Github user zentol commented on a diff in the pull request:
https://github.com/apache/flink/pull/4353#discussion_r127708906
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/OperatorSubtaskState.java
---
@@ -18,20 +18,40 @@
package org.apache.flink.runtime.checkpoint;
+import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.runtime.state.CompositeStateHandle;
import org.apache.flink.runtime.state.KeyedStateHandle;
import org.apache.flink.runtime.state.OperatorStateHandle;
import org.apache.flink.runtime.state.SharedStateRegistry;
import org.apache.flink.runtime.state.StateObject;
import org.apache.flink.runtime.state.StateUtil;
import org.apache.flink.runtime.state.StreamStateHandle;
+import org.apache.flink.util.Preconditions;
+
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
-import java.util.Arrays;
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
/**
- * Container for the state of one parallel subtask of an operator. This is
part of the {@link OperatorState}.
+ * This class encapsulates the state for one parallel instance of an
operator. The complete state of a (logical)
+ * operator (e.g. a flatmap operator) consists of the union of all {@link
OperatorSubtaskState}s from all
+ * parallel tasks that physically execute parallelized, physical instances
of the operator.
+ * <p>The full state of the logical operator is represented by {@link
OperatorState} which consists of
+ * {@link OperatorSubtaskState}s.
+ * <p>Typically, we expect all collections in this class to be of size 0
or 1, because there up to one state handle
+ * produced per state type (e.g. managed-keyed, raw-operator, ...). In
particular, this holds when taking a snapshot.
+ * The purpose of having the state handles in collections is that this
class is also reused in restoring state.
+ * Under normal circumstances, the expected size of each collection is
still 0 or 1, except for scale-down. In
+ * scale-down, one operator subtask can become responsible for the state
of multiple previous subtasks. The collections
+ * can then store all the state handles that are relevant to build up the
new subtask state.
+ * <p>There is no collection for legacy state because it is nor rescalable.
--- End diff --
typo: nor -> not
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