Github user JPercivall commented on a diff in the pull request:
https://github.com/apache/nifi/pull/563#discussion_r72325489
--- Diff:
nifi-api/src/main/java/org/apache/nifi/components/state/ExternalStateManager.java
---
@@ -0,0 +1,90 @@
+/*
+ * 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.nifi.components.state;
+
+import org.apache.nifi.annotation.behavior.Stateful;
+
+import java.io.IOException;
+
+/**
+ * <p>
+ * The ExternalStateManager is responsible for providing NiFi a mechanism
for retrieving
+ * and clearing state stored in an external system a NiFi component
interact with.
+ * </p>
+ *
+ * <p>
+ * When calling methods in this class, the state is always
retrieved/cleared from external system
+ * regardless NiFi instance is a part of a cluster or standalone.
+ * </p>
+ *
+ * <p>
+ * This mechanism is designed to allow developers to easily store and
retrieve small amounts of state.
+ * Since implementation of this interface interacts with remote system,
one should consider the cost of
+ * retrieving this data, and the amount of data should be kept to the
minimum required.
--- End diff --
This paragraph doesn't make sense for External state since the user
probably isn't choosing what to store
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