diff --git a/lib/MySQL_Monitor.cpp b/lib/MySQL_Monitor.cpp
index 5a0988ef23..45d2e1b550 100644
--- a/lib/MySQL_Monitor.cpp
+++ b/lib/MySQL_Monitor.cpp
@@ -107,124 +107,63 @@ static void close_mysql(MYSQL *my) {
 }
 
 
-
-struct cmp_str {
-	bool operator()(char const *a, char const *b) const
-	{
-		return strcmp(a, b) < 0;
-	}
-};
-
 class MySQL_Monitor_Connection_Pool {
-	private:
-	pthread_mutex_t mutex;
-	int size;
-	std::map<char *, PtrArray *, cmp_str> my_connections;
-	public:
-	MySQL_Monitor_Connection_Pool();
-	~MySQL_Monitor_Connection_Pool();
+private:
+	std::mutex mutex;
+	std::map<std::pair<std::string, int>, std::vector<MYSQL*> > my_connections;
+public:
 	MYSQL * get_connection(char *hostname, int port);
 	void put_connection(char *hostname, int port, MYSQL *my);
 	void purge_idle_connections();
 };
 
-MySQL_Monitor_Connection_Pool::MySQL_Monitor_Connection_Pool() {
-	size=0;
-	pthread_mutex_init(&mutex,NULL);
-}
-
-MySQL_Monitor_Connection_Pool::~MySQL_Monitor_Connection_Pool() {
-	// FIXME: destructor is yet not completed
-	std::map<char *, PtrArray *>::iterator it;
-	for(it = my_connections.begin(); it != my_connections.end(); it++) {
-		PtrArray *lst=it->second;
-		delete lst;
-		char *host=it->first;
-		free(host);
-	}
-	my_connections.erase(my_connections.begin(),my_connections.end());
-}
-
 void MySQL_Monitor_Connection_Pool::purge_idle_connections() {
-	unsigned long long now=monotonic_time();
-	pthread_mutex_lock(&mutex);
-	std::map<char *, PtrArray *>::iterator it;
-	//fprintf(stderr,"conn pool size: %d\n",my_connections.size());
-	unsigned int totconn;
-	totconn=0;
-	for(it = my_connections.begin(); it != my_connections.end(); it++) {
-		PtrArray *lst=it->second;
-		totconn+=lst->len;
-	}
-__loop_purge_idle_connections:
-	//fprintf(stderr,"tot conn in pool: %d\n",totconn);
-	for(it = my_connections.begin(); it != my_connections.end(); it++) {
-		PtrArray *lst=it->second;
-		if (lst->len) {
-			unsigned int it3;
-			for(it3 = 0; it3 < lst->len; it3++) {
-				MYSQL *my=(MYSQL *)(lst->index(it3));
-				unsigned long long then=0;
-				memcpy(&then,my->net.buff,sizeof(unsigned long long));
-				if (now > (then + mysql_thread___monitor_ping_interval*1000 * 3)) {
-					MySQL_Monitor_State_Data *mmsd= new MySQL_Monitor_State_Data((char *)"",0,NULL,false);
-					mmsd->mysql=my;
-					WorkItem *item;
-					item=new WorkItem(mmsd,NULL);
-					GloMyMon->queue.add(item);
-					lst->remove_index_fast(it3);
-					it3--;
-				}
-			}
+	unsigned long long now = monotonic_time();
+	std::lock_guard<std::mutex> lock(mutex);
+	for(auto it = my_connections.begin(); it != my_connections.end();) {
+		auto& lst = it->second;
+		for(auto it3 = lst.begin(); it3 != lst.end();) {
+			MYSQL *my = *it3;
+			unsigned long long then = *(unsigned long long*)my->net.buff;
+			if (now > (then + mysql_thread___monitor_ping_interval*1000 * 3)) {
+				MySQL_Monitor_State_Data *mmsd= new MySQL_Monitor_State_Data((char *)"",0,NULL,false);
+				mmsd->mysql=my;
+				GloMyMon->queue.add(new WorkItem(mmsd,NULL));
+				std::swap(*it3, lst.back());
+				if(it3 == lst.end() - 1)
+					it3 = lst.erase(it3);
+				else
+					lst.pop_back();
+			} else
+				++it3;
+		}
+		if (lst.size()) {
+			++it;
 		} else {
-			my_connections.erase(it);
-			goto __loop_purge_idle_connections;
+			it = my_connections.erase(it);
 		}
 	}
-	pthread_mutex_unlock(&mutex);
 }
 
