[GitHub] [flink] aljoscha commented on a change in pull request #13550: [FLINK-19474] Implement a state backend that holds a single key at a time

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aljoscha commented on a change in pull request #13550:
URL: https://github.com/apache/flink/pull/13550#discussion_r502326914



##########
File path: flink-state-backends/flink-statebackend-rocksdb/src/test/java/org/apache/flink/contrib/streaming/state/benchmark/StateBackendBenchmarkUtils.java
##########
@@ -74,6 +75,24 @@
 			case ROCKSDB:
 				rootDir = prepareDirectory(rootDirName, null);
 				return createRocksDBKeyedStateBackend(rootDir);
+			case SINGLE_KEY:
+				try {
+					return new SingleKeyStateBackend().createKeyedStateBackend(

Review comment:
       Should also be in a method like for the other backends.

##########
File path: flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/sorted/state/SingleKeyKeyGroupedInternalPriorityQueue.java
##########
@@ -0,0 +1,78 @@
+/*
+ * 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.sorted.state;
+
+import org.apache.flink.runtime.state.KeyGroupedInternalPriorityQueue;
+import org.apache.flink.runtime.state.PriorityComparator;
+import org.apache.flink.runtime.state.heap.HeapPriorityQueue;
+import org.apache.flink.runtime.state.heap.HeapPriorityQueueElement;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Very similar implementation to {@link org.apache.flink.runtime.state.heap.HeapPriorityQueueSet}. The only difference
+ * is it keeps track of elements for a single key at a time.
+ */
+class SingleKeyKeyGroupedInternalPriorityQueue<T extends HeapPriorityQueueElement>
+		extends HeapPriorityQueue<T>
+		implements KeyGroupedInternalPriorityQueue<T> {
+
+	private final Map<T, T> dedupMap = new HashMap<>();

Review comment:
       Why do we need to additionally deduplicate?

##########
File path: flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/sorted/state/SingleKeyKeyedStateBackend.java
##########
@@ -0,0 +1,260 @@
+/*
+ * 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.sorted.state;
+
+import org.apache.flink.api.common.ExecutionConfig;
+import org.apache.flink.api.common.state.AggregatingStateDescriptor;
+import org.apache.flink.api.common.state.ListStateDescriptor;
+import org.apache.flink.api.common.state.MapStateDescriptor;
+import org.apache.flink.api.common.state.ReducingStateDescriptor;
+import org.apache.flink.api.common.state.State;
+import org.apache.flink.api.common.state.StateDescriptor;
+import org.apache.flink.api.common.state.ValueStateDescriptor;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.runtime.checkpoint.CheckpointOptions;
+import org.apache.flink.runtime.state.CheckpointStreamFactory;
+import org.apache.flink.runtime.state.CheckpointableKeyedStateBackend;
+import org.apache.flink.runtime.state.KeyGroupRange;
+import org.apache.flink.runtime.state.KeyGroupedInternalPriorityQueue;
+import org.apache.flink.runtime.state.Keyed;
+import org.apache.flink.runtime.state.KeyedStateFunction;
+import org.apache.flink.runtime.state.KeyedStateHandle;
+import org.apache.flink.runtime.state.PriorityComparable;
+import org.apache.flink.runtime.state.PriorityComparator;
+import org.apache.flink.runtime.state.SnapshotResult;
+import org.apache.flink.runtime.state.StateSnapshotTransformer;
+import org.apache.flink.runtime.state.heap.HeapPriorityQueueElement;
+import org.apache.flink.runtime.state.internal.InternalKvState;
+import org.apache.flink.util.FlinkRuntimeException;
+
+import javax.annotation.Nonnull;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.RunnableFuture;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * A simple {@link CheckpointableKeyedStateBackend} which keeps values for a single key at a time.

