Github user JPercivall commented on a diff in the pull request:
https://github.com/apache/nifi/pull/563#discussion_r69829659
--- Diff:
nifi-api/src/main/java/org/apache/nifi/components/state/ExternalStateManager.java
---
@@ -0,0 +1,63 @@
+/*
+ * 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.
+ * </p>
+ *
+ * <p>
+ * Any component that wishes to implement ExternalStateManager should also
use the {@link Stateful} annotation
+ * with {@link Scope#EXTERNAL} to provide a description of what state is
being stored.
+ * If this annotation is not present, the UI will not expose such
information or allow DFMs to clear the state.
+ * </p>
+ */
+public interface ExternalStateManager {
+
+ /**
+ * Returns the current state for the component. This return value may
be <code>null</code>.
+ *
+ * @return the current state for the component or null if there is no
state is retrieved
+ * @throws IOException if unable to communicate with the underlying
storage mechanism
+ */
+ StateMap getState() throws IOException;
+
+ /**
+ * Clears all keys and values from the component's state
+ *
+ * @throws IOException if unable to communicate with the underlying
storage mechanism
+ */
+ void clear() throws IOException;
--- End diff --
This should be renamed to "clearState" to match the corresponding
"getState" and to better identified the method. Since it's an interface you
never know what else the processor may want to clear and "clear" could end up
being ambiguous.
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