diff --git a/autoload/youcompleteme.vim b/autoload/youcompleteme.vim
index b09c4d38..f7c99c88 100644
--- a/autoload/youcompleteme.vim
+++ b/autoload/youcompleteme.vim
@@ -479,15 +479,13 @@ function! s:OnFileReadyToParse( ... )
 
   " We only want to send a new FileReadyToParse event notification if the buffer
   " has changed since the last time we sent one, or if forced.
-  if force_parsing || b:changedtick != get( b:, 'ycm_changedtick', -1 )
+  if force_parsing || s:Pyeval( "ycm_state.NeedsReparse()" )
     exec s:python_command "ycm_state.OnFileReadyToParse()"
 
     call timer_stop( s:pollers.file_parse_response.id )
     let s:pollers.file_parse_response.id = timer_start(
           \ s:pollers.file_parse_response.wait_milliseconds,
           \ function( 's:PollFileParseResponse' ) )
-
-    let b:ycm_changedtick = b:changedtick
   endif
 endfunction
 
diff --git a/python/ycm/buffer.py b/python/ycm/buffer.py
new file mode 100644
index 00000000..3fdedbe9
--- /dev/null
+++ b/python/ycm/buffer.py
@@ -0,0 +1,105 @@
+# Copyright (C) 2016, Davit Samvelyan
+#
+# This file is part of YouCompleteMe.
+#
+# YouCompleteMe is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# YouCompleteMe is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
+
+from __future__ import unicode_literals
+from __future__ import print_function
+from __future__ import division
+from __future__ import absolute_import
+from future import standard_library
+standard_library.install_aliases()
+from builtins import *  # noqa
+
+from ycm import vimsupport
+from ycm.client.event_notification import EventNotification
+from ycm.diagnostic_interface import DiagnosticInterface
+
+
+class Buffer( object ):
+
+  def __init__( self, bufnr, user_options ):
+    self.number = bufnr
+    self._parse_tick = 0
+    self._handled_tick = 0
+    self._parse_request = None
+    self._diag_interface = DiagnosticInterface( bufnr, user_options )
+
+
+  def FileParseRequestReady( self, block = False ):
+    return bool( self._parse_request and
+                 ( block or self._parse_request.Done() ) )
+
+
+  def SendParseRequest( self, extra_data ):
+    self._parse_request = EventNotification( 'FileReadyToParse',
+                                             extra_data = extra_data )
+    self._parse_request.Start()
+    # Decrement handled tick to ensure correct handling when we are forcing
+    # reparse on buffer visit and changed tick remains the same.
+    self._handled_tick -= 1
+    self._parse_tick = self._ChangedTick()
+
+
+  def NeedsReparse( self ):
+    return self._parse_tick != self._ChangedTick()
+
+
+  def UpdateDiagnostics( self ):
+    self._diag_interface.UpdateWithNewDiagnostics(
+      self._parse_request.Response() )
+
+
+  def PopulateLocationList( self ):
+    return self._diag_interface.PopulateLocationList()
+
+
+  def GetResponse( self ):
+    return self._parse_request.Response()
+
+
+  def IsResponseHandled( self ):
+    return self._handled_tick == self._parse_tick
+
+
+  def MarkResponseHandled( self ):
+    self._handled_tick = self._parse_tick
+
+
+  def OnCursorMoved( self ):
+    self._diag_interface.OnCursorMoved()
+
+
+  def GetErrorCount( self ):
+    return self._diag_interface.GetErrorCount()
+
+
+  def GetWarningCount( self ):
+    return self._diag_interface.GetWarningCount()
+
+
+  def _ChangedTick( self ):
+    return vimsupport.GetBufferChangedTick(self.number)
+
+
+class BufferDict( dict ):
+
+  def __init__( self, user_options ):
+    self._user_options = user_options
+
+
+  def __missing__( self, key ):
+    value = self[ key ] = Buffer( key, self._user_options )
+    return value
diff --git a/python/ycm/diagnostic_interface.py b/python/ycm/diagnostic_interface.py
index 89051635..7e5e59bf 100644
--- a/python/ycm/diagnostic_interface.py
+++ b/python/ycm/diagnostic_interface.py
@@ -30,16 +30,17 @@ import vim
 
 
 class DiagnosticInterface( object ):
-  def __init__( self, user_options ):
+  def __init__( self, bufnr, user_options ):
+    self._bufnr = bufnr
     self._user_options = user_options
+    self._diagnostics = []
     self._diag_filter = DiagnosticFilter.CreateFromOptions( user_options )
     # Line and column numbers are 1-based
-    self._buffer_number_to_line_to_diags = defaultdict(
-      lambda: defaultdict( list ) )
+    self._line_to_diags = defaultdict( list )
+    self._placed_signs = []
     self._next_sign_id = 1
     self._previous_line_number = -1
     self._diag_message_needs_clearing = False
-    self._placed_signs = []
 
