diff --git a/hio/bin/t04.c b/hio/bin/t04.c
index aa9aae1..c4d789e 100644
--- a/hio/bin/t04.c
+++ b/hio/bin/t04.c
@@ -237,7 +237,7 @@ int main (int argc, char* argv[])
 	HIO_INIT_NTIME (&tmout.w, -1,  0);
 */
 
-	marc = hio_svc_marc_start(hio, &ci, HIO_NULL);
+	marc = hio_svc_marc_start(hio, &ci, HIO_NULL, HIO_NULL);
 	if (!marc)
 	{
 		printf ("Cannot start a mariadb client service\n");
diff --git a/hio/lib/hio-mar.h b/hio/lib/hio-mar.h
index 82345ec..c2bafa7 100644
--- a/hio/lib/hio-mar.h
+++ b/hio/lib/hio-mar.h
@@ -125,6 +125,8 @@ struct hio_dev_mar_make_t
 {
 	int flags;
 	hio_dev_mar_tmout_t tmout;
+	const hio_bch_t* default_group;
+
 	hio_dev_mar_on_write_t on_write; /* mandatory */
 	hio_dev_mar_on_read_t on_read; /* mandatory */
 	hio_dev_mar_on_connect_t on_connect; /* optional */
@@ -251,7 +253,8 @@ HIO_EXPORT hio_oow_t hio_dev_mar_escapebchars (
 HIO_EXPORT hio_svc_marc_t* hio_svc_marc_start (
 	hio_t*                        hio,
 	const hio_svc_marc_connect_t* ci,
-	const hio_svc_marc_tmout_t*   tmout
+	const hio_svc_marc_tmout_t*   tmout,
+	const hio_bch_t*              default_group
 );
 
 HIO_EXPORT void hio_svc_marc_stop (
diff --git a/hio/lib/hio.h b/hio/lib/hio.h
index ba314da..248a00f 100644
--- a/hio/lib/hio.h
+++ b/hio/lib/hio.h
@@ -222,7 +222,8 @@ struct hio_dev_mth_t
 	void          (*fail_before_make)     (void* ctx);
 
 	/* ------------------------------------------------------------------ */
-	hio_syshnd_t (*getsyshnd)    (hio_dev_t* dev); /* mandatory. called in hio_dev_make() after successful make() */
+	hio_syshnd_t (*getsyshnd)      (hio_dev_t* dev); /* mandatory. called in hio_dev_make() after successful make() */
+	int          (*issyshndbroken) (hio_dev_t* dev); /* the device whose underlying system handle can get closed before kill() must implement this */
 
 	/* ------------------------------------------------------------------ */
 	/* return -1 on failure, 0 if no data is availble, 1 otherwise.
diff --git a/hio/lib/mar-cli.c b/hio/lib/mar-cli.c
index 5e09387..ed7b973 100644
--- a/hio/lib/mar-cli.c
+++ b/hio/lib/mar-cli.c
@@ -45,6 +45,8 @@ struct hio_svc_marc_t
 	int stopping;
 	int tmout_set;
 
+	const hio_bch_t* default_group;
+
 	hio_svc_marc_connect_t ci;
 	hio_svc_marc_tmout_t tmout;
 
@@ -90,7 +92,7 @@ struct dev_xtn_t
 
 #define INVALID_SID HIO_TYPE_MAX(hio_oow_t)
 
-hio_svc_marc_t* hio_svc_marc_start (hio_t* hio, const hio_svc_marc_connect_t* ci, const hio_svc_marc_tmout_t* tmout)
+hio_svc_marc_t* hio_svc_marc_start (hio_t* hio, const hio_svc_marc_connect_t* ci, const hio_svc_marc_tmout_t* tmout, const hio_bch_t* default_group)
 {
 	hio_svc_marc_t* marc = HIO_NULL;
 
@@ -108,6 +110,7 @@ hio_svc_marc_t* hio_svc_marc_start (hio_t* hio, const hio_svc_marc_connect_t* ci
 		marc->tmout = *tmout;
 		marc->tmout_set = 1;
 	}
+	marc->default_group = default_group;
 
