diff --git a/src/java/org/apache/hadoop/conf/Reconfigurable.java b/src/java/org/apache/hadoop/conf/Reconfigurable.java
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+++ b/src/java/org/apache/hadoop/conf/Reconfigurable.java
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+/**
+ * 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.hadoop.conf;
+
+import java.util.Collection;
+
+/**
+ * Something whose {@link Configuration} can be changed at run time.
+ */
+public interface Reconfigurable extends Configurable {
+
+  /**
+   * Change a configuration property on this object to the value specified.
+   *
+   * Change a configuration property on this object to the value specified 
+   * and return the previous value that the configuration property was set to
+   * (or null if it was not previously set). If newVal is null, set the property
+   * to its default value;
+   *
+   * If the property cannot be changed, throw a 
+   * {@link ReconfigurationException}.
+   */
+  public String reconfigureProperty(String property, String newVal) 
+    throws ReconfigurationException;
+
+  /**
+   * Return whether a given property is changeable at run time.
+   *
+   * If isPropertyReconfigurable returns true for a property,
+   * then changeConf should not throw an exception when changing
+   * this property.
+   */
+  public boolean isPropertyReconfigurable(String property);
+
+  /**
+   * Return all the properties that can be changed at run time.
+   */
+  public Collection<String> getReconfigurableProperties();
+
+
+}