 
   def OnCursorMoved( self ):
@@ -53,54 +54,43 @@ class DiagnosticInterface( object ):
 
 
   def GetErrorCount( self ):
-    return len( self._FilterDiagnostics( _DiagnosticIsError ) )
+    return self._DiagnosticsCount( _DiagnosticIsError )
 
 
   def GetWarningCount( self ):
-    return len( self._FilterDiagnostics( _DiagnosticIsWarning ) )
+    return self._DiagnosticsCount( _DiagnosticIsWarning )
 
 
-  def PopulateLocationList( self, diags ):
-    vimsupport.SetLocationList(
-      vimsupport.ConvertDiagnosticsToQfList(
-          self._ApplyDiagnosticFilter( diags ) ) )
+  def PopulateLocationList( self ):
+    # Do nothing if loc list is already populated by diag_interface
+    if not self._user_options[ 'always_populate_location_list' ]:
+      self._UpdateLocationList()
+    return bool( self._diagnostics )
 
 
   def UpdateWithNewDiagnostics( self, diags ):
-    normalized_diags = [ _NormalizeDiagnostic( x ) for x in
-            self._ApplyDiagnosticFilter( diags ) ]
-    self._buffer_number_to_line_to_diags = _ConvertDiagListToDict(
-        normalized_diags )
+    self._diagnostics = [ _NormalizeDiagnostic( x ) for x in
+                            self._ApplyDiagnosticFilter( diags ) ]
+    self._ConvertDiagListToDict()
 
     if self._user_options[ 'enable_diagnostic_signs' ]:
-      self._placed_signs, self._next_sign_id = _UpdateSigns(
-        self._placed_signs,
-        self._buffer_number_to_line_to_diags,
-        self._next_sign_id )
+      self._UpdateSigns()
 
     if self._user_options[ 'enable_diagnostic_highlighting' ]:
-      _UpdateSquiggles( self._buffer_number_to_line_to_diags )
+      self._UpdateSquiggles()
 
     if self._user_options[ 'always_populate_location_list' ]:
-      self.PopulateLocationList( normalized_diags )
+      self._UpdateLocationList()
 
 
-  def _ApplyDiagnosticFilter( self, diags, extra_predicate = None ):
-    filetypes = vimsupport.CurrentFiletypes()
+  def _ApplyDiagnosticFilter( self, diags ):
+    filetypes = vimsupport.GetBufferFiletypes( self._bufnr )
     diag_filter = self._diag_filter.SubsetForTypes( filetypes )
-    predicate = diag_filter.IsAllowed
-    if extra_predicate is not None:
-      def Filter( diag ):
-        return extra_predicate( diag ) and diag_filter.IsAllowed( diag )
-
-      predicate = Filter
-
-    return filter( predicate, diags )
+    return filter( diag_filter.IsAllowed, diags )
 
 
   def _EchoDiagnosticForLine( self, line_num ):
-    buffer_num = vim.current.buffer.number
-    diags = self._buffer_number_to_line_to_diags[ buffer_num ][ line_num ]
+    diags = self._line_to_diags[ line_num ]
     if not diags:
       if self._diag_message_needs_clearing:
         # Clear any previous diag echo
@@ -117,144 +107,105 @@ class DiagnosticInterface( object ):
     self._diag_message_needs_clearing = True
 
 
-  def _FilterDiagnostics( self, predicate ):
-    matched_diags = []
-    line_to_diags = self._buffer_number_to_line_to_diags[
-      vim.current.buffer.number ]
-
-    for diags in itervalues( line_to_diags ):
-      matched_diags.extend( list(
-        self._ApplyDiagnosticFilter( diags, predicate ) ) )
-    return matched_diags
+  def _DiagnosticsCount( self, predicate ):
+    count = 0
+    for diags in itervalues( self._line_to_diags ):
+      count += sum( 1 for d in diags if predicate( d ) )
+    return count
 
 
-def _UpdateSquiggles( buffer_number_to_line_to_diags ):
-  vimsupport.ClearYcmSyntaxMatches()
-  line_to_diags = buffer_number_to_line_to_diags[ vim.current.buffer.number ]
+  def _ConvertDiagListToDict( self ):
+    self._line_to_diags = defaultdict( list )
+    for diag in self._diagnostics:
+      location = diag[ 'location' ]
+      bufnr = vimsupport.GetBufferNumberForFilename( location[ 'filepath' ] )
+      if bufnr != self._bufnr:
+          continue
+      line_number = location[ 'line_num' ]
+      self._line_to_diags[ line_number ].append( diag )
 