 	HIO_SVCL_APPEND_SVC (&hio->actsvc, (hio_svc_t*)marc);
 	return marc;
@@ -368,6 +371,7 @@ static hio_dev_mar_t* alloc_device (hio_svc_marc_t* marc, hio_oow_t sid)
 		mi.flags = HIO_DEV_MAR_USE_TMOUT;
 		mi.tmout = marc->tmout;
 	}
+	mi.default_group = marc->default_group;
 
 	mi.on_connect = mar_on_connect;
 	mi.on_disconnect = mar_on_disconnect;
@@ -378,11 +382,12 @@ static hio_dev_mar_t* alloc_device (hio_svc_marc_t* marc, hio_oow_t sid)
 	if (HIO_UNLIKELY(!mar)) return HIO_NULL;
 
 	xtn = (dev_xtn_t*)hio_dev_mar_getxtn(mar);
-	xtn->sid = sid;
 	xtn->svc = marc;
+	xtn->sid = sid;
 
 	if (hio_dev_mar_connect(mar, &marc->ci) <= -1) 
 	{
+		/* connection failed immediately */
 		xtn->sid = INVALID_SID;
 		hio_dev_mar_halt (mar);
 		return HIO_NULL;
@@ -422,8 +427,8 @@ static sess_t* get_session (hio_svc_marc_t* marc, hio_oow_t sid)
 	}
 
 	sess = &marc->sess.ptr[sid];
-	HIO_ASSERT (hio, sess->sid == sid);
 	HIO_ASSERT (hio, sess->svc == marc);
+	HIO_ASSERT (hio, sess->sid == sid);
 
 	if (!sess->dev)
 	{
diff --git a/hio/lib/mar.c b/hio/lib/mar.c
index 60f1f1a..cebe5b5 100644
--- a/hio/lib/mar.c
+++ b/hio/lib/mar.c
@@ -52,6 +52,9 @@ static int dev_mar_make (hio_dev_t* dev, void* ctx)
 		return -1;
 	}
 
+	if (mi->default_group)  /* don't care about success/failure */
+		mysql_options(rdev->hnd, MYSQL_READ_DEFAULT_GROUP, mi->default_group);
+
 	if (mysql_options(rdev->hnd, MYSQL_OPT_NONBLOCK, 0) != 0)
 	{
 		hio_seterrbfmt (hio, HIO_ESYSERR, "%hs", mysql_error(rdev->hnd));
@@ -137,6 +140,7 @@ static int dev_mar_kill (hio_dev_t* dev, int force)
 
 	rdev->connected = 0;
 	rdev->broken = 0;
+	rdev->broken_syshnd = HIO_SYSHND_INVALID;
 
 	return 0;
 }
@@ -148,6 +152,12 @@ static hio_syshnd_t dev_mar_getsyshnd (hio_dev_t* dev)
 	return (hio_syshnd_t)mysql_get_socket(rdev->hnd);
 }
 
+static int dev_mar_issyshndbroken (hio_dev_t* dev)
+{
+	hio_dev_mar_t* rdev = (hio_dev_mar_t*)dev;
+	return rdev->broken;
+}
+
 static int events_to_mysql_wstatus (int events)
 {
 	int wstatus = 0;
@@ -235,12 +245,17 @@ static int dev_mar_ioctl (hio_dev_t* dev, int cmd, void* arg)
 				if (HIO_UNLIKELY(!tmp)) /* connection attempt failed immediately */
 				{
 					/* immediate failure doesn't invoke on_discoonect(). 
-					 * the caller must check the return code of this function.  */
+					 * the caller must check the return code of this function. */
+
+					rdev->connected = 0;
+					rdev->broken = 1;
+					rdev->broken_syshnd = HIO_SYSHND_INVALID;
+
 					hio_seterrbfmt (hio, HIO_ESYSERR, "%hs", mysql_error(rdev->hnd));
 					return -1;
 				}
 
