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diff --git a/src/common/config_obs.h b/src/common/config_obs.h
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2004-2006 Sage Weil <sage@newdream.net>
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ *
+ */
+
+#ifndef CEPH_CONFIG_OBS_H
+#define CEPH_CONFIG_OBS_H
+
+#include <set>
+#include <string>
+
+#include "common/config_fwd.h"
+
+namespace ceph {
+/** @brief Base class for configuration observers.
+ * Use this as a base class for your object if it has to respond to configuration changes,
+ * for example by updating some values or modifying its behavior.
+ * Subscribe for configuration changes by calling the md_config_t::add_observer() method
+ * and unsubscribe using md_config_t::remove_observer().
+ */
+template<class ConfigProxy>
+class md_config_obs_impl {
+public:
+  virtual ~md_config_obs_impl() {}
+  /** @brief Get a table of strings specifying the configuration keys in which the object is interested.
+   * This is called when the object is subscribed to configuration changes with add_observer().
+   * The returned table should not be freed until the observer is removed with remove_observer().
+   * Note that it is not possible to change the set of tracked keys without re-subscribing. */
+  virtual const char** get_tracked_conf_keys() const = 0;
+  /// React to a configuration change.
+  virtual void handle_conf_change(const ConfigProxy& conf,
+				  const std::set <std::string> &changed) = 0;
+  /// Unused for now
+  virtual void handle_subsys_change(const ConfigProxy& conf,
+				    const std::set<int>& changed) { }
+};
+}
+
+using md_config_obs_t = ceph::md_config_obs_impl<ConfigProxy>;
+
+#endif