diff --git a/update_manager/variable.h b/update_manager/variable.h
new file mode 100644
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--- /dev/null
+++ b/update_manager/variable.h
@@ -0,0 +1,203 @@
+// Copyright (c) 2014 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_UPDATE_MANAGER_VARIABLE_H_
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_UPDATE_MANAGER_VARIABLE_H_
+
+#include <algorithm>
+#include <list>
+#include <string>
+
+#include <base/bind.h>
+#include <base/logging.h>
+#include <base/time/time.h>
+#include <gtest/gtest_prod.h>  // for FRIEND_TEST
+
+#include "update_engine/update_manager/event_loop.h"
+
+namespace chromeos_update_manager {
+
+// The VariableMode specifies important behavior of the variable in terms of
+// whether, how and when the value of the variable changes.
+enum VariableMode {
+  // Const variables never changes during the life of a policy request, so the
+  // EvaluationContext caches the value even between different evaluations of
+  // the same policy request.
+  kVariableModeConst,
+
+  // Poll variables, or synchronous variables, represent a variable with a value
+  // that can be queried at any time, but it is not known when the value
+  // changes on the source of information. In order to detect if the value of
+  // the variable changes, it has to be queried again.
+  kVariableModePoll,
+
+  // Async variables are able to produce a signal or callback whenever the
+  // value changes. This means that it's not required to poll the value to
+  // detect when it changes, instead, you should register an observer to get
+  // a notification when that happens.
+  kVariableModeAsync,
+};
+
+// This class is a base class with the common functionality that doesn't
+// deppend on the variable's type, implemented by all the variables.
+class BaseVariable {
+ public:
+  // Interface for observing changes on variable value.
+  class ObserverInterface {
+   public:
+    virtual ~ObserverInterface() {}
+
+    // Called when the value on the variable changes.
+    virtual void ValueChanged(BaseVariable* variable) = 0;
+  };
+
+  virtual ~BaseVariable() {
+    if (!observer_list_.empty()) {
+      LOG(WARNING) << "Variable " << name_ << " deleted with "
+                   << observer_list_.size() << " observers.";
+    }
+    DCHECK(observer_list_.empty()) << "Don't destroy the variable without "
+                                      "removing the observers.";
+  }
+
+  // Returns the variable name as a string.
+  const std::string& GetName() const {
+    return name_;
+  }
+
+  // Returns the variable mode.
+  VariableMode GetMode() const {
+    return mode_;
+  }
+
+  // For VariableModePoll variables, it returns the polling interval of this
+  // variable. In other case, it returns 0.
+  base::TimeDelta GetPollInterval() const {
+    return poll_interval_;
+  }
+
+  // Adds and removes observers for value changes on the variable. This only
+  // works for kVariableAsync variables since the other modes don't track value
+  // changes. Adding the same observer twice has no effect.
+  virtual void AddObserver(BaseVariable::ObserverInterface* observer) {
+    if (std::find(observer_list_.begin(), observer_list_.end(), observer) ==
+        observer_list_.end()) {
+      observer_list_.push_back(observer);
+    }
+  }
+
+  virtual void RemoveObserver(BaseVariable::ObserverInterface* observer) {
+    observer_list_.remove(observer);
+  }
+
+ protected:
+  // Creates a BaseVariable using the default polling interval (5 minutes).
+  BaseVariable(const std::string& name, VariableMode mode)
+      : BaseVariable(name, mode,
+                     base::TimeDelta::FromMinutes(kDefaultPollMinutes)) {}
+
+  // Creates a BaseVariable with mode kVariableModePoll and the provided
+  // polling interval.
+  BaseVariable(const std::string& name, base::TimeDelta poll_interval)
+      : BaseVariable(name, kVariableModePoll, poll_interval) {}
+
+  // Calls ValueChanged on all the observers.
+  void NotifyValueChanged() {
+    // Fire all the observer methods from the main loop as single call. In order
+    // to avoid scheduling these callbacks when it is not needed, we check
+    // first the list of observers.
+    if (!observer_list_.empty()) {
+        RunFromMainLoop(base::Bind(&BaseVariable::OnValueChangedNotification,
+                                  base::Unretained(this)));
+    }
+  }
+
+ private:
+  friend class UmEvaluationContextTest;
+  FRIEND_TEST(UmBaseVariableTest, RepeatedObserverTest);
+  FRIEND_TEST(UmBaseVariableTest, NotifyValueChangedTest);
+  FRIEND_TEST(UmBaseVariableTest, NotifyValueRemovesObserversTest);
+
+  BaseVariable(const std::string& name, VariableMode mode,
+               base::TimeDelta poll_interval)
+    : name_(name), mode_(mode),
+      poll_interval_(mode == kVariableModePoll ?
+                     poll_interval : base::TimeDelta()) {}
+
+  void OnValueChangedNotification() {
+    // A ValueChanged() method can change the list of observers, for example
+    // removing itself and invalidating the iterator, so we create a snapshot
+    // of the observers first. Also, to support the case when *another* observer
+    // is removed, we check for them.
+    std::list<BaseVariable::ObserverInterface*> observer_list_copy(
+        observer_list_);
+
+    for (auto& observer : observer_list_copy) {
+      if (std::find(observer_list_.begin(), observer_list_.end(), observer) !=
+          observer_list_.end()) {
+        observer->ValueChanged(this);
+      }
+    }
+  }
+
+  // The default PollInterval in minutes.
+  static constexpr int kDefaultPollMinutes = 5;
+
+  // The variable's name as a string.
+  const std::string name_;
+
+  // The variable's mode.
+  const VariableMode mode_;
+
+  // The variable's polling interval for VariableModePoll variable and 0 for
+  // other modes.
+  const base::TimeDelta poll_interval_;
+
+  // The list of value changes observers.
+  std::list<BaseVariable::ObserverInterface*> observer_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(BaseVariable);
+};
+
+// Interface to an Update Manager variable of a given type. Implementation
+// internals are hidden as protected members, since policies should not be
+// using them directly.
+template<typename T>
+class Variable : public BaseVariable {
+ public:
+  virtual ~Variable() {}
+
+ protected:
+  // Only allow to get values through the EvaluationContext class and not
+  // directly from the variable.
+  friend class EvaluationContext;
+
+  // Needed to be able to verify variable contents during unit testing.
+  friend class UmTestUtils;
+  FRIEND_TEST(UmRealRandomProviderTest, GetRandomValues);
+
+  Variable(const std::string& name, VariableMode mode)
+      : BaseVariable(name, mode) {}
+
+  Variable(const std::string& name, const base::TimeDelta poll_interval)
+      : BaseVariable(name, poll_interval) {}
+
+  // Gets the current value of the variable. The current value is copied to a
+  // new object and returned. The caller of this method owns the object and
+  // should delete it.
+  //
+  // In case of and error getting the current value or the |timeout| timeout is
+  // exceeded, a NULL value is returned and the |errmsg| is set.
+  //
+  // The caller can pass a NULL value for |errmsg|, in which case the error
+  // message won't be set.
+  virtual const T* GetValue(base::TimeDelta timeout, std::string* errmsg) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(Variable);
+};
+
+}  // namespace chromeos_update_manager
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_UPDATE_MANAGER_VARIABLE_H_