diff --git a/common/prefs_unittest.cc b/common/prefs_unittest.cc
new file mode 100644
index 0000000..354b05b
--- /dev/null
+++ b/common/prefs_unittest.cc
@@ -0,0 +1,337 @@
+//
+// 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.
+//
+
+#include "update_engine/common/prefs.h"
+
+#include <inttypes.h>
+
+#include <string>
+
+#include <base/files/file_util.h>
+#include <base/macros.h>
+#include <base/strings/string_util.h>
+#include <base/strings/stringprintf.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+using std::string;
+using testing::Eq;
+using testing::_;
+
+namespace {
+// Test key used along the tests.
+const char kKey[] = "test-key";
+}
+
+namespace chromeos_update_engine {
+
+class PrefsTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    ASSERT_TRUE(base::CreateNewTempDirectory("auprefs", &prefs_dir_));
+    ASSERT_TRUE(prefs_.Init(prefs_dir_));
+  }
+
+  void TearDown() override {
+    base::DeleteFile(prefs_dir_, true);  // recursive
+  }
+
+  bool SetValue(const string& key, const string& value) {
+    return base::WriteFile(prefs_dir_.Append(key), value.data(),
+                           value.length()) == static_cast<int>(value.length());
+  }
+
+  base::FilePath prefs_dir_;
+  Prefs prefs_;
+};
+
+TEST_F(PrefsTest, GetFileNameForKey) {
+  const char kAllvalidCharsKey[] =
+      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_-";
+  base::FilePath path;
+  EXPECT_TRUE(prefs_.GetFileNameForKey(kAllvalidCharsKey, &path));
+  EXPECT_EQ(prefs_dir_.Append(kAllvalidCharsKey).value(), path.value());
+}
+
+TEST_F(PrefsTest, GetFileNameForKeyBadCharacter) {
+  base::FilePath path;
+  EXPECT_FALSE(prefs_.GetFileNameForKey("ABC abc", &path));
+}
+
+TEST_F(PrefsTest, GetFileNameForKeyEmpty) {
+  base::FilePath path;
+  EXPECT_FALSE(prefs_.GetFileNameForKey("", &path));
+}
+
+TEST_F(PrefsTest, GetString) {
+  const string test_data = "test data";
+  ASSERT_TRUE(SetValue(kKey, test_data));
+  string value;
+  EXPECT_TRUE(prefs_.GetString(kKey, &value));
+  EXPECT_EQ(test_data, value);
+}
+
+TEST_F(PrefsTest, GetStringBadKey) {
+  string value;
+  EXPECT_FALSE(prefs_.GetString(",bad", &value));
+}
+
+TEST_F(PrefsTest, GetStringNonExistentKey) {
+  string value;
+  EXPECT_FALSE(prefs_.GetString("non-existent-key", &value));
+}
+
+TEST_F(PrefsTest, SetString) {
+  const char kValue[] = "some test value\non 2 lines";
+  EXPECT_TRUE(prefs_.SetString(kKey, kValue));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ(kValue, value);
+}
+
+TEST_F(PrefsTest, SetStringBadKey) {
+  const char kKeyWithDots[] = ".no-dots";
+  EXPECT_FALSE(prefs_.SetString(kKeyWithDots, "some value"));
+  EXPECT_FALSE(base::PathExists(prefs_dir_.Append(kKeyWithDots)));
+}
+
+TEST_F(PrefsTest, SetStringCreateDir) {
+  const char kValue[] = "test value";
+  base::FilePath subdir = prefs_dir_.Append("subdir1").Append("subdir2");
+  EXPECT_TRUE(prefs_.Init(subdir));
+  EXPECT_TRUE(prefs_.SetString(kKey, kValue));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(subdir.Append(kKey), &value));
+  EXPECT_EQ(kValue, value);
+}
+
+TEST_F(PrefsTest, SetStringDirCreationFailure) {
+  EXPECT_TRUE(prefs_.Init(base::FilePath("/dev/null")));
+  EXPECT_FALSE(prefs_.SetString(kKey, "test value"));
+}
+
+TEST_F(PrefsTest, SetStringFileCreationFailure) {
+  base::CreateDirectory(prefs_dir_.Append(kKey));
+  EXPECT_FALSE(prefs_.SetString(kKey, "test value"));
+  EXPECT_TRUE(base::DirectoryExists(prefs_dir_.Append(kKey)));