-				/* connected_deferred immediately. postpone actual handling to the ready() callback */
+				/* connected immediately. postpone actual handling to the ready() callback */
 				HIO_DEV_MAR_SET_PROGRESS (rdev, HIO_DEV_MAR_CONNECTING);
 				rdev->connected_deferred = 1; /* to let the ready() handler to trigger on_connect() */
 				/* regiter it in the multiplexer so that the ready() handler is
@@ -291,8 +306,12 @@ static int dev_mar_ioctl (hio_dev_t* dev, int cmd, void* arg)
 						/* the underlying socket must have gotten closed by mysql_real_query_start() */
 						const hio_ooch_t* prev_errmsg;
 						prev_errmsg = hio_backuperrmsg(hio);
+
+						rdev->connected = 0;
 						rdev->broken = 1;
-						rdev->broken_syshnd = syshnd;
+						/* remember the previous handle - this may be needed by the poll/select based multiplexer */
+						rdev->broken_syshnd = syshnd; 
+
 						watch_mysql (rdev, 0);
 						hio_dev_mar_halt (rdev); /* i can't keep this device alive regardless of the caller's post-action */
 						hio_seterrbfmt (hio, HIO_ESYSERR, "%js", prev_errmsg);
@@ -335,6 +354,7 @@ static hio_dev_mth_t dev_mar_methods =
 	dev_mar_kill,
 	HIO_NULL,
 	dev_mar_getsyshnd,
+	dev_mar_issyshndbroken,
 
 	HIO_NULL,
 	HIO_NULL,
@@ -416,8 +436,16 @@ static int dev_evcb_mar_ready (hio_dev_t* dev, int events)
 					{
 						/* connection attempt failed */
 
+						/* the mysql client library closes the underlying socket handle 
+						 * whenever the connection attempt fails. this prevent hio from 
+						 * managing the the mysql connections properly. this also causes
+						 * race condition if this library is used in multi-threaded programs. */
+
+						rdev->connected = 0;
 						rdev->broken = 1; /* trick dev_mar_getsyshnd() to return rdev->broken_syshnd. */
-						rdev->broken_syshnd = syshnd; /* mysql_get_socket() over a failed mariadb handle ends up with segfault */
+						/* remember the previous handle - this may be needed by the poll/select based multiplexer
+						 * mysql_get_socket() over a failed mariadb handle ends up with segfault */
+						rdev->broken_syshnd = syshnd; 
 
 						/* this attempts to trigger the low-level multiplxer to delete 'syshnd' closed by mysql_real_connect_cont().
 						 * the underlying low-level operation may fail. but i don't care. the best is not to open 
diff --git a/hio/lib/pipe.c b/hio/lib/pipe.c
index 880273d..e0f7320 100644
--- a/hio/lib/pipe.c
+++ b/hio/lib/pipe.c
@@ -376,6 +376,7 @@ static hio_dev_mth_t dev_pipe_methods =
 	dev_pipe_kill_master,
 	HIO_NULL,
 	dev_pipe_getsyshnd,
+	HIO_NULL,
 
 	HIO_NULL,
 	HIO_NULL,
@@ -389,6 +390,7 @@ static hio_dev_mth_t dev_pipe_methods_slave =
 	dev_pipe_kill_slave,
 	dev_pipe_fail_before_make_slave,
 	dev_pipe_getsyshnd_slave,
+	HIO_NULL,
 
 	dev_pipe_read_slave,
 	dev_pipe_write_slave,
diff --git a/hio/lib/pro.c b/hio/lib/pro.c
index cdeb251..67b2934 100644
--- a/hio/lib/pro.c
+++ b/hio/lib/pro.c
@@ -738,6 +738,7 @@ static hio_dev_mth_t dev_pro_methods =
 	dev_pro_kill_master,
 	HIO_NULL,
 	dev_pro_getsyshnd,
+	HIO_NULL,
 