-  for diags in itervalues( line_to_diags ):
-    # Insert squiggles in reverse order so that errors overlap warnings.
-    for diag in reversed( diags ):
-      location_extent = diag[ 'location_extent' ]
-      is_error = _DiagnosticIsError( diag )
-
-      if location_extent[ 'start' ][ 'line_num' ] <= 0:
-        location = diag[ 'location' ]
-        vimsupport.AddDiagnosticSyntaxMatch(
-          location[ 'line_num' ],
-          location[ 'column_num' ],
-          is_error = is_error )
-      else:
-        vimsupport.AddDiagnosticSyntaxMatch(
-          location_extent[ 'start' ][ 'line_num' ],
-          location_extent[ 'start' ][ 'column_num' ],
-          location_extent[ 'end' ][ 'line_num' ],
-          location_extent[ 'end' ][ 'column_num' ],
-          is_error = is_error )
-
-      for diag_range in diag[ 'ranges' ]:
-        vimsupport.AddDiagnosticSyntaxMatch(
-          diag_range[ 'start' ][ 'line_num' ],
-          diag_range[ 'start' ][ 'column_num' ],
-          diag_range[ 'end' ][ 'line_num' ],
-          diag_range[ 'end' ][ 'column_num' ],
-          is_error = is_error )
-
-
-def _UpdateSigns( placed_signs, buffer_number_to_line_to_diags, next_sign_id ):
-  new_signs, kept_signs, next_sign_id = _GetKeptAndNewSigns(
-    placed_signs, buffer_number_to_line_to_diags, next_sign_id
-  )
-  # Dummy sign used to prevent "flickering" in Vim when last mark gets
-  # deleted from buffer. Dummy sign prevents Vim to collapsing the sign column
-  # in that case.
-  # There's also a vim bug which causes the whole window to redraw in some
-  # conditions (vim redraw logic is very complex). But, somehow, if we place a
-  # dummy sign before placing other "real" signs, it will not redraw the
-  # buffer (patch to vim pending).
-  dummy_sign_needed = not kept_signs and new_signs
-
-  if dummy_sign_needed:
-    vimsupport.PlaceDummySign( next_sign_id + 1,
-                               vim.current.buffer.number,
-                               new_signs[ 0 ].line )
-
-  # We place only those signs that haven't been placed yet.
-  new_placed_signs = _PlaceNewSigns( kept_signs, new_signs )
-
-  # We use incremental placement, so signs that already placed on the correct
-  # lines will not be deleted and placed again, which should improve performance
-  # in case of many diags. Signs which don't exist in the current diag should be
-  # deleted.
-  _UnplaceObsoleteSigns( kept_signs, placed_signs )
-
-  if dummy_sign_needed:
-    vimsupport.UnPlaceDummySign( next_sign_id + 1, vim.current.buffer.number )
-
-  return new_placed_signs, next_sign_id
-
-
-def _GetKeptAndNewSigns( placed_signs, buffer_number_to_line_to_diags,
-                         next_sign_id ):
-  new_signs = []
-  kept_signs = []
-  for buffer_number, line_to_diags in iteritems(
-                                            buffer_number_to_line_to_diags ):
-    if not vimsupport.BufferIsVisible( buffer_number ):
-      continue
-
-    for line, diags in iteritems( line_to_diags ):
-      # Only one sign is visible by line.
-      first_diag = diags[ 0 ]
-      sign = _DiagSignPlacement( next_sign_id,
-                                 line,
-                                 buffer_number,
-                                 _DiagnosticIsError( first_diag ) )
-      if sign not in placed_signs:
-        new_signs.append( sign )
-        next_sign_id += 1
-      else:
-        # We use .index here because `sign` contains a new id, but
-        # we need the sign with the old id to unplace it later on.
-        # We won't be placing the new sign.
-        kept_signs.append( placed_signs[ placed_signs.index( sign ) ] )
-  return new_signs, kept_signs, next_sign_id
-
-
-def _PlaceNewSigns( kept_signs, new_signs ):
-  placed_signs = kept_signs[:]
-  for sign in new_signs:
-    # Do not set two signs on the same line, it will screw up storing sign
-    # locations.
