diff --git a/common/fake_prefs.cc b/common/fake_prefs.cc
new file mode 100644
index 0000000..5a0a3af
--- /dev/null
+++ b/common/fake_prefs.cc
@@ -0,0 +1,168 @@
+//
+// Copyright (C) 2014 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/fake_prefs.h"
+
+#include <algorithm>
+
+#include <gtest/gtest.h>
+
+using std::string;
+
+using chromeos_update_engine::FakePrefs;
+
+namespace {
+
+void CheckNotNull(const string& key, void* ptr) {
+  EXPECT_NE(nullptr, ptr)
+      << "Called Get*() for key \"" << key << "\" with a null parameter.";
+}
+
+}  // namespace
+
+namespace chromeos_update_engine {
+
+FakePrefs::~FakePrefs() {
+  EXPECT_TRUE(observers_.empty());
+}
+
+// Compile-time type-dependent constants definitions.
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<string>::type =
+    FakePrefs::PrefType::kString;
+template<>
+string FakePrefs::PrefValue::* const  // NOLINT(runtime/string), not static str.
+    FakePrefs::PrefConsts<string>::member = &FakePrefs::PrefValue::as_str;
+
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<int64_t>::type =
+    FakePrefs::PrefType::kInt64;
+template<>
+int64_t FakePrefs::PrefValue::* const FakePrefs::PrefConsts<int64_t>::member =
+    &FakePrefs::PrefValue::as_int64;
+
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<bool>::type =
+    FakePrefs::PrefType::kBool;
+template<>
+bool FakePrefs::PrefValue::* const FakePrefs::PrefConsts<bool>::member =
+    &FakePrefs::PrefValue::as_bool;
+
+bool FakePrefs::GetString(const string& key, string* value) const {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetString(const string& key, const string& value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::GetInt64(const string& key, int64_t* value) const {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetInt64(const string& key, const int64_t value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::GetBoolean(const string& key, bool* value) const {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetBoolean(const string& key, const bool value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::Exists(const string& key) const {
+  return values_.find(key) != values_.end();
+}
+
+bool FakePrefs::Delete(const string& key) {
+  if (values_.find(key) == values_.end())
+    return false;
+  values_.erase(key);
+  const auto observers_for_key = observers_.find(key);
+  if (observers_for_key != observers_.end()) {
+    std::vector<ObserverInterface*> copy_observers(observers_for_key->second);
+    for (ObserverInterface* observer : copy_observers)
+      observer->OnPrefDeleted(key);
+  }
+  return true;
+}
+
+string FakePrefs::GetTypeName(PrefType type) {
+  switch (type) {
+    case PrefType::kString:
+      return "string";
+    case PrefType::kInt64:
+      return "int64_t";
+    case PrefType::kBool:
+      return "bool";
+  }
+  return "Unknown";
+}
+
+void FakePrefs::CheckKeyType(const string& key, PrefType type) const {
+  auto it = values_.find(key);
+  EXPECT_TRUE(it == values_.end() || it->second.type == type)
+      << "Key \"" << key << "\" if defined as " << GetTypeName(it->second.type)
+      << " but is accessed as a " << GetTypeName(type);
+}
+
+template<typename T>
+void FakePrefs::SetValue(const string& key, const T& value) {
+  CheckKeyType(key, PrefConsts<T>::type);
+  values_[key].type = PrefConsts<T>::type;
+  values_[key].value.*(PrefConsts<T>::member) = value;
+  const auto observers_for_key = observers_.find(key);
+  if (observers_for_key != observers_.end()) {
+    std::vector<ObserverInterface*> copy_observers(observers_for_key->second);
+    for (ObserverInterface* observer : copy_observers)
+      observer->OnPrefSet(key);
+  }
+}
+
+template<typename T>
+bool FakePrefs::GetValue(const string& key, T* value) const {
+  CheckKeyType(key, PrefConsts<T>::type);
+  auto it = values_.find(key);
+  if (it == values_.end())
+    return false;
+  CheckNotNull(key, value);
+  *value = it->second.value.*(PrefConsts<T>::member);
+  return true;
+}
+
+void FakePrefs::AddObserver(const string& key, ObserverInterface* observer) {
+  observers_[key].push_back(observer);
+}
+
+void FakePrefs::RemoveObserver(const string& key, ObserverInterface* observer) {
+  std::vector<ObserverInterface*>& observers_for_key = observers_[key];
+  auto observer_it =
+      std::find(observers_for_key.begin(), observers_for_key.end(), observer);
+  EXPECT_NE(observer_it, observers_for_key.end())
+      << "Trying to remove an observer instance not watching the key "
+      << key;
+  if (observer_it != observers_for_key.end())
+    observers_for_key.erase(observer_it);
+  if (observers_for_key.empty())
+    observers_.erase(key);
+}
+
+}  // namespace chromeos_update_engine