diff --git a/common/prefs.h b/common/prefs.h
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--- /dev/null
+++ b/common/prefs.h
@@ -0,0 +1,81 @@
+//
+// Copyright (C) 2012 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_H_
+#define UPDATE_ENGINE_COMMON_PREFS_H_
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include <base/files/file_path.h>
+
+#include "gtest/gtest_prod.h"  // for FRIEND_TEST
+#include "update_engine/common/prefs_interface.h"
+
+namespace chromeos_update_engine {
+
+// Implements a preference store by storing the value associated with
+// a key in a separate file named after the key under a preference
+// store directory.
+
+class Prefs : public PrefsInterface {
+ public:
+  Prefs() = default;
+
+  // Initializes the store by associating this object with |prefs_dir|
+  // as the preference store directory. Returns true on success, false
+  // otherwise.
+  bool Init(const base::FilePath& prefs_dir);
+
+  // PrefsInterface methods.
+  bool GetString(const std::string& key, std::string* value) const override;
+  bool SetString(const std::string& key, const std::string& value) override;
+  bool GetInt64(const std::string& key, int64_t* value) const override;
+  bool SetInt64(const std::string& key, const int64_t value) override;
+  bool GetBoolean(const std::string& key, bool* value) const override;
+  bool SetBoolean(const std::string& key, const bool value) override;
+
+  bool Exists(const std::string& key) const override;
+  bool Delete(const std::string& key) override;
+
+  void AddObserver(const std::string& key,
+                   ObserverInterface* observer) override;
+  void RemoveObserver(const std::string& key,
+                      ObserverInterface* observer) override;
+
+ private:
+  FRIEND_TEST(PrefsTest, GetFileNameForKey);
+  FRIEND_TEST(PrefsTest, GetFileNameForKeyBadCharacter);
+  FRIEND_TEST(PrefsTest, GetFileNameForKeyEmpty);
+
+  // Sets |filename| to the full path to the file containing the data
+  // associated with |key|. Returns true on success, false otherwise.
+  bool GetFileNameForKey(const std::string& key,
+                         base::FilePath* filename) const;
+
+  // Preference store directory.
+  base::FilePath prefs_dir_;
+
+  // The registered observers watching for changes.
+  std::map<std::string, std::vector<ObserverInterface*>> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(Prefs);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_COMMON_PREFS_H_