-    if sign in placed_signs:
-      continue
-    vimsupport.PlaceSign( sign.id, sign.line, sign.buffer, sign.is_error )
-    placed_signs.append( sign )
-  return placed_signs
-
-
-def _UnplaceObsoleteSigns( kept_signs, placed_signs ):
-  for sign in placed_signs:
-    if sign not in kept_signs:
-      vimsupport.UnplaceSignInBuffer( sign.buffer, sign.id )
-
-
-def _ConvertDiagListToDict( diag_list ):
-  buffer_to_line_to_diags = defaultdict( lambda: defaultdict( list ) )
-  for diag in diag_list:
-    location = diag[ 'location' ]
-    buffer_number = vimsupport.GetBufferNumberForFilename(
-      location[ 'filepath' ] )
-    line_number = location[ 'line_num' ]
-    buffer_to_line_to_diags[ buffer_number ][ line_number ].append( diag )
-
-  for line_to_diags in itervalues( buffer_to_line_to_diags ):
-    for diags in itervalues( line_to_diags ):
-      # We want errors to be listed before warnings so that errors aren't hidden
-      # by the warnings.
+    for diags in itervalues( self._line_to_diags ):
+      # We also want errors to be listed before warnings so that errors aren't
+      # hidden by the warnings; Vim won't place a sign over an existing one.
       diags.sort( key = lambda diag: ( diag[ 'kind' ],
                                        diag[ 'location' ][ 'column_num' ] ) )
-  return buffer_to_line_to_diags
+
+
+  def _UpdateSigns( self ):
+    new_signs, obsolete_signs = self._GetNewAndObsoleteSigns()
+
+    self._PlaceNewSigns( new_signs )
+
+    self._UnplaceObsoleteSigns( obsolete_signs )
+
+
+  def _GetNewAndObsoleteSigns( self ):
+    new_signs = []
+    obsolete_signs = self._placed_signs[:]
+    for line, diags in iteritems( self._line_to_diags ):
+      # We always go for the first diagnostic on line,
+      # because it is sorted giving priority to the Errors.
+      diag = diags[ 0 ]
+      sign = _DiagSignPlacement( self._next_sign_id,
+                                 line, _DiagnosticIsError( diag ) )
+      try:
+        obsolete_signs.remove( sign )
+      except ValueError:
+        new_signs.append( sign )
+        self._next_sign_id += 1
+
+    return new_signs, obsolete_signs
+
+
+  def _PlaceNewSigns( self, new_signs ):
+    for sign in new_signs:
+      vimsupport.PlaceSign( sign.id, sign.line, self._bufnr, sign.is_error )
+      self._placed_signs.append( sign )
+
+
+  def _UnplaceObsoleteSigns( self, obsolete_signs ):
+    for sign in obsolete_signs:
+      self._placed_signs.remove( sign )
+      vimsupport.UnplaceSignInBuffer( self._bufnr, sign.id )
+
+
+  def _UpdateSquiggles( self ):
+    if self._bufnr != vim.current.buffer.number:
+      return
+
+    vimsupport.ClearYcmSyntaxMatches()
+
+    for diags in itervalues( self._line_to_diags ):
+      for diag in reversed( diags ):
+        location_extent = diag[ 'location_extent' ]
+        is_error = _DiagnosticIsError( diag )
+
+        if location_extent[ 'start' ][ 'line_num' ] <= 0:
+          location = diag[ 'location' ]
+          vimsupport.AddDiagnosticSyntaxMatch(
+            location[ 'line_num' ],
+            location[ 'column_num' ],
+            is_error = is_error )
+        else:
+          vimsupport.AddDiagnosticSyntaxMatch(
+            location_extent[ 'start' ][ 'line_num' ],
+            location_extent[ 'start' ][ 'column_num' ],
+            location_extent[ 'end' ][ 'line_num' ],
+            location_extent[ 'end' ][ 'column_num' ],
+            is_error = is_error )
+
+        for diag_range in diag[ 'ranges' ]:
+          vimsupport.AddDiagnosticSyntaxMatch(
+            diag_range[ 'start' ][ 'line_num' ],
+            diag_range[ 'start' ][ 'column_num' ],
+            diag_range[ 'end' ][ 'line_num' ],
+            diag_range[ 'end' ][ 'column_num' ],
+            is_error = is_error )
+
+
+  def _UpdateLocationList( self ):
+    vimsupport.SetLocationList(
+      vimsupport.ConvertDiagnosticsToQfList( self._diagnostics ) )
 
