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.
---
