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From StefanRRichter <...@git.apache.org>
Subject [GitHub] flink pull request #5239: [FLINK-8360] Implement task-local state recovery
Date Thu, 15 Feb 2018 17:13:54 GMT
Github user StefanRRichter commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5239#discussion_r168545164
  
    --- Diff: flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/BackendRestorerProcedure.java
---
    @@ -0,0 +1,143 @@
    +/*
    + * 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.streaming.api.operators;
    +
    +import org.apache.flink.core.fs.CloseableRegistry;
    +import org.apache.flink.runtime.checkpoint.StateObjectCollection;
    +import org.apache.flink.runtime.state.Snapshotable;
    +import org.apache.flink.runtime.state.StateObject;
    +import org.apache.flink.util.Disposable;
    +import org.apache.flink.util.ExceptionUtils;
    +import org.apache.flink.util.Preconditions;
    +import org.apache.flink.util.ThrowingSupplier;
    +
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +import javax.annotation.Nonnull;
    +
    +import java.io.Closeable;
    +import java.io.IOException;
    +import java.util.Iterator;
    +
    +/**
    + * This class implements the logic that creates (and potentially restores) a state backend.
The restore logic
    + * considers multiple, prioritized options of snapshots to restore from, where all of
the options should recreate
    + * the same state for the backend. When we fail to restore from the snapshot with the
highest priority (typically
    + * the "fastest" to restore), we fallback to the next snapshot with the next highest
priority. We also take care
    + * of cleaning up from failed restore attempts. We only reattempt when the problem occurs
during the restore call
    + * and will only stop after all snapshot alternatives are exhausted and all failed.
    + *
    + * @param <T> type of the restored backend.
    + * @param <S> type of the supplied snapshots from which the backend restores.
    + */
    +public class BackendRestorerProcedure<
    +	T extends Closeable & Disposable & Snapshotable<?, StateObjectCollection<S>>,
    +	S extends StateObject> {
    +
    +	/** Logger for this class. */
    +	private static final Logger LOG = LoggerFactory.getLogger(BackendRestorerProcedure.class);
    +
    +	/** Factory for new, fresh backends without state. */
    +	private final ThrowingSupplier<T, Exception> instanceSupplier;
    +
    +	/** This registry is used so that recovery can participate in the task lifecycle, i.e.
can be canceled. */
    +	private final CloseableRegistry backendCloseableRegistry;
    +
    +	/**
    +	 * Creates a new backend restorer using the given backend supplier and the closeable
registry.
    +	 *
    +	 * @param instanceSupplier factory function for new, empty backend instances.
    +	 * @param backendCloseableRegistry registry to allow participation in task lifecycle,
e.g. react to cancel.
    +	 */
    +	public BackendRestorerProcedure(
    +		@Nonnull ThrowingSupplier<T, Exception> instanceSupplier,
    +		@Nonnull CloseableRegistry backendCloseableRegistry) {
    +
    +		this.instanceSupplier = Preconditions.checkNotNull(instanceSupplier);
    +		this.backendCloseableRegistry = Preconditions.checkNotNull(backendCloseableRegistry);
    +	}
    +
    +	/**
    +	 * Creates a new state backend and restores it from the provided set of state snapshot
alternatives.
    +	 *
    +	 * @param restoreOptions iterator over a prioritized set of state snapshot alternatives
for recovery.
    +	 * @return the created (and restored) state backend.
    +	 * @throws Exception if the backend could not be created or restored.
    +	 */
    +	public @Nonnull T createAndRestore(@Nonnull Iterator<StateObjectCollection<S>>
restoreOptions) throws Exception {
    +
    +		StateObjectCollection<S> restoreState = null;
    +
    +		boolean retry;
    +
    +		while (true) {
    --- End diff --
    
    It is not quiet that easy, because the restore method must always be called at least once
and also the log message might differ between iterations (the "will retry" part). But I agree
it could be simplified.


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