diff --git a/buffet/states/mock_state_change_queue_interface.h b/buffet/states/mock_state_change_queue_interface.h
index c984dbb..127e01d 100644
--- a/buffet/states/mock_state_change_queue_interface.h
+++ b/buffet/states/mock_state_change_queue_interface.h
@@ -20,8 +20,17 @@
                bool(base::Time timestamp,
                     native_types::Object changed_properties));
   MOCK_METHOD0(GetAndClearRecordedStateChanges, std::vector<StateChange>());
-  MOCK_CONST_METHOD0(GetLastStateChangeId,
-                     StateChangeQueueInterface::UpdateID());
+  MOCK_CONST_METHOD0(GetLastStateChangeId, UpdateID());
+  MOCK_METHOD1(MockAddOnStateUpdatedCallback,
+               base::CallbackList<void(UpdateID)>::Subscription*(
+                  const base::Callback<void(UpdateID)>&));
+  MOCK_METHOD1(NotifyStateUpdatedOnServer, void(UpdateID));
+
+ private:
+  Token AddOnStateUpdatedCallback(
+      const base::Callback<void(UpdateID)>& callback) override {
+    return Token{MockAddOnStateUpdatedCallback(callback)};
+  }
 };
 
 }  // namespace buffet
diff --git a/buffet/states/state_change_queue.cc b/buffet/states/state_change_queue.cc
index a6e1a72..00ec41a 100644
--- a/buffet/states/state_change_queue.cc
+++ b/buffet/states/state_change_queue.cc
@@ -57,4 +57,15 @@
   return changes;
 }
 
+StateChangeQueueInterface::Token StateChangeQueue::AddOnStateUpdatedCallback(
+    const base::Callback<void(UpdateID)>& callback) {
+  if (state_changes_.empty())
+    callback.Run(last_change_id_);
+  return Token{callbacks_.Add(callback).release()};
+}
+
+void StateChangeQueue::NotifyStateUpdatedOnServer(UpdateID update_id) {
+  callbacks_.Notify(update_id);
+}
+
 }  // namespace buffet
diff --git a/buffet/states/state_change_queue.h b/buffet/states/state_change_queue.h
index 83161fe..26ea9cd 100644
--- a/buffet/states/state_change_queue.h
+++ b/buffet/states/state_change_queue.h
@@ -27,6 +27,9 @@
       native_types::Object changed_properties) override;
   std::vector<StateChange> GetAndClearRecordedStateChanges() override;
   UpdateID GetLastStateChangeId() const override { return last_change_id_; }
+  Token AddOnStateUpdatedCallback(
+      const base::Callback<void(UpdateID)>& callback) override;
+  void NotifyStateUpdatedOnServer(UpdateID update_id) override;
 
  private:
   // To make sure we do not call NotifyPropertiesUpdated() and
@@ -45,6 +48,9 @@
   // invocation increments this value by 1.
   UpdateID last_change_id_{0};
 
+  // Callback list for state change queue even sinks.
+  base::CallbackList<void(UpdateID)> callbacks_;
+
   DISALLOW_COPY_AND_ASSIGN(StateChangeQueue);
 };
 
diff --git a/buffet/states/state_change_queue_interface.h b/buffet/states/state_change_queue_interface.h
index 7c67829..2f11e6b 100644
--- a/buffet/states/state_change_queue_interface.h
+++ b/buffet/states/state_change_queue_interface.h
@@ -7,6 +7,7 @@
 
 #include <vector>
 
+#include <base/callback_list.h>
 #include <base/time/time.h>
 #include <chromeos/variant_dictionary.h>
 
@@ -30,6 +31,8 @@
 class StateChangeQueueInterface {
  public:
   using UpdateID = uint64_t;
+  using Token =
+      std::unique_ptr<base::CallbackList<void(UpdateID)>::Subscription>;
 
   // Returns true if the state change notification queue is empty.
   virtual bool IsEmpty() const = 0;
@@ -46,6 +49,16 @@
   // invocation increments this value by 1.
   virtual UpdateID GetLastStateChangeId() const = 0;
 
+  // Subscribes for device state update notifications from cloud server.
+  // The |callback| will be called every time a state patch with given ID is
+  // successfully received and processed by GCD server.
+  // Returns a subscription token. As soon as this token is destroyed, the
+  // respective callback is removed from the callback list.
+  virtual Token AddOnStateUpdatedCallback(
+      const base::Callback<void(UpdateID)>& callback) WARN_UNUSED_RESULT = 0;
+
+  virtual void NotifyStateUpdatedOnServer(UpdateID update_id) = 0;
+
  protected:
   // No one should attempt do destroy the queue through the interface.
   virtual ~StateChangeQueueInterface() {}
diff --git a/buffet/states/state_change_queue_unittest.cc b/buffet/states/state_change_queue_unittest.cc
index 1a30f97..091e5c3 100644
--- a/buffet/states/state_change_queue_unittest.cc
+++ b/buffet/states/state_change_queue_unittest.cc
@@ -5,6 +5,7 @@
 
