diff --git a/main.rkt b/main.rkt
index 3810d93..20e7b81 100644
--- a/main.rkt
+++ b/main.rkt
@@ -13,6 +13,7 @@
          sl-log-to-file&display
          sl-set-log-level
          sl-log-level
+         sl-sync
          )
 
 (define (iso-timestamp)
@@ -26,8 +27,15 @@
             (date-second d))))
 
 (define sync-count 0)
-(define sync-sem (make-semaphore))
+(define sync-sems (make-hash))
 
+(define (sl-sync l topic)
+  (let ((sem (hash-ref sync-sems topic #f)))
+    (when (eq? sem #f)
+      (set! sem (make-semaphore))
+      (hash-set! sync-sems topic sem))
+    (log-message l 'fatal #f "@@SYNC@@" (list (iso-timestamp) topic))
+    (semaphore-wait sem)))
 
 (define log-callbacks #f)
 (set! log-callbacks '())
@@ -76,11 +84,17 @@
                    )
 
               (when (string=? msg "@@SYNC@@")
-                (set! sync-count (- sync-count 1))
-                (semaphore-post sync-sem)
-                (set! msg "synchronization received")
-                (set! level 'info)
-                )
+                (let ((sem (hash-ref sync-sems topic #f)))
+                  (if (eq? sem #f)
+                      (begin
+                        (set! msg (format "Cannot synchronize on topic ~a" topic))
+                        (set! level 'error))
+                      (begin
+                        (semaphore-post sem)
+                        (set! msg (format "Synchronization received on topic ~a" topic))
+                        (set! level 'info)
+                        )
+                      )))
 
               (enqueue! log-queue (list topic level dt msg))
               (unless (null? lc)
@@ -160,12 +174,7 @@
        (define (warnn msg . args) (log 'warning msg args))
        (define (errn msg . args) (log 'error msg args))
        (define (fataln msg . args) (log 'fatal msg args))
-       (define (syncn) (begin
-                         (log-message l 'fatal #f "@@SYNC@@" (list (iso-timestamp) 'id))
-                         (set! sync-count (+ sync-count 1))
-                         (semaphore-wait sync-sem)
-                         )
-         )
+       (define (syncn) (sl-sync l 'id))
        (log-to* receiver (λ () log-callbacks))
        )
      )