+}
+
+TEST_F(PrefsTest, GetInt64) {
+  ASSERT_TRUE(SetValue(kKey, " \n 25 \t "));
+  int64_t value;
+  EXPECT_TRUE(prefs_.GetInt64(kKey, &value));
+  EXPECT_EQ(25, value);
+}
+
+TEST_F(PrefsTest, GetInt64BadValue) {
+  ASSERT_TRUE(SetValue(kKey, "30a"));
+  int64_t value;
+  EXPECT_FALSE(prefs_.GetInt64(kKey, &value));
+}
+
+TEST_F(PrefsTest, GetInt64Max) {
+  ASSERT_TRUE(SetValue(kKey, base::StringPrintf("%" PRIi64, kint64max)));
+  int64_t value;
+  EXPECT_TRUE(prefs_.GetInt64(kKey, &value));
+  EXPECT_EQ(kint64max, value);
+}
+
+TEST_F(PrefsTest, GetInt64Min) {
+  ASSERT_TRUE(SetValue(kKey, base::StringPrintf("%" PRIi64, kint64min)));
+  int64_t value;
+  EXPECT_TRUE(prefs_.GetInt64(kKey, &value));
+  EXPECT_EQ(kint64min, value);
+}
+
+TEST_F(PrefsTest, GetInt64Negative) {
+  ASSERT_TRUE(SetValue(kKey, " \t -100 \n "));
+  int64_t value;
+  EXPECT_TRUE(prefs_.GetInt64(kKey, &value));
+  EXPECT_EQ(-100, value);
+}
+
+TEST_F(PrefsTest, GetInt64NonExistentKey) {
+  int64_t value;
+  EXPECT_FALSE(prefs_.GetInt64("random-key", &value));
+}
+
+TEST_F(PrefsTest, SetInt64) {
+  EXPECT_TRUE(prefs_.SetInt64(kKey, -123));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ("-123", value);
+}
+
+TEST_F(PrefsTest, SetInt64BadKey) {
+  const char kKeyWithSpaces[] = "s p a c e s";
+  EXPECT_FALSE(prefs_.SetInt64(kKeyWithSpaces, 20));
+  EXPECT_FALSE(base::PathExists(prefs_dir_.Append(kKeyWithSpaces)));
+}
+
+TEST_F(PrefsTest, SetInt64Max) {
+  EXPECT_TRUE(prefs_.SetInt64(kKey, kint64max));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ(base::StringPrintf("%" PRIi64, kint64max), value);
+}
+
+TEST_F(PrefsTest, SetInt64Min) {
+  EXPECT_TRUE(prefs_.SetInt64(kKey, kint64min));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ(base::StringPrintf("%" PRIi64, kint64min), value);
+}
+
+TEST_F(PrefsTest, GetBooleanFalse) {
+  ASSERT_TRUE(SetValue(kKey, " \n false \t "));
+  bool value;
+  EXPECT_TRUE(prefs_.GetBoolean(kKey, &value));
+  EXPECT_FALSE(value);
+}
+
+TEST_F(PrefsTest, GetBooleanTrue) {
+  const char kKey[] = "test-key";
+  ASSERT_TRUE(SetValue(kKey, " \t true \n "));
+  bool value;
+  EXPECT_TRUE(prefs_.GetBoolean(kKey, &value));
+  EXPECT_TRUE(value);
+}
+
+TEST_F(PrefsTest, GetBooleanBadValue) {
+  const char kKey[] = "test-key";
+  ASSERT_TRUE(SetValue(kKey, "1"));
+  bool value;
+  EXPECT_FALSE(prefs_.GetBoolean(kKey, &value));
+}
+
+TEST_F(PrefsTest, GetBooleanBadEmptyValue) {
+  const char kKey[] = "test-key";
+  ASSERT_TRUE(SetValue(kKey, ""));
+  bool value;
+  EXPECT_FALSE(prefs_.GetBoolean(kKey, &value));
+}
+
+TEST_F(PrefsTest, GetBooleanNonExistentKey) {
+  bool value;
+  EXPECT_FALSE(prefs_.GetBoolean("random-key", &value));
+}
+
+TEST_F(PrefsTest, SetBooleanTrue) {
+  const char kKey[] = "test-bool";
+  EXPECT_TRUE(prefs_.SetBoolean(kKey, true));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ("true", value);
+}
+
+TEST_F(PrefsTest, SetBooleanFalse) {
+  const char kKey[] = "test-bool";
+  EXPECT_TRUE(prefs_.SetBoolean(kKey, false));
+  string value;
+  EXPECT_TRUE(base::ReadFileToString(prefs_dir_.Append(kKey), &value));
+  EXPECT_EQ("false", value);
+}
+
+TEST_F(PrefsTest, SetBooleanBadKey) {
+  const char kKey[] = "s p a c e s";
+  EXPECT_FALSE(prefs_.SetBoolean(kKey, true));
+  EXPECT_FALSE(base::PathExists(prefs_dir_.Append(kKey)));
+}
+
+TEST_F(PrefsTest, ExistsWorks) {
+  // test that the key doesn't exist before we set it.