 
 MYSQL * MySQL_Monitor_Connection_Pool::get_connection(char *hostname, int port) {
-	std::map<char *, PtrArray * , cmp_str >::iterator it;
-	char *buf=(char *)malloc(16+strlen(hostname));
-	sprintf(buf,"%s:%d",hostname,port);
-	pthread_mutex_lock(&mutex);
-	it = my_connections.find(buf);
-	free(buf);
-	if (it != my_connections.end()) {
-		PtrArray *lst=it->second;
-		if (lst->len) {
-			MYSQL *ret=(MYSQL *)lst->remove_index_fast(0);
-			size--;
-			pthread_mutex_unlock(&mutex);
-			memset(ret->net.buff,0,sizeof(unsigned long long)); // reset what was polluted
-			return ret;
-		}
-	}
-	pthread_mutex_unlock(&mutex);
-	return NULL;
+	std::lock_guard<std::mutex> lock(mutex);
+	auto it = my_connections.find(std::make_pair(hostname, port));
+	if (it == my_connections.end() || !it->second.size())
+		return NULL;
+	MYSQL *my = it->second.back();
+	it->second.pop_back();
+	*(unsigned long long*)my->net.buff = 0;
+	return my;
 }
 
 void MySQL_Monitor_Connection_Pool::put_connection(char *hostname, int port, MYSQL *my) {
-	size++;
-	std::map<char *, PtrArray * , cmp_str >::iterator it;
-	char * buf=(char *)malloc(16+strlen(hostname));
-	sprintf(buf,"%s:%d",hostname,port);
-	unsigned long long now=monotonic_time();
-	memcpy(my->net.buff,&now,sizeof(unsigned long long));	//mark insert time
-	pthread_mutex_lock(&mutex);
-	it = my_connections.find(buf);
-	PtrArray *lst=NULL;
-	if (it==my_connections.end()) {
-		lst=new PtrArray();
-		my_connections.insert(my_connections.begin(), std::pair<char *, PtrArray *>(buf,lst));
-	} else {
-		free(buf);
-		lst=it->second;
-	}
-	lst->add(my);
-	pthread_mutex_unlock(&mutex);
+	unsigned long long now = monotonic_time();
+	std::lock_guard<std::mutex> lock(mutex);
+	*(unsigned long long*)my->net.buff = now;
+	auto it = my_connections.emplace(std::piecewise_construct,
+		std::forward_as_tuple(hostname, port), std::forward_as_tuple()).first;
+	it->second.push_back(my);
 }
 
 MySQL_Monitor_State_Data::MySQL_Monitor_State_Data(char *h, int p, struct event_base *b, bool _use_ssl, int g) {
diff --git a/lib/MySQL_Session.cpp b/lib/MySQL_Session.cpp
index ea95accf96..12c23ac4a2 100644
--- a/lib/MySQL_Session.cpp
+++ b/lib/MySQL_Session.cpp
@@ -2336,7 +2336,11 @@ int MySQL_Session::handler() {
 						myconn->async_state_machine=ASYNC_IDLE;
 						myds->DSS=STATE_MARIADB_GENERIC;
 						if (transaction_persistent==true) {
-							transaction_persistent_hostgroup=current_hostgroup;
+							if (transaction_persistent_hostgroup==-1) { // change only if not set already, do not allow to change it again
+								if (myds->myconn->IsActiveTransaction()==true) { // only active transaction is important here. Ignore other criterias
+									transaction_persistent_hostgroup=current_hostgroup;
+								}
+							}
 						}
 					}
 				} else {