Review comment:
       ```suggestion
    * A {@link CheckpointableKeyedStateBackend} which keeps values for a single key at a time.
   ```
   
   I think `simple` rarely adds much information.

##########
File path: flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/sorted/state/SingleKeyKeyedStateBackend.java
##########
@@ -0,0 +1,260 @@
+/*
+ * 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.sorted.state;
+
+import org.apache.flink.api.common.ExecutionConfig;
+import org.apache.flink.api.common.state.AggregatingStateDescriptor;
+import org.apache.flink.api.common.state.ListStateDescriptor;
+import org.apache.flink.api.common.state.MapStateDescriptor;
+import org.apache.flink.api.common.state.ReducingStateDescriptor;
+import org.apache.flink.api.common.state.State;
+import org.apache.flink.api.common.state.StateDescriptor;
+import org.apache.flink.api.common.state.ValueStateDescriptor;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.runtime.checkpoint.CheckpointOptions;
+import org.apache.flink.runtime.state.CheckpointStreamFactory;
+import org.apache.flink.runtime.state.CheckpointableKeyedStateBackend;
+import org.apache.flink.runtime.state.KeyGroupRange;
+import org.apache.flink.runtime.state.KeyGroupedInternalPriorityQueue;
+import org.apache.flink.runtime.state.Keyed;
+import org.apache.flink.runtime.state.KeyedStateFunction;
+import org.apache.flink.runtime.state.KeyedStateHandle;
+import org.apache.flink.runtime.state.PriorityComparable;
+import org.apache.flink.runtime.state.PriorityComparator;
+import org.apache.flink.runtime.state.SnapshotResult;
+import org.apache.flink.runtime.state.StateSnapshotTransformer;
+import org.apache.flink.runtime.state.heap.HeapPriorityQueueElement;
+import org.apache.flink.runtime.state.internal.InternalKvState;
+import org.apache.flink.util.FlinkRuntimeException;
+
+import javax.annotation.Nonnull;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.RunnableFuture;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * A simple {@link CheckpointableKeyedStateBackend} which keeps values for a single key at a time.
+ *
+ * <p><b>IMPORTANT:</b> Requires the incoming records to be sorted/grouped by the key. Used in a BATCH style execution.
+ */
+class SingleKeyKeyedStateBackend<K> implements CheckpointableKeyedStateBackend<K> {
+	@SuppressWarnings("rawtypes")
+	private static final Map<Class<? extends StateDescriptor>, StateFactory> STATE_FACTORIES =
+		Stream.of(
+			Tuple2.of(ValueStateDescriptor.class, (StateFactory) SingleKeyValueState::create),
+			Tuple2.of(ListStateDescriptor.class, (StateFactory) SingleKeyListState::create),
+			Tuple2.of(MapStateDescriptor.class, (StateFactory) SingleKeyMapState::create),
+			Tuple2.of(AggregatingStateDescriptor.class, (StateFactory) SingleKeyAggregatingState::create),
+			Tuple2.of(ReducingStateDescriptor.class, (StateFactory) SingleKeyReducingState::create)
+		).collect(Collectors.toMap(t -> t.f0, t -> t.f1));
+
+	private K currentKey = null;
+	private final TypeSerializer<K> keySerializer;
+	private final List<KeySelectionListener<K>> keySelectionListeners = new ArrayList<>();
+	private final Map<String, State> states = new HashMap<>();
+	private final Map<String, KeyGroupedInternalPriorityQueue<?>> priorityQueues = new HashMap<>();
+	private final KeyGroupRange keyGroupRange;
+
+	public SingleKeyKeyedStateBackend(
+			TypeSerializer<K> keySerializer,
+			KeyGroupRange keyGroupRange) {
+		this.keySerializer = keySerializer;
+		this.keyGroupRange = keyGroupRange;
+	}
+
+	@Override
+	public void setCurrentKey(K newKey) {
+		if (!Objects.equals(newKey, currentKey)) {
+			notifyKeySelected(newKey);
+			for (State value : states.values()) {
+				((AbstractSingleKeyState<?, ?, ?>) value).clearAllNamespaces();
+			}
+			for (KeyGroupedInternalPriorityQueue<?> value : priorityQueues.values()) {
+				while (value.poll() != null) {
+					// remove everything for the key
+				}
+			}
+			this.currentKey = newKey;
+		}
+	}
+
+	@Override
+	public K getCurrentKey() {
+		return currentKey;
+	}
+
+	@Override
+	public TypeSerializer<K> getKeySerializer() {
+		return keySerializer;
+	}
+
+	@Override
+	public <N, S extends State, T> void applyToAllKeys(
+			N namespace,
+			TypeSerializer<N> namespaceSerializer,
+			StateDescriptor<S, T> stateDescriptor,
+			KeyedStateFunction<K, S> function) {
+		throw new UnsupportedOperationException("applyToAllKeys() is not supported in BATCH execution mode.");

Review comment:
       I'm wondering about the name here. If we always refer to `BATCH execution mode` in the exceptions, maybe the state backend should be called `BatchExecutionStateBackend`. Or do we think it could be used for any other purpose.




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