From 3733574105ddf81f8755f7583bb35bf32b3c9667 Mon Sep 17 00:00:00 2001
From: Feng <ery.lee@gmail.com>
Date: Tue, 12 Apr 2016 15:11:01 +0800
Subject: [PATCH] fix issue  #500

---
 src/emqttd_backend.erl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/emqttd_backend.erl b/src/emqttd_backend.erl
index f50e22bd2..ead4c59d5 100644
--- a/src/emqttd_backend.erl
+++ b/src/emqttd_backend.erl
@@ -96,7 +96,9 @@ expire_messages(Time) when is_integer(Time) ->
                             when Time > (MegaSecs * 1000000 + Secs) -> Topic
                         end),
             Topics = mnesia:select(retained_message, Match, write),
-            lists:foreach(fun(Topic) -> mnesia:delete({retained_message, Topic}) end, Topics)
+            lists:foreach(fun(<<"$SYS/", _/binary>>) -> ok; %% ignore $SYS/# messages
+                             (Topic) -> mnesia:delete({retained_message, Topic})
+                           end, Topics)
         end).
 
 -spec(retained_count() -> non_neg_integer()).