 
 _DiagnosticIsError = CompileLevel( 'error' )
@@ -271,12 +222,10 @@ def _NormalizeDiagnostic( diag ):
   return diag
 
 
-class _DiagSignPlacement(
-                    namedtuple( "_DiagSignPlacement",
-                                [ 'id', 'line', 'buffer', 'is_error' ] ) ):
+class _DiagSignPlacement( namedtuple( "_DiagSignPlacement",
+                                      [ 'id', 'line', 'is_error' ] ) ):
   # We want two signs that have different ids but the same location to compare
   # equal. ID doesn't matter.
   def __eq__( self, other ):
     return ( self.line == other.line and
-             self.buffer == other.buffer and
              self.is_error == other.is_error )
diff --git a/python/ycm/tests/event_notification_test.py b/python/ycm/tests/event_notification_test.py
index fa4e082a..f7cf5dfc 100644
--- a/python/ycm/tests/event_notification_test.py
+++ b/python/ycm/tests/event_notification_test.py
@@ -105,7 +105,10 @@ def MockEventNotification( response_method, native_filetype_completer = True ):
         'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
         return_value = native_filetype_completer ):
 
-        yield
+        with patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+                    return_value = True ):
+
+          yield
 
 
 @patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
@@ -293,7 +296,6 @@ def _Check_FileReadyToParse_Diagnostic_Error( ycm, vim_command ):
       # Consequent calls to HandleFileParseRequest shouldn't mess with
       # existing diagnostics, when there is no new parse request.
       vim_command.reset_mock()
-      ok_( not ycm.FileParseRequestReady() )
       ycm.HandleFileParseRequest()
       vim_command.assert_not_called()
       eq_( ycm.GetErrorCount(), 1 )
@@ -327,7 +329,6 @@ def _Check_FileReadyToParse_Diagnostic_Warning( ycm, vim_command ):
       # Consequent calls to HandleFileParseRequest shouldn't mess with
       # existing diagnostics, when there is no new parse request.
       vim_command.reset_mock()
-      ok_( not ycm.FileParseRequestReady() )
       ycm.HandleFileParseRequest()
       vim_command.assert_not_called()
       eq_( ycm.GetErrorCount(), 0 )
@@ -351,6 +352,8 @@ def _Check_FileReadyToParse_Diagnostic_Clean( ycm, vim_command ):
 
 
 @patch( 'ycm.youcompleteme.YouCompleteMe._AddUltiSnipsDataIfNeeded' )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @YouCompleteMeInstance( { 'collect_identifiers_from_tags_files': 1 } )
 def EventNotification_FileReadyToParse_TagFiles_UnicodeWorkingDirectory_test(
     ycm, *args ):
@@ -495,6 +498,8 @@ def EventNotification_BufferUnload_BuildRequestForDeletedAndUnsavedBuffers_test(
 
 @patch( 'ycm.syntax_parse.SyntaxKeywordsForCurrentBuffer',
         return_value = [ 'foo', 'bar' ] )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @YouCompleteMeInstance( { 'seed_identifiers_with_syntax': 1 } )
 def EventNotification_FileReadyToParse_SyntaxKeywords_SeedWithCache_test(
     ycm, *args ):
@@ -529,6 +534,8 @@ def EventNotification_FileReadyToParse_SyntaxKeywords_SeedWithCache_test(
 
 @patch( 'ycm.syntax_parse.SyntaxKeywordsForCurrentBuffer',
         return_value = [ 'foo', 'bar' ] )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @YouCompleteMeInstance( { 'seed_identifiers_with_syntax': 1 } )
 def EventNotification_FileReadyToParse_SyntaxKeywords_ClearCacheIfRestart_test(
     ycm, *args ):
diff --git a/python/ycm/tests/test_utils.py b/python/ycm/tests/test_utils.py
index 36ce2478..30740188 100644
--- a/python/ycm/tests/test_utils.py
+++ b/python/ycm/tests/test_utils.py
@@ -40,7 +40,7 @@ BUFWINNR_REGEX = re.compile( '^bufwinnr\((?P<buffer_number>[0-9]+)\)$' )
 BWIPEOUT_REGEX = re.compile(
   '^(?:silent! )bwipeout!? (?P<buffer_number>[0-9]+)$' )
 GETBUFVAR_REGEX = re.compile(
-  '^getbufvar\((?P<buffer_number>[0-9]+), "&(?P<option>.+)"\)$' )
+  '^getbufvar\((?P<buffer_number>[0-9]+), "(?P<option>.+)"\)$' )
 MATCHADD_REGEX = re.compile(
   '^matchadd\(\'(?P<group>.+)\', \'(?P<pattern>.+)\'\)$' )
 MATCHDELETE_REGEX = re.compile( '^matchdelete\((?P<id>)\)$' )
@@ -85,10 +85,12 @@ def _MockGetBufferWindowNumber( buffer_number ):
 def _MockGetBufferVariable( buffer_number, option ):
   for vim_buffer in VIM_MOCK.buffers:
     if vim_buffer.number == buffer_number:
-      if option == 'mod':
+      if option == '&mod':
         return vim_buffer.modified
-      if option == 'ft':
+      if option == '&ft':
         return vim_buffer.filetype
+      if option == 'changedtick':
+        return vim_buffer.changedtick
       return ''
   return ''
 
