diff --git a/lib/RON/Storage.hs b/lib/RON/Storage.hs
--- a/lib/RON/Storage.hs
+++ b/lib/RON/Storage.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
@@ -9,6 +10,7 @@
 -- | RON File Storage. For usage, see "RON.Storage.IO".
 module RON.Storage (
     Collection (..),
+    CollectionDocId (..),
     CollectionName,
     DocId (..),
     Document (..),
@@ -47,6 +49,8 @@
 instance Collection a => Show (DocId a) where
     show (DocId file) = collectionName @a </> file
 
+data CollectionDocId = forall a. Collection a => CollectionDocId (DocId a)
+
 -- | Collection (directory name)
 type CollectionName = FilePath
 
@@ -190,7 +194,7 @@
         let Document{value = oldObj, versions, isTouched = IsTouched isTouched}
                 = oldDoc
         when (newObj /= oldObj || length versions /= 1 || isTouched) $
-            save docid versions
+            save docid $ toList versions
   where
     Object{id, frame} = newObj
 
diff --git a/lib/RON/Storage/IO.hs b/lib/RON/Storage/IO.hs
--- a/lib/RON/Storage/IO.hs
+++ b/lib/RON/Storage/IO.hs
@@ -25,14 +25,17 @@
     module X,
     -- * Handle
     Handle,
-    OnDocumentChanged (..),
     newHandle,
-    setOnDocumentChanged,
     -- * Storage
     Storage,
     runStorage,
+    subscribeForever,
 ) where
 
+import           Control.Concurrent.STM (TChan, atomically, dupTChan,
+                                         newBroadcastTChanIO, readTChan,
+                                         writeTChan)
+import           Control.Monad (forever)
 import           Data.Bits (shiftL)
 import qualified Data.ByteString.Lazy as BSL
 import           Network.Info (MAC (MAC), getNetworkInterfaces, mac)
@@ -125,21 +128,13 @@
     { hClock             :: IORef EpochTime
     , hDataDir           :: FilePath
     , hReplica           :: ReplicaId
-    , hOnDocumentChanged :: IORef (Maybe OnDocumentChanged)
+    , hOnDocumentChanged :: TChan CollectionDocId
     }
 
--- | The handler is called as @onDocumentChanged docid@, where
--- @docid@ is the changed document id.
-newtype OnDocumentChanged =
-    OnDocumentChanged (forall a . Collection a => DocId a -> IO ())
-
 emitDocumentChanged :: Collection a => DocId a -> Storage ()
 emitDocumentChanged docid = Storage $ do
     Handle{hOnDocumentChanged} <- ask
-    liftIO $ do
-        mOnDocumentChanged <- readIORef hOnDocumentChanged
-        whenJust mOnDocumentChanged $ \(OnDocumentChanged onDocumentChanged) ->
-            onDocumentChanged docid
+    liftIO . atomically $ writeTChan hOnDocumentChanged $ CollectionDocId docid
 
 -- | Create new storage handle
 newHandle :: FilePath -> IO Handle
@@ -147,13 +142,9 @@
     time <- getCurrentEpochTime
     hClock <- newIORef time
     hReplica <- applicationSpecific <$> getMacAddress
-    hOnDocumentChanged <- newIORef Nothing
+    hOnDocumentChanged <- newBroadcastTChanIO
     pure Handle{hDataDir, hClock, hReplica, hOnDocumentChanged}
 
-setOnDocumentChanged :: Handle -> OnDocumentChanged -> IO ()
-setOnDocumentChanged h handler =
-    writeIORef (hOnDocumentChanged h) $ Just handler
-
 listDirectoryIfExists :: FilePath -> Storage [FilePath]
 listDirectoryIfExists relpath = Storage $ do
     Handle{hDataDir} <- ask
@@ -183,3 +174,10 @@
         + fromIntegral b2 `shiftL` 16
         + fromIntegral b1 `shiftL` 8
         + fromIntegral b0
+
+subscribeForever :: Handle -> (CollectionDocId -> IO ()) -> IO ()
+subscribeForever Handle{hOnDocumentChanged} action = do
+    childChan <- atomically $ dupTChan hOnDocumentChanged
+    forever $ do
+        docId <- atomically $ readTChan childChan
+        action docId
diff --git a/ron-storage.cabal b/ron-storage.cabal
--- a/ron-storage.cabal
+++ b/ron-storage.cabal
@@ -1,7 +1,7 @@
 cabal-version:  2.2
 
 name:           ron-storage
-version:        0.5
+version:        0.6
 
 bug-reports:    https://github.com/ff-notes/ron/issues
 category:       Distributed Systems, Protocol, Database
@@ -58,6 +58,7 @@
         filepath,
         mtl,
         network-info,
+        stm,
         text,
         transformers,
         -- project