+  EXPECT_FALSE(prefs_.Exists(kKey));
+
+  // test that the key exists after we set it.
+  ASSERT_TRUE(prefs_.SetInt64(kKey, 8));
+  EXPECT_TRUE(prefs_.Exists(kKey));
+}
+
+TEST_F(PrefsTest, DeleteWorks) {
+  // test that it's alright to delete a non-existent key.
+  EXPECT_TRUE(prefs_.Delete(kKey));
+
+  // delete the key after we set it.
+  ASSERT_TRUE(prefs_.SetInt64(kKey, 0));
+  EXPECT_TRUE(prefs_.Delete(kKey));
+
+  // make sure it doesn't exist anymore.
+  EXPECT_FALSE(prefs_.Exists(kKey));
+}
+
+class MockPrefsObserver : public PrefsInterface::ObserverInterface {
+ public:
+  MOCK_METHOD1(OnPrefSet, void(const string&));
+  MOCK_METHOD1(OnPrefDeleted, void(const string& key));
+};
+
+TEST_F(PrefsTest, ObserversCalled) {
+  MockPrefsObserver mock_obserser;
+  prefs_.AddObserver(kKey, &mock_obserser);
+
+  EXPECT_CALL(mock_obserser, OnPrefSet(Eq(kKey)));
+  EXPECT_CALL(mock_obserser, OnPrefDeleted(_)).Times(0);
+  prefs_.SetString(kKey, "value");
+  testing::Mock::VerifyAndClearExpectations(&mock_obserser);
+
+  EXPECT_CALL(mock_obserser, OnPrefSet(_)).Times(0);
+  EXPECT_CALL(mock_obserser, OnPrefDeleted(Eq(kKey)));
+  prefs_.Delete(kKey);
+  testing::Mock::VerifyAndClearExpectations(&mock_obserser);
+
+  prefs_.RemoveObserver(kKey, &mock_obserser);
+}
+
+TEST_F(PrefsTest, OnlyCalledOnObservedKeys) {
+  MockPrefsObserver mock_obserser;
+  const char kUnusedKey[] = "unused-key";
+  prefs_.AddObserver(kUnusedKey, &mock_obserser);
+
+  EXPECT_CALL(mock_obserser, OnPrefSet(_)).Times(0);
+  EXPECT_CALL(mock_obserser, OnPrefDeleted(_)).Times(0);
+  prefs_.SetString(kKey, "value");
+  prefs_.Delete(kKey);
+
+  prefs_.RemoveObserver(kUnusedKey, &mock_obserser);
+}
+
+TEST_F(PrefsTest, RemovedObserversNotCalled) {
+  MockPrefsObserver mock_obserser_a, mock_obserser_b;
+  prefs_.AddObserver(kKey, &mock_obserser_a);
+  prefs_.AddObserver(kKey, &mock_obserser_b);
+  EXPECT_CALL(mock_obserser_a, OnPrefSet(_)).Times(2);
+  EXPECT_CALL(mock_obserser_b, OnPrefSet(_)).Times(1);
+  EXPECT_TRUE(prefs_.SetString(kKey, "value"));
+  prefs_.RemoveObserver(kKey, &mock_obserser_b);
+  EXPECT_TRUE(prefs_.SetString(kKey, "other value"));
+  prefs_.RemoveObserver(kKey, &mock_obserser_a);
+  EXPECT_TRUE(prefs_.SetString(kKey, "yet another value"));
+}
+
+TEST_F(PrefsTest, UnsuccessfulCallsNotObserved) {
+  MockPrefsObserver mock_obserser;
+  const char kInvalidKey[] = "no spaces or .";
+  prefs_.AddObserver(kInvalidKey, &mock_obserser);
+
+  EXPECT_CALL(mock_obserser, OnPrefSet(_)).Times(0);
+  EXPECT_CALL(mock_obserser, OnPrefDeleted(_)).Times(0);
+  EXPECT_FALSE(prefs_.SetString(kInvalidKey, "value"));
+  EXPECT_FALSE(prefs_.Delete(kInvalidKey));
+
+  prefs_.RemoveObserver(kInvalidKey, &mock_obserser);
+}
+
+}  // namespace chromeos_update_engine