diff --git a/src/datascript/conn.cljc b/src/datascript/conn.cljc
index bb1c3e30..d45a7472 100644
--- a/src/datascript/conn.cljc
+++ b/src/datascript/conn.cljc
@@ -96,10 +96,11 @@
    (transact! conn tx-data nil))
   ([conn tx-data tx-meta]
    {:pre [(conn? conn)]}
-   (let [report (-transact! conn tx-data tx-meta)]
-     (doseq [[_ callback] (some-> (:listeners (meta conn)) (deref))]
-       (callback report))
-     report)))
+   (locking conn
+     (let [report (-transact! conn tx-data tx-meta)]
+       (doseq [[_ callback] (some-> (:listeners (meta conn)) (deref))]
+         (callback report))
+       report))))
 
 (defn reset-conn!
   ([conn db]
diff --git a/src/datascript/sync/client.cljc b/src/datascript/sync/client.cljc
index 9b895fc3..9369b07b 100644
--- a/src/datascript/sync/client.cljc
+++ b/src/datascript/sync/client.cljc
@@ -5,7 +5,7 @@
     [datascript.serialize :as serialize]))
 
 (defn client-id []
-  (long (* (rand) 9007199254740991)))
+  (long (* (rand) 0x1FFFFFFFFFFFFF)))
 
 (def *last-tx-id
   (atom 0))
diff --git a/src/datascript/sync/server.cljc b/src/datascript/sync/server.cljc
index 14c2bab7..7fbfee87 100644
--- a/src/datascript/sync/server.cljc
+++ b/src/datascript/sync/server.cljc
@@ -8,15 +8,20 @@
   (get @(:clients (meta conn)) channel))
 
 (defn on-tx [conn report]
-  ;; TODO filter what to send where
-  (let [msg {:message    :transacted
-             :tx-data    (db/tx-from-datoms (:tx-data report))
-             :tx-id      (:tx-id (:tx-meta report))
-             :server-idx (:db/current-tx (:tempids report))}]
-    (doseq [[channel {:keys [status send-fn]}] @(:clients (meta conn))
-            ; :let [_ (prn "broadcasting to" channel status)]
-            :when (= :active status)]
-      (send-fn channel msg))))
+  (let [*clients (:clients (meta conn))
+        msg      {:message    :transacted
+                  :tx-data    (db/tx-from-datoms (:tx-data report))
+                  :tx-id      (:tx-id (:tx-meta report))
+                  :server-idx (:db/current-tx (:tempids report))}]
+    (doseq [[channel {:keys [status send-fn pending]}] @*clients]
+      (if (= :active status)
+        (do
+          (when pending
+            (doseq [msg pending]
+              (send-fn channel msg))
+            (swap! *clients update client dissoc :pending))
+          (send-fn channel msg))
+        (swap! *clients update client update :pending (fnil conj []) msg)))))
 
 (defn client-connected [conn channel send-fn]
   (let [*clients (:clients (meta conn))
@@ -27,18 +32,26 @@
       (conn/listen! conn :sync #(on-tx conn %)))
     nil))
 
+(defn drop-before [txs server-idx]
+  (vec
+    (drop-while #(<= (:server-idx %) server-idx) txs)))
+
 (defn client-message [conn channel body]
   (case (:message body)
     :catching-up
     (let [{:keys [patterns server-idx]} body ;; TODO delta from server-idx
           {:keys [send-fn]}             (client conn channel)
-          db                            @conn]
+          db                            @conn
+          server-idx                    (:max-tx db)]
       (send-fn channel
         {:message    :catched-up
          :snapshot   (serialize/serializable db) ;; TODO patterns
-         :server-idx (:max-tx db)})
-      ;; TODO race - external txs between (:max-tx db) and after :status :active
-      (swap! (:clients (meta conn)) update channel assoc :status :active))
+         :server-idx server-idx})
+      (swap! (:clients (meta conn)) update channel
+        (fn [client]
+          (-> client
+            (assoc :status :active)
+            (update :pending drop-before server-idx)))))
     
     :transacting
     (doseq [{:keys [tx-data tx-id]} (:txs body)]
diff --git a/test/datascript/test/sync.clj b/test/datascript/test/sync.clj
index f9b7c6f5..0c405c91 100644
--- a/test/datascript/test/sync.clj
+++ b/test/datascript/test/sync.clj
@@ -22,14 +22,10 @@
         (throw (ex-info "Timeout" {}))))))
 
 (def freeze
-  ; identity
-  tdc/transit-write-str
-  )
+  tdc/transit-write-str)
 
 (def thaw
-  ; identity
-  tdc/transit-read-str
-  )
+  tdc/transit-read-str)
 
 (defn setup []
   (let [server (d/create-conn)
@@ -80,7 +76,7 @@
           (tdc/all-datoms @c2)))
     (is (= #{[1 :name "Ivan"]}
           (tdc/all-datoms @server)))))
-  
+
 
 ; (t/test-ns *ns*)
 ; (t/run-test-var #'test-conn)