 	HIO_NULL, /* read */
 	HIO_NULL, /* write */
@@ -752,6 +753,7 @@ static hio_dev_mth_t dev_pro_methods_slave =
 	dev_pro_kill_slave,
 	HIO_NULL,
 	dev_pro_getsyshnd_slave,
+	HIO_NULL,
 
 	dev_pro_read_slave,
 	dev_pro_write_slave,
diff --git a/hio/lib/sck.c b/hio/lib/sck.c
index 0be43fc..1666ab9 100644
--- a/hio/lib/sck.c
+++ b/hio/lib/sck.c
@@ -1398,6 +1398,7 @@ static hio_dev_mth_t dev_mth_sck_stateless =
 	dev_sck_kill,
 	HIO_NULL,
 	dev_sck_getsyshnd,
+	HIO_NULL,
 
 	dev_sck_read_stateless,
 	dev_sck_write_stateless,
@@ -1413,6 +1414,7 @@ static hio_dev_mth_t dev_mth_sck_stateful =
 	dev_sck_kill,
 	HIO_NULL,
 	dev_sck_getsyshnd,
+	HIO_NULL,
 
 	dev_sck_read_stateful,
 	dev_sck_write_stateful,
@@ -1427,6 +1429,7 @@ static hio_dev_mth_t dev_mth_clisck =
 	dev_sck_kill,
 	dev_sck_fail_before_make_client,
 	dev_sck_getsyshnd,
+	HIO_NULL,
 
 	dev_sck_read_stateful,
 	dev_sck_write_stateful,
@@ -1441,6 +1444,7 @@ static hio_dev_mth_t dev_mth_sck_bpf =
 	dev_sck_kill,
 	HIO_NULL,
 	dev_sck_getsyshnd,
+	HIO_NULL,
 
 	dev_sck_read_bpf,
 	dev_sck_write_bpf,
diff --git a/hio/lib/sys-mux.c b/hio/lib/sys-mux.c
index 7d5b609..e83a1aa 100644
--- a/hio/lib/sys-mux.c
+++ b/hio/lib/sys-mux.c
@@ -419,6 +419,11 @@ int hio_sys_ctrlmux (hio_t* hio, hio_sys_mux_cmd_t cmd, hio_dev_t* dev, int dev_
 	int x;
 
 	HIO_ASSERT (hio, hio == dev->hio);
+
+	/* no operation over a broken(closed) handle to prevent multiplexer from failing.
+	 * close of the handle leads to auto-deletion from the kqueue multiplexer. 
+	 * the closed handle must not be fed to the multiplexer */
+	if (dev->dev_mth->issyshndbroken && dev->dev_mth->issyshndbroken(dev)) return 0;
 	hnd = dev->dev_mth->getsyshnd(dev);
 
 	switch (cmd)
@@ -495,6 +500,11 @@ int hio_sys_ctrlmux (hio_t* hio, hio_sys_mux_cmd_t cmd, hio_dev_t* dev, int dev_
 	int x;
 
 	HIO_ASSERT (hio, hio == dev->hio);
+
+	/* no operation over a broken(closed) handle to prevent multiplexer from failing.
+	 * close of the handle leads to auto-deletion from the epoll multiplexer. 
+	 * the closed handle must not be fed to the multiplexer */
+	if (dev->dev_mth->issyshndbroken && dev->dev_mth->issyshndbroken(dev)) return 0;
 	hnd = dev->dev_mth->getsyshnd(dev);
 
 	events = 0;
diff --git a/hio/lib/thr.c b/hio/lib/thr.c
index f36dabb..12f8fd4 100644
--- a/hio/lib/thr.c
+++ b/hio/lib/thr.c
@@ -568,6 +568,7 @@ static hio_dev_mth_t dev_thr_methods =
 	dev_thr_kill_master,
 	HIO_NULL,
 	dev_thr_getsyshnd,
+	HIO_NULL,
 
 	HIO_NULL,
 	HIO_NULL,
@@ -581,6 +582,7 @@ static hio_dev_mth_t dev_thr_methods_slave =
 	dev_thr_kill_slave,
 	dev_thr_fail_before_make_slave,
 	dev_thr_getsyshnd_slave,
+	HIO_NULL,
 
 	dev_thr_read_slave,
 	dev_thr_write_slave,