 #include "buffet/states/state_change_queue.h"
 
+#include <chromeos/bind_lambda.h>
 #include <gtest/gtest.h>
 
 #include "buffet/commands/unittest_utils.h"
@@ -161,4 +162,29 @@
   EXPECT_EQ(expected2, changes[1].changed_properties);
 }
 
+TEST_F(StateChangeQueueTest, ImmediateStateChangeNotification) {
+  // When queue is empty, registering a new callback will trigger it.
+  bool called = false;
+  auto callback = [&called](StateChangeQueueInterface::UpdateID id) {
+    called = true;
+  };
+  queue_->AddOnStateUpdatedCallback(base::Bind(callback));
+  EXPECT_TRUE(called);
+}
+
+TEST_F(StateChangeQueueTest, DelayedStateChangeNotification) {
+  // When queue is not empty, registering a new callback will not trigger it.
+  ASSERT_TRUE(queue_->NotifyPropertiesUpdated(
+      base::Time::Now(),
+      native_types::Object{
+        {"prop.name1", unittests::make_int_prop_value(1)},
+        {"prop.name2", unittests::make_int_prop_value(2)},
+      }));
+
+  auto callback = [](StateChangeQueueInterface::UpdateID id) {
+    FAIL() << "This should not be called";
+  };
+  queue_->AddOnStateUpdatedCallback(base::Bind(callback));
+}
+
 }  // namespace buffet
diff --git a/buffet/states/state_manager.cc b/buffet/states/state_manager.cc
index 57330f4..2ce78c1 100644
--- a/buffet/states/state_manager.cc
+++ b/buffet/states/state_manager.cc
@@ -166,8 +166,15 @@
   return true;
 }
 
-std::vector<StateChange> StateManager::GetAndClearRecordedStateChanges() {
-  return state_change_queue_->GetAndClearRecordedStateChanges();
+std::pair<StateChangeQueueInterface::UpdateID, std::vector<StateChange>>
+StateManager::GetAndClearRecordedStateChanges() {
+  return std::make_pair(state_change_queue_->GetLastStateChangeId(),
+                        state_change_queue_->GetAndClearRecordedStateChanges());
+}
+
+void StateManager::NotifyStateUpdatedOnServer(
+    StateChangeQueueInterface::UpdateID id) {
+  state_change_queue_->NotifyStateUpdatedOnServer(id);
 }
 
 bool StateManager::LoadStateDefinition(const base::DictionaryValue& json,
diff --git a/buffet/states/state_manager.h b/buffet/states/state_manager.h
index a34f7c1..914daeb 100644
--- a/buffet/states/state_manager.h
+++ b/buffet/states/state_manager.h
@@ -9,6 +9,7 @@
 #include <memory>
 #include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include <base/callback.h>
@@ -58,7 +59,16 @@
 
   // Returns the recorded state changes since last time this method has been
   // called.
-  std::vector<StateChange> GetAndClearRecordedStateChanges();
+  std::pair<StateChangeQueueInterface::UpdateID, std::vector<StateChange>>
+  GetAndClearRecordedStateChanges();
+
+  // Called to notify that the state patch with |id| has been successfully sent
+  // to the server and processed.
+  void NotifyStateUpdatedOnServer(StateChangeQueueInterface::UpdateID id);
+
+  StateChangeQueueInterface* GetStateChangeQueue() const {
+    return state_change_queue_;
+  }
 
  private:
   friend class BaseApiHandlerTest;
diff --git a/buffet/states/state_manager_unittest.cc b/buffet/states/state_manager_unittest.cc
index 50e6a5f..4437a18 100644
--- a/buffet/states/state_manager_unittest.cc
+++ b/buffet/states/state_manager_unittest.cc
@@ -198,10 +198,10 @@
   EXPECT_CALL(mock_state_change_queue_, GetAndClearRecordedStateChanges())
       .WillOnce(Return(expected_val));
   auto changes = mgr_->GetAndClearRecordedStateChanges();
-  ASSERT_EQ(1, changes.size());
-  EXPECT_EQ(expected_val.back().timestamp, changes.back().timestamp);
+  ASSERT_EQ(1, changes.second.size());
+  EXPECT_EQ(expected_val.back().timestamp, changes.second.back().timestamp);
   EXPECT_EQ(expected_val.back().changed_properties,
-            changes.back().changed_properties);
+            changes.second.back().changed_properties);
 }
 
 TEST_F(StateManagerTest, SetProperties) {