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diff --git a/src/main/java/com/google/devtools/build/skyframe/ShareabilityOfValue.java b/src/main/java/com/google/devtools/build/skyframe/ShareabilityOfValue.java
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+++ b/src/main/java/com/google/devtools/build/skyframe/ShareabilityOfValue.java
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+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// Licensed 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 com.google.devtools.build.skyframe;
+
+/**
+ * When the {@link NodeEntry#getValue} corresponding to a given {@link SkyFunctionName} is
+ * shareable: always, sometimes (depending on the specific key argument and/or value), or never.
+ *
+ * <p>Values may be unshareable because they are just not serializable, or because they contain data
+ * that cannot safely be re-used as-is by another invocation.
+ *
+ * <p>Unshareable data should not be serialized, since it will never be re-used. Attempts to fetch
+ * serialized data will check this value and only perform the fetch if the value is not {@link
+ * #NEVER}.
+ */
+public enum ShareabilityOfValue {
+  ALWAYS,
+  SOMETIMES,
+  NEVER
+}