@@ -230,6 +232,7 @@ class VimBuffer( object ):
     self.modified = modified
     self.window = window
     self.omnifunc = omnifunc
+    self.changedtick = 1
 
 
   def __getitem__( self, index ):
@@ -250,6 +253,10 @@ class VimBuffer( object ):
     return [ ToUnicode( x ) for x in self.contents ]
 
 
+def EmulateCurrentBufferChange():
+  VIM_MOCK.current.buffer.changedtick += 1
+
+
 class VimMatch( object ):
 
   def __init__( self, group, pattern ):
diff --git a/python/ycm/tests/youcompleteme_test.py b/python/ycm/tests/youcompleteme_test.py
index 53a95b87..a55eca25 100644
--- a/python/ycm/tests/youcompleteme_test.py
+++ b/python/ycm/tests/youcompleteme_test.py
@@ -349,6 +349,8 @@ def YouCompleteMe_ShowDiagnostics_NoDiagnosticsDetected_test(
                           'open_loclist_on_ycm_diags': 0 } )
 @patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
         return_value = True )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
 @patch( 'ycm.vimsupport.SetLocationList', new_callable = ExtendedMock )
 def YouCompleteMe_ShowDiagnostics_DiagnosticsFound_DoNotOpenLocationList_test(
@@ -388,6 +390,8 @@ def YouCompleteMe_ShowDiagnostics_DiagnosticsFound_DoNotOpenLocationList_test(
 @YouCompleteMeInstance( { 'open_loclist_on_ycm_diags': 1 } )
 @patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
         return_value = True )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
 @patch( 'ycm.vimsupport.SetLocationList', new_callable = ExtendedMock )
 @patch( 'ycm.vimsupport.OpenLocationList', new_callable = ExtendedMock )
@@ -431,6 +435,8 @@ def YouCompleteMe_ShowDiagnostics_DiagnosticsFound_OpenLocationList_test(
                           'enable_diagnostic_highlighting': 1 } )
 @patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
         return_value = True )
+@patch( 'ycm.youcompleteme.YouCompleteMe.IsServerReadyWithCache',
+        return_value = True )
 @patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
 @patch( 'vim.command', new_callable = ExtendedMock )
 def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
@@ -526,11 +532,7 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
 
     # Only the error sign is placed.
     vim_command.assert_has_exact_calls( [
-      call( 'sign define ycm_dummy_sign' ),
-      call( 'sign place 3 name=ycm_dummy_sign line=3 buffer=5' ),
       call( 'sign place 1 name=YcmError line=3 buffer=5' ),
-      call( 'sign undefine ycm_dummy_sign' ),
-      call( 'sign unplace 3 buffer=5' )
     ] )
 
     # When moving the cursor on the diagnostics, the error is displayed to the
@@ -540,3 +542,29 @@ def YouCompleteMe_UpdateDiagnosticInterface_PrioritizeErrorsOverWarnings_test(
       call( "expected ';' after expression (FixIt)",
             truncate = True, warning = False )
     ] )
+
+  # Test the case when error is fixed, but warning remains.
+  with MockVimBuffers( [ current_buffer ], current_buffer, ( 3, 1 ) ):
+    with patch( 'ycm.client.event_notification.EventNotification.Response',
+                return_value = diagnostics[ 1 : 2 ] ):
+      with patch( 'ycm.vimsupport.ClearYcmSyntaxMatches' ) as clear_matches:
+        test_utils.VIM_MATCHES = []
+        vim_command.reset_mock()
+        ycm.OnFileReadyToParse()
+        ycm.HandleFileParseRequest( block = True )
+        clear_matches.assert_has_calls( [ call() ] )
+
+    # Error match is added after warning matches.
+    assert_that(
+      test_utils.VIM_MATCHES,
+      contains(
+        VimMatch( 'YcmWarningSection', '\%3l\%5c\_.\{-}\%3l\%7c' ),
+        VimMatch( 'YcmWarningSection', '\%3l\%3c\_.\{-}\%3l\%9c' ),
+      )
+    )
+
+    # Only the error sign is placed.
+    vim_command.assert_has_exact_calls( [
+      call( 'sign place 2 name=YcmWarning line=3 buffer=5' ),
+      call( 'try | exec "sign unplace 1 buffer=5" | catch /E158/ | endtry' ),
+    ] )
diff --git a/python/ycm/vimsupport.py b/python/ycm/vimsupport.py
index 25c0392f..a37538de 100644
--- a/python/ycm/vimsupport.py
+++ b/python/ycm/vimsupport.py
@@ -147,6 +147,14 @@ def GetBufferFilepath( buffer_object ):
   return os.path.join( GetCurrentDirectory(), str( buffer_object.number ) )
 
