer(container, replicas);
+
+ } catch (ContainerNotFoundException ex) {
+ LOG.warn("Missing container {}.", id);
+ } finally {
+ lockManager.unlock(id);
+ }
+ }
+
+ /**
+ * Reconciles the InflightActions for a given container.
+ *
+ * @param container Container to update
+ * @param inflightActions inflightReplication (or) inflightDeletion
+ * @param filter filter to check if the operation is completed
+ */
+ private void updateInflightAction(final ContainerInfo container,
+ final Map<ContainerID, List<InflightAction>> inflightActions,
+ final Predicate<InflightAction> filter) {
+ final ContainerID id = container.containerID();
+ final long deadline = Time.monotonicNow() - eventTimeout;
+ if (inflightActions.containsKey(id)) {
+ final List<InflightAction> actions = inflightActions.get(id);
+ actions.removeIf(action -> action.time < deadline);
+ actions.removeIf(filter);
+ if (actions.isEmpty()) {
+ inflightActions.remove(id);
+ }
+ }
+ }
+
+ /**
+ * Returns true if the container is healthy according to ReplicationMonitor.
+ *
+ * According to ReplicationMonitor container is considered healthy if
+ * it has exact number of replicas in the same state as the container.
+ *
+ * @param container Container to check
+ * @param replicas Set of ContainerReplicas
+ * @return true if the container is healthy, false otherwise
+ */
+ private boolean isContainerHealthy(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ return container.getReplicationFactor().getNumber() == replicas.size() &&
+ replicas.stream().allMatch(
+ r -> compareState(container.getState(), r.getState()));
+ }
+
+ /**
+ * Checks if the container is under replicated or not.
+ *
+ * @param container Container to check
+ * @param replicas Set of ContainerReplicas
+ * @return true if the container is under replicated, false otherwise
+ */
+ private boolean isContainerUnderReplicated(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ return container.getReplicationFactor().getNumber() >
+ getReplicaCount(container.containerID(), replicas);
+ }
+
+ /**
+ * Checks if the container is over replicated or not.
+ *
+ * @param container Container to check
+ * @param replicas Set of ContainerReplicas
+ * @return true if the container if over replicated, false otherwise
+ */
+ private boolean isContainerOverReplicated(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ return container.getReplicationFactor().getNumber() <
+ getReplicaCount(container.containerID(), replicas);
+ }
+
+ /**
+ * Returns the replication count of the given container. This also
+ * considers inflight replication and deletion.
+ *
+ * @param id ContainerID
+ * @param replicas Set of existing replicas
+ * @return number of estimated replicas for this container
+ */
+ private int getReplicaCount(final ContainerID id,
+ final Set<ContainerReplica> replicas) {
+ return replicas.size()
+ + inflightReplication.getOrDefault(id, Collections.emptyList()).size()
+ - inflightDeletion.getOrDefault(id, Collections.emptyList()).size();
+ }
+
+ /**
+ * Force close the container replica(s) if possible.
+ *
+ * <p>
+ * If <50% of container replicas are in QUASI_CLOSED state and all
+ * the other replica are either in OPEN or CLOSING state, do nothing.
+ * We cannot identify the correct replica since we don't have quorum
+ * yet.
+ * </p>
+ *
+ * <p>
+ * If >50% (quorum) of replicas are in QUASI_CLOSED state, try to identify
+ * the latest container replica using originNodeId and sequenceId.
+ * Force close those replica(s) which have the latest sequenceId.
+ * </p>
+ *
+ * @param container ContainerInfo
+ * @param replicas Set of ContainerReplicas
+ */
+ private void forceCloseContainer(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ Preconditions.assertTrue(container.getState() ==
+ LifeCycleState.QUASI_CLOSED);
+ final int replicationFactor = container.getReplicationFactor().getNumber();
+ final List<ContainerReplica> quasiClosedReplicas = replicas.stream()
+ .filter(r -> r.getState() == State.QUASI_CLOSED)
+ .collect(Collectors.toList());
+ final long uniqueQuasiClosedReplicaCount = quasiClosedReplicas
+ .stream()
+ .map(ContainerReplica::getOriginDatanodeId)
+ .distinct()
+ .count();
+ if (uniqueQuasiClosedReplicaCount > (replicationFactor / 2)) {
+ // Quorum of unique replica has been QUASI_CLOSED
+ final Long sequenceId = quasiClosedReplicas.stream()
+ .map(ContainerReplica::getSequenceId)
+ .max(Long::compare)
+ .orElse(-1L);
+ quasiClosedReplicas.stream()
+ .filter(r -> sequenceId != -1L)
+ .filter(replica -> replica.getSequenceId().equals(sequenceId))
+ .forEach(replica -> sendCloseCommand(
+ container, replica.getDatanodeDetails(), true));
+ }
+ }
+
+ /**
+ * If the given container is under replicated, identify a new set of
+ * datanode(s) to replicate the container using ContainerPlacementPolicy
+ * and send replicate container command to the identified datanode(s).
