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https://issues.apache.org/jira/browse/FLINK-8360?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16364126#comment-16364126
 ] 

ASF GitHub Bot commented on FLINK-8360:
---------------------------------------

Github user sihuazhou commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5239#discussion_r168184247
  
    --- Diff: 
flink-runtime/src/main/java/org/apache/flink/runtime/state/TaskLocalStateStoreImpl.java
 ---
    @@ -0,0 +1,298 @@
    +/*
    + * 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.state;
    +
    +import org.apache.flink.annotation.VisibleForTesting;
    +import org.apache.flink.api.common.JobID;
    +import org.apache.flink.core.fs.FileSystem;
    +import org.apache.flink.core.fs.Path;
    +import org.apache.flink.runtime.checkpoint.TaskStateSnapshot;
    +import org.apache.flink.runtime.clusterframework.types.AllocationID;
    +import org.apache.flink.runtime.jobgraph.JobVertexID;
    +
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +import javax.annotation.Nonnegative;
    +import javax.annotation.Nonnull;
    +import javax.annotation.Nullable;
    +import javax.annotation.concurrent.GuardedBy;
    +
    +import java.io.File;
    +import java.io.IOException;
    +import java.util.ArrayList;
    +import java.util.Collection;
    +import java.util.HashMap;
    +import java.util.Iterator;
    +import java.util.List;
    +import java.util.Map;
    +import java.util.SortedMap;
    +import java.util.TreeMap;
    +import java.util.concurrent.ConcurrentHashMap;
    +import java.util.concurrent.Executor;
    +
    +/**
    + * Main implementation of a {@link TaskLocalStateStore}.
    + */
    +public class TaskLocalStateStoreImpl implements TaskLocalStateStore {
    +
    +   /** Logger for this class. */
    +   private static final Logger LOG = 
LoggerFactory.getLogger(TaskLocalStateStoreImpl.class);
    +
    +   /** Maximum number of retained snapshots. */
    +   @VisibleForTesting
    +   static final int MAX_RETAINED_SNAPSHOTS = 5;
    +
    +   /** Dummy value to use instead of null to satisfy {@link 
ConcurrentHashMap}. */
    +   private final TaskStateSnapshot NULL_DUMMY = new TaskStateSnapshot();
    +
    +   /** JobID from the owning subtask. */
    +   @Nonnull
    +   private final JobID jobID;
    +
    +   /** AllocationID of the owning slot. */
    +   @Nonnull
    +   private final AllocationID allocationID;
    +
    +   /** JobVertexID of the owning subtask. */
    +   @Nonnull
    +   private final JobVertexID jobVertexID;
    +
    +   /** Subtask index of the owning subtask. */
    +   @Nonnegative
    +   private final int subtaskIndex;
    +
    +   /** The configured mode for local recovery. */
    +   @Nonnull
    +   private final LocalRecoveryConfig localRecoveryConfig;
    +
    +   /** Executor that runs the discarding of released state objects. */
    +   @Nonnull
    +   private final Executor discardExecutor;
    +
    +   /** Lock for synchronisation on the storage map and the discarded 
status. */
    +   @Nonnull
    +   private final Object lock;
    +
    +   /** Status flag if this store was already discarded. */
    +   @GuardedBy("lock")
    +   private boolean discarded;
    +
    +   /** Maps checkpoint ids to local TaskStateSnapshots. */
    +   @Nonnull
    +   @GuardedBy("lock")
    +   private final SortedMap<Long, TaskStateSnapshot> 
storedTaskStateByCheckpointID;
    +
    +   public TaskLocalStateStoreImpl(
    +           @Nonnull JobID jobID,
    +           @Nonnull AllocationID allocationID,
    +           @Nonnull JobVertexID jobVertexID,
    +           @Nonnegative int subtaskIndex,
    +           @Nonnull LocalRecoveryConfig localRecoveryConfig,
    +           @Nonnull Executor discardExecutor) {
    +
    +           this.jobID = jobID;
    +           this.allocationID = allocationID;
    +           this.jobVertexID = jobVertexID;
    +           this.subtaskIndex = subtaskIndex;
    +           this.discardExecutor = discardExecutor;
    +           this.lock = new Object();
    +           this.storedTaskStateByCheckpointID = new TreeMap<>();
    +           this.discarded = false;
    +           this.localRecoveryConfig = localRecoveryConfig;
    +   }
    +
    +   @Override
    +   public void storeLocalState(
    +           @Nonnegative long checkpointId,
    +           @Nullable TaskStateSnapshot localState) {
    +
    +           if (localState == null) {
    +                   localState = NULL_DUMMY;
    +           }
    +
    +           LOG.info("Storing local state for checkpoint {}.", 
checkpointId);
    +           LOG.debug("Local state for checkpoint {} is {}.", checkpointId, 
localState);
    +
    +           Map<Long, TaskStateSnapshot> toDiscard = new 
HashMap<>(MAX_RETAINED_SNAPSHOTS);
    +
    +           synchronized (lock) {
    +                   if (discarded) {
    +                           // we ignore late stores and simply discard the 
state.
    +                           toDiscard.put(checkpointId, localState);
    +                   } else {
    +                           TaskStateSnapshot previous =
    +                                   
storedTaskStateByCheckpointID.put(checkpointId, localState);
    +
    +                           if (previous != null) {
    +                                   toDiscard.put(checkpointId, previous);
    +                           }
    +
    +                           // remove from history.
    +                           while (storedTaskStateByCheckpointID.size() > 
MAX_RETAINED_SNAPSHOTS) {
    +                                   Long removeCheckpointID = 
storedTaskStateByCheckpointID.firstKey();
    --- End diff --
    
    I found this can be problematic, even this method has been invoked but it 
doesn't mean the checkpoint is truly completed. If we perform remove local task 
state here we may removed the `really completed local checkpoint data`. Image 
the situation when `max concurrent checkpoint number = MAX_RETAINED_SNAPSHOTS + 
1`...


> Implement task-local state recovery
> -----------------------------------
>
>                 Key: FLINK-8360
>                 URL: https://issues.apache.org/jira/browse/FLINK-8360
>             Project: Flink
>          Issue Type: New Feature
>          Components: State Backends, Checkpointing
>            Reporter: Stefan Richter
>            Assignee: Stefan Richter
>            Priority: Major
>             Fix For: 1.5.0
>
>
> This issue tracks the development of recovery from task-local state. The main 
> idea is to have a secondary, local copy of the checkpointed state, while 
> there is still a primary copy in DFS that we report to the checkpoint 
> coordinator.
> Recovery can attempt to restore from the secondary local copy, if available, 
> to save network bandwidth. This requires that the assignment from tasks to 
> slots is as sticky is possible.
> For starters, we will implement this feature for all managed keyed states and 
> can easily enhance it to all other state types (e.g. operator state) later.



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