Github user StephanEwen commented on a diff in the pull request:
https://github.com/apache/flink/pull/3524#discussion_r108671962
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/state/SharedStateRegistry.java
---
@@ -0,0 +1,192 @@
+/*
+ * 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.java.tuple.Tuple2;
+
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * A {@code SharedStateRegistry} will be deployed in the
+ * {@link org.apache.flink.runtime.checkpoint.CheckpointCoordinator} to
+ * maintain the reference count of those state objects shared among
different
+ * checkpoints. Each shared state object must be identified by a unique
key.
+ */
+public class SharedStateRegistry implements Serializable {
+
+ private static final long serialVersionUID = -8357254413007773970L;
+
+ /** All registered state objects */
+ private final Map<String, Tuple2<StateObject, Integer>>
registeredStates = new HashMap<>();
+
+ /** All state objects that are not referenced any more */
+ private transient final List<StateObject> discardedStates = new
ArrayList<>();
+
+ /**
+ * Register the state in the registry
+ *
+ * @param key The key of the state to register
+ * @param state The state to register
+ */
+ public void register(String key, StateObject state) {
+ Tuple2<StateObject, Integer> stateAndRefCnt =
registeredStates.get(key);
+
+ if (stateAndRefCnt == null) {
+ registeredStates.put(key, new Tuple2<>(state, 1));
+ } else {
+ if (!stateAndRefCnt.f0.equals(state)) {
--- End diff --
This is a nice idea, but I think that `equals()` on state objects does not
always work very well. On `ByteStreamHandle`, it is a bit expensive (compare
all bytes, which may be up to a few megabytes) and on `FileStreamHandle` it
assumes that the paths are normalized (upper/lower case, slashes, authority or
no authority, ...). That's why I suggested to have a dedicated key. Then we
only requite the key to be "normalized" so it behaved well for `equals()`.
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