+ *
+ * @param container ContainerInfo
+ * @param replicas Set of ContainerReplicas
+ */
+ private void handleUnderReplicatedContainer(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ try {
+ final ContainerID id = container.containerID();
+ final List<DatanodeDetails> deletionInFlight = inflightDeletion
+ .getOrDefault(id, Collections.emptyList())
+ .stream()
+ .map(action -> action.datanode)
+ .collect(Collectors.toList());
+ final List<DatanodeDetails> source = replicas.stream()
+ .filter(r ->
+ r.getState() == State.QUASI_CLOSED ||
+ r.getState() == State.CLOSED)
+ .filter(r -> !deletionInFlight.contains(r.getDatanodeDetails()))
+ .sorted((r1, r2) -> r2.getSequenceId().compareTo(r1.getSequenceId()))
+ .map(ContainerReplica::getDatanodeDetails)
+ .collect(Collectors.toList());
+ if (source.size() > 0) {
+ final int deficit = container.getReplicationFactor().getNumber() -
+ getReplicaCount(id, replicas);
+ final List<DatanodeDetails> selectedDatanodes = containerPlacement
+ .chooseDatanodes(source, deficit, container.getUsedBytes());
+ inflightReplication.computeIfAbsent(id, k -> new ArrayList<>());
+ for (DatanodeDetails datanode : selectedDatanodes) {
+ sendReplicateCommand(container, datanode, source);
+ }
+ } else {
+ LOG.warn("Cannot replicate container {}, no healthy replica found.",
+ container.containerID());
+ }
+ } catch (IOException ex) {
+ LOG.warn("Exception while replicating container {}.",
+ container.getContainerID(), ex);
+ }
+ }
+
+ /**
+ * If the given container is over replicated, identify the datanode(s)
+ * to delete the container and send delete container command to the
+ * identified datanode(s).
+ *
+ * @param container ContainerInfo
+ * @param replicas Set of ContainerReplicas
+ */
+ private void handleOverReplicatedContainer(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+
+ final ContainerID id = container.containerID();
+ final int replicationFactor = container.getReplicationFactor().getNumber();
+ // Dont consider inflight replication while calculating deficit here.
+ final int deficit = replicas.size() - replicationFactor -
+ inflightDeletion.getOrDefault(id, Collections.emptyList()).size();
+
+ if (deficit > 0) {
+ final Map<UUID, ContainerReplica> uniqueReplicas =
+ new LinkedHashMap<>();
+
+ replicas.stream()
+ .filter(r -> compareState(container.getState(), r.getState()))
+ .forEach(r -> uniqueReplicas
+ .putIfAbsent(r.getOriginDatanodeId(), r));
+
+ // Retain one healthy replica per origin node Id.
+ final List<ContainerReplica> eligibleReplicas = new ArrayList<>(replicas);
+ eligibleReplicas.removeAll(uniqueReplicas.values());
+
+ final List<ContainerReplica> unhealthyReplicas = eligibleReplicas
+ .stream()
+ .filter(r -> !compareState(container.getState(), r.getState()))
+ .collect(Collectors.toList());
+
+ //Move the unhealthy replicas to the front of eligible replicas to delete
+ eligibleReplicas.removeAll(unhealthyReplicas);
+ eligibleReplicas.addAll(0, unhealthyReplicas);
+
+ inflightDeletion.computeIfAbsent(id, k -> new ArrayList<>());
+ for (int i = 0; i < deficit; i++) {
+ sendDeleteCommand(container,
+ eligibleReplicas.get(i).getDatanodeDetails(), true);
+ }
+ }
+ }
+
+ /**
+ * Handles inconsistent container.
+ * A container is inconsistent if any of the replica state doesn't
+ * match the container state. We have to take appropriate action
+ * based on state of the replica.
+ *
+ * @param container ContainerInfo
+ * @param replicas Set of ContainerReplicas
+ */
+ private void handleInconsistentContainer(final ContainerInfo container,
+ final Set<ContainerReplica> replicas) {
+ // Find unhealthy replicas
+ List<ContainerReplica> unhealthyReplicas = replicas.stream()
+ .filter(r -> !compareState(container.getState(), r.getState()))
+ .collect(Collectors.toList());
+
+ Iterator<ContainerReplica> iterator = unhealthyReplicas.iterator();
+ while (iterator.hasNext()) {
+ final ContainerReplica replica = iterator.next();
+ final State state = replica.getState();
+ if (state == State.OPEN || state == State.CLOSING) {
+ sendCloseCommand(container, replica.getDatanodeDetails(), false);
+ iterator.remove();
+ }
+
+ if (state == State.QUASI_CLOSED) {
+ // Send force close command if the BCSID matches
+ if (container.getSequenceId() == replica.getSequenceId()) {
+ sendCloseCommand(container, replica.getDatanodeDetails(), true);
+ iterator.remove();
+ }
+ }
+ }
+
+ // Now we are left with the replicas which are either unhealthy or
+ // the BCSID doesn't match. These replicas should be deleted.