 
+def GetCurrentBufferNumber():
+  return vim.current.buffer.number
+
+
+def GetBufferChangedTick( bufnr ):
+  return GetIntValue( 'getbufvar({0}, "changedtick")'.format( bufnr ) )
+
+
 def UnplaceSignInBuffer( buffer_number, sign_id ):
   if buffer_number < 0:
     return
@@ -584,6 +592,11 @@ def CurrentFiletypes():
   return VimExpressionToPythonType( "&filetype" ).split( '.' )
 
 
+def GetBufferFiletypes( bufnr ):
+  command = 'getbufvar({0}, "&ft")'.format( bufnr )
+  return VimExpressionToPythonType( command ).split( '.' )
+
+
 def FiletypesForBuffer( buffer_object ):
   # NOTE: Getting &ft for other buffers only works when the buffer has been
   # visited by the user at least once, which is true for modified buffers
diff --git a/python/ycm/youcompleteme.py b/python/ycm/youcompleteme.py
index 254ce200..5c17252c 100644
--- a/python/ycm/youcompleteme.py
+++ b/python/ycm/youcompleteme.py
@@ -33,10 +33,10 @@ import vim
 from subprocess import PIPE
 from tempfile import NamedTemporaryFile
 from ycm import base, paths, vimsupport
+from ycm.buffer import BufferDict
 from ycmd import utils
 from ycmd import server_utils
 from ycmd.request_wrap import RequestWrap
-from ycm.diagnostic_interface import DiagnosticInterface
 from ycm.omni_completer import OmniCompleter
 from ycm import syntax_parse
 from ycm.client.ycmd_keepalive import YcmdKeepalive
@@ -49,8 +49,7 @@ from ycm.client.completion_request import ( CompletionRequest,
 from ycm.client.debug_info_request import ( SendDebugInfoRequest,
                                             FormatDebugInfoResponse )
 from ycm.client.omni_completion_request import OmniCompletionRequest
-from ycm.client.event_notification import ( SendEventNotificationAsync,
-                                            EventNotification )
+from ycm.client.event_notification import SendEventNotificationAsync
 from ycm.client.shutdown_request import SendShutdownRequest
 
 
@@ -114,11 +113,9 @@ class YouCompleteMe( object ):
     self._available_completers = {}
     self._user_options = user_options
     self._user_notified_about_crash = False
-    self._diag_interface = DiagnosticInterface( user_options )
     self._omnicomp = OmniCompleter( user_options )
-    self._latest_file_parse_request = None
+    self._buffers = BufferDict( user_options )
     self._latest_completion_request = None
-    self._latest_diagnostics = []
     self._logger = logging.getLogger( 'ycm' )
     self._client_logfile = None
     self._server_stdout = None
@@ -134,6 +131,11 @@ class YouCompleteMe( object ):
       'cs': lambda self: self._OnCompleteDone_Csharp()
     }
 
+
+  def _GetCurrentBuffer( self ):
+    return self._buffers[ vimsupport.GetCurrentBufferNumber() ]
+
+
   def _SetupServer( self ):
     self._available_completers = {}
     self._user_notified_about_crash = False
@@ -230,6 +232,10 @@ class YouCompleteMe( object ):
     return self._server_is_ready_with_cache
 
 
+  def IsServerReadyWithCache( self ):
+    return self._server_is_ready_with_cache
+
+
   def _NotifyUserIfServerCrashed( self ):
     if self._user_notified_about_crash or self.IsServerAlive():
       return
@@ -353,11 +359,18 @@ class YouCompleteMe( object ):
              self.NativeFiletypeCompletionAvailable() )
 
 
+  def NeedsReparse( self ):
+    return self._GetCurrentBuffer().NeedsReparse()
+
+
   def OnFileReadyToParse( self ):
     if not self.IsServerAlive():
       self._NotifyUserIfServerCrashed()
       return
 
