diff --git a/common/prefs_interface.h b/common/prefs_interface.h
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+++ b/common/prefs_interface.h
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+//
+// Copyright (C) 2011 The Android Open Source Project
+//
+// 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.
+//
+
+#ifndef UPDATE_ENGINE_COMMON_PREFS_INTERFACE_H_
+#define UPDATE_ENGINE_COMMON_PREFS_INTERFACE_H_
+
+#include <stdint.h>
+
+#include <string>
+
+namespace chromeos_update_engine {
+
+// The prefs interface allows access to a persistent preferences
+// store. The two reasons for providing this as an interface are
+// testing as well as easier switching to a new implementation in the
+// future, if necessary.
+
+class PrefsInterface {
+ public:
+  // Observer class to be notified about key value changes.
+  class ObserverInterface {
+   public:
+    virtual ~ObserverInterface() = default;
+
+    // Called when the value is set for the observed |key|.
+    virtual void OnPrefSet(const std::string& key) = 0;
+
+    // Called when the observed |key| is deleted.
+    virtual void OnPrefDeleted(const std::string& key) = 0;
+  };
+
+  virtual ~PrefsInterface() = default;
+
+  // Gets a string |value| associated with |key|. Returns true on
+  // success, false on failure (including when the |key| is not
+  // present in the store).
+  virtual bool GetString(const std::string& key, std::string* value) const = 0;
+
+  // Associates |key| with a string |value|. Returns true on success,
+  // false otherwise.
+  virtual bool SetString(const std::string& key, const std::string& value) = 0;
+
+  // Gets an int64_t |value| associated with |key|. Returns true on
+  // success, false on failure (including when the |key| is not
+  // present in the store).
+  virtual bool GetInt64(const std::string& key, int64_t* value) const = 0;
+
+  // Associates |key| with an int64_t |value|. Returns true on success,
+  // false otherwise.
+  virtual bool SetInt64(const std::string& key, const int64_t value) = 0;
+
+  // Gets a boolean |value| associated with |key|. Returns true on
+  // success, false on failure (including when the |key| is not
+  // present in the store).
+  virtual bool GetBoolean(const std::string& key, bool* value) const = 0;
+
+  // Associates |key| with a boolean |value|. Returns true on success,
+  // false otherwise.
+  virtual bool SetBoolean(const std::string& key, const bool value) = 0;
+
+  // Returns true if the setting exists (i.e. a file with the given key
+  // exists in the prefs directory)
+  virtual bool Exists(const std::string& key) const = 0;
+
+  // Returns true if successfully deleted the file corresponding to
+  // this key. Calling with non-existent keys does nothing.
+  virtual bool Delete(const std::string& key) = 0;
+
+  // Add an observer to watch whenever the given |key| is modified. The
+  // OnPrefSet() and OnPrefDelete() methods will be called whenever any of the
+  // Set*() methods or the Delete() method are called on the given key,
+  // respectively.
+  virtual void AddObserver(const std::string& key,
+                           ObserverInterface* observer) = 0;
+
+  // Remove an observer added with AddObserver(). The observer won't be called
+  // anymore for future Set*() and Delete() method calls.
+  virtual void RemoveObserver(const std::string& key,
+                              ObserverInterface* observer) = 0;
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_COMMON_PREFS_INTERFACE_H_