+
+ /*
+ * If we have unhealthy replicas we go under replicated and then
+ * replicate the healthy copy.
+ *
+ * We also make sure that we delete only one unhealthy replica at a time.
+ *
+ * If there are two unhealthy replica:
+ * - Delete first unhealthy replica
+ * - Re-replicate the healthy copy
+ * - Delete second unhealthy replica
+ * - Re-replicate the healthy copy
+ *
+ * Note: Only one action will be executed in a single ReplicationMonitor
+ * iteration. So to complete all the above actions we need four
+ * ReplicationMonitor iterations.
+ */
+
+ unhealthyReplicas.stream().findFirst().ifPresent(replica ->
+ sendDeleteCommand(container, replica.getDatanodeDetails(), false));
+
+ }
+
+ /**
+ * Sends close container command for the given container to the given
+ * datanode.
+ *
+ * @param container Container to be closed
+ * @param datanode The datanode on which the container
+ * has to be closed
+ * @param force Should be set to true if we want to close a
+ * QUASI_CLOSED container
+ */
+ private void sendCloseCommand(final ContainerInfo container,
+ final DatanodeDetails datanode,
+ final boolean force) {
+ CloseContainerCommand closeContainerCommand =
+ new CloseContainerCommand(container.getContainerID(),
+ container.getPipelineID(), force);
+ eventPublisher.fireEvent(SCMEvents.DATANODE_COMMAND,
+ new CommandForDatanode<>(datanode.getUuid(), closeContainerCommand));
+ }
+
+ /**
+ * Sends replicate container command for the given container to the given
+ * datanode.
+ *
+ * @param container Container to be replicated
+ * @param datanode The destination datanode to replicate
+ * @param sources List of source nodes from where we can replicate
+ */
+ private void sendReplicateCommand(final ContainerInfo container,
+ final DatanodeDetails datanode,
+ final List<DatanodeDetails> sources) {
+
+ final ContainerID id = container.containerID();
+ final ReplicateContainerCommand replicateCommand =
+ new ReplicateContainerCommand(id.getId(), sources);
+ eventPublisher.fireEvent(SCMEvents.DATANODE_COMMAND,
+ new CommandForDatanode<>(datanode.getUuid(), replicateCommand));
+ inflightReplication.computeIfAbsent(id, k -> new ArrayList<>());
+ inflightReplication.get(id).add(
+ new InflightAction(datanode, Time.monotonicNow()));
+ }
+
+ /**
+ * Sends delete container command for the given container to the given
+ * datanode.
+ *
+ * @param container Container to be deleted
+ * @param datanode The datanode on which the replica should be deleted
+ * @param force Should be set to true to delete an OPEN replica
+ */
+ private void sendDeleteCommand(final ContainerInfo container,
+ final DatanodeDetails datanode,
+ final boolean force) {
+ final ContainerID id = container.containerID();
+ final DeleteContainerCommand deleteCommand =
+ new DeleteContainerCommand(container.getContainerID(), force);
+ eventPublisher.fireEvent(SCMEvents.DATANODE_COMMAND,
+ new CommandForDatanode<>(datanode.getUuid(), deleteCommand));
+ inflightDeletion.computeIfAbsent(id, k -> new ArrayList<>());
+ inflightDeletion.get(id).add(
+ new InflightAction(datanode, Time.monotonicNow()));
+ }
+
+ /**
+ * Compares the container state with the replica state.
+ *
+ * @param containerState ContainerState
+ * @param replicaState ReplicaState
+ * @return true if the state matches, false otherwise
+ */
+ private static boolean compareState(final LifeCycleState containerState,
+ final State replicaState) {
+ switch (containerState) {
+ case OPEN:
+ return replicaState == State.OPEN;
+ case CLOSING:
+ return replicaState == State.CLOSING;
+ case QUASI_CLOSED:
+ return replicaState == State.QUASI_CLOSED;
+ case CLOSED:
+ return replicaState == State.CLOSED;
+ case DELETING:
+ return false;
+ case DELETED:
+ return false;
+ default:
+ return false;
+ }
+ }
+
+ /**
+ * Wrapper class to hold the InflightAction with its start time.
+ */
+ private static final class InflightAction {
+
+ private final DatanodeDetails datanode;
Review comment:
For in-flight replication, is this the source datanode or the destination datanode? It will probably be clearer from reading the rest of the code.
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