+    if not self.IsServerReadyWithCache():
+      return
+
     self._omnicomp.OnFileReadyToParse( None )
 
     extra_data = {}
@@ -365,9 +378,7 @@ class YouCompleteMe( object ):
     self._AddSyntaxDataIfNeeded( extra_data )
     self._AddExtraConfDataIfNeeded( extra_data )
 
-    self._latest_file_parse_request = EventNotification(
-      'FileReadyToParse', extra_data = extra_data )
-    self._latest_file_parse_request.Start()
+    self._GetCurrentBuffer().SendParseRequest( extra_data )
 
 
   def OnBufferUnload( self, deleted_buffer_file ):
@@ -385,7 +396,7 @@ class YouCompleteMe( object ):
 
 
   def OnCursorMoved( self ):
-    self._diag_interface.OnCursorMoved()
+    self._GetCurrentBuffer().OnCursorMoved()
 
 
   def _CleanLogfile( self ):
@@ -512,11 +523,11 @@ class YouCompleteMe( object ):
 
 
   def GetErrorCount( self ):
-    return self._diag_interface.GetErrorCount()
+    return self._GetCurrentBuffer().GetErrorCount()
 
 
   def GetWarningCount( self ):
-    return self._diag_interface.GetWarningCount()
+    return self._GetCurrentBuffer().GetWarningCount()
 
 
   def DiagnosticUiSupportedForCurrentFiletype( self ):
@@ -530,31 +541,33 @@ class YouCompleteMe( object ):
 
 
   def _PopulateLocationListWithLatestDiagnostics( self ):
-    # Do nothing if loc list is already populated by diag_interface
-    if not self._user_options[ 'always_populate_location_list' ]:
-      self._diag_interface.PopulateLocationList( self._latest_diagnostics )
-    return bool( self._latest_diagnostics )
+    return self._GetCurrentBuffer().PopulateLocationList()
 
 
   def UpdateDiagnosticInterface( self ):
     self._diag_interface.UpdateWithNewDiagnostics( self._latest_diagnostics )
 
 
-  def FileParseRequestReady( self, block = False ):
-    return bool( self._latest_file_parse_request and
-                 ( block or self._latest_file_parse_request.Done() ) )
+  def FileParseRequestReady( self ):
+    # Return True if server is not ready yet, to stop repeating check timer.
+    return ( not self.IsServerReadyWithCache() or
+             self._GetCurrentBuffer().FileParseRequestReady() )
 
 
   def HandleFileParseRequest( self, block = False ):
+    if not self.IsServerReadyWithCache():
+      return
+
+    current_buffer = self._GetCurrentBuffer()
     # Order is important here:
     # FileParseRequestReady has a low cost, while
     # NativeFiletypeCompletionUsable is a blocking server request
-    if ( self.FileParseRequestReady( block ) and
+    if ( not current_buffer.IsResponseHandled() and
+         current_buffer.FileParseRequestReady( block ) and
          self.NativeFiletypeCompletionUsable() ):
 
       if self.ShouldDisplayDiagnostics():
-        self._latest_diagnostics = self._latest_file_parse_request.Response()
-        self.UpdateDiagnosticInterface()
+        current_buffer.UpdateDiagnostics()
       else:
         # YCM client has a hard-coded list of filetypes which are known
         # to support diagnostics, self.DiagnosticUiSupportedForCurrentFiletype()
@@ -563,18 +576,9 @@ class YouCompleteMe( object ):
         # the _latest_file_parse_request for any exception or UnknownExtraConf
         # response, to allow the server to raise configuration warnings, etc.
         # to the user. We ignore any other supplied data.
-        self._latest_file_parse_request.Response()
+        current_buffer.GetResponse()
 
-      # We set the file parse request to None because we want to prevent
-      # repeated issuing of the same warnings/errors/prompts. Setting this to
-      # None makes FileParseRequestReady return False until the next
-      # request is created.
-      #
-      # Note: it is the server's responsibility to determine the frequency of
-      # error/warning/prompts when receiving a FileReadyToParse event, but
-      # it our responsibility to ensure that we only apply the
-      # warning/error/prompt received once (for each event).
-      self._latest_file_parse_request = None
+      current_buffer.MarkResponseHandled()
 
 
   def DebugInfo( self ):
@@ -709,7 +713,7 @@ class YouCompleteMe( object ):
     if filetype in self._filetypes_with_keywords_loaded:
       return
 
-    if self.IsServerReady():
+    if self.IsServerReadyWithCache():
       self._filetypes_with_keywords_loaded.add( filetype )
     extra_data[ 'syntax_keywords' ] = list(
        syntax_parse.SyntaxKeywordsForCurrentBuffer() )