diff --git a/indexer/index.cpp b/indexer/index.cpp
index 68ecf412d6..b07c6375ec 100644
--- a/indexer/index.cpp
+++ b/indexer/index.cpp
@@ -163,13 +163,14 @@ int Index::UpdateMap(string const & fileName, m2::RectD & rect)
     if (id != INVALID_MWM_ID && m_info[id].m_lockCount > 0)
     {
       m_info[id].SetStatus(MwmInfo::STATUS_PENDING_UPDATE);
-      return -2;
+      rv = -2;
+    } else {
+      ReplaceFileWithReady(fileName);
+      rv = RegisterImpl(fileName, rect);
     }
-
-    ReplaceFileWithReady(fileName);
-
-    rv = RegisterImpl(fileName, rect);
   }
+  if (rv != -1)
+    m_observers.ForEach(&Observer::OnMapUpdateIsReady, fileName);
   if (rv >= 0)
     m_observers.ForEach(&Observer::OnMapUpdated, fileName);
   return rv;
diff --git a/indexer/index.hpp b/indexer/index.hpp
index 5f89228bc5..1e1f97080d 100644
--- a/indexer/index.hpp
+++ b/indexer/index.hpp
@@ -40,13 +40,25 @@ protected:
   virtual void UpdateMwmInfo(MwmId id);
 
 public:
+  /// An Observer interface to MwmSet. Note that these functions can
+  /// be called from *ANY* thread because most signals are sent when
+  /// some thread releases its MwmLock, so overrides must be as fast
+  /// as possible and non-blocking when it's possible.
   class Observer
   {
   public:
     virtual ~Observer() {}
 
+    /// Called when a map is registered for a first time.
     virtual void OnMapRegistered(string const & file) {}
+
+    /// Called when update for a map is downloaded.
+    virtual void OnMapUpdateIsReady(string const & file) {}
+
+    /// Called when update for a map is applied.
     virtual void OnMapUpdated(string const & file) {}
+
+    /// Called when map is deleted.
     virtual void OnMapDeleted(string const & file) {}
   };
 
diff --git a/indexer/indexer_tests/index_test.cpp b/indexer/indexer_tests/index_test.cpp
index df3448941d..8d09c4d653 100644
--- a/indexer/indexer_tests/index_test.cpp
+++ b/indexer/indexer_tests/index_test.cpp
@@ -24,7 +24,11 @@ class Observer : public Index::Observer
 {
 public:
   Observer(string const & file)
-      : m_file(file), m_map_registered_calls(0), m_map_updated_calls(0), m_map_deleted_calls(0)
+      : m_file(file),
+        m_map_registered_calls(0),
+        m_map_update_is_ready_calls(0),
+        m_map_updated_calls(0),
+        m_map_deleted_calls(0)
   {
   }
 
@@ -34,11 +38,19 @@ public:
     CHECK_EQUAL(m_file, file, ());
     ++m_map_registered_calls;
   }
+
+  void OnMapUpdateIsReady(string const & file) override
+  {
+    CHECK_EQUAL(m_file, file, ());
+    ++m_map_update_is_ready_calls;
+  }
+
   void OnMapUpdated(string const & file) override
   {
     CHECK_EQUAL(m_file, file, ());
     ++m_map_updated_calls;
   }
+
   void OnMapDeleted(string const & file) override
   {
     CHECK_EQUAL(m_file, file, ());
@@ -46,12 +58,14 @@ public:
   }
 
   int map_registered_calls() const { return m_map_registered_calls; }
+  int map_update_is_ready_calls() const { return m_map_update_is_ready_calls; }
   int map_updated_calls() const { return m_map_updated_calls; }
   int map_deleted_calls() const { return m_map_deleted_calls; }
 
 private:
   string const m_file;
   int m_map_registered_calls;
+  int m_map_update_is_ready_calls;
   int m_map_updated_calls;
   int m_map_deleted_calls;
 };
@@ -100,8 +114,10 @@ UNIT_TEST(Index_MwmStatusNotifications)
   MY_SCOPE_GUARD(testMapUpdateGuard, bind(&CheckedDeleteFile, testMapUpdatePath));
 
   // Check that observers are notified when map is deleted.
+  TEST_EQUAL(0, observer.map_update_is_ready_calls(), ());
   TEST_EQUAL(0, observer.map_updated_calls(), ());
   TEST_LESS_OR_EQUAL(0, index.UpdateMap(testMapName, dummyRect), ());
+  TEST_EQUAL(1, observer.map_update_is_ready_calls(), ());
   TEST_EQUAL(1, observer.map_updated_calls(), ());
 
   // Check that observers are notified when map is deleted.