Add sticky backend query hint

include/MySQL_Session.h

@@ -305,6 +305,12 @@ class MySQL_Session
 	PtrArray *mybes;
 	MySQL_Data_Stream *client_myds;
 	MySQL_Data_Stream *server_myds;
+
+	// Storage for per-hostgroup backend server hints
+	std::map<unsigned int, MySrvC *> sticky_backend_hint;
+
+	int sticky_backend;
+
 	/*
 	 * @brief Store the hostgroups that hold connections that have been flagged as 'expired' by the
 	 *  maintenance thread. These values will be used to release the retained connections in the specific

include/query_processor.h

@@ -131,6 +131,7 @@ class Query_Processor_Output {
 	int timeout;
 	int retries;
 	int delay;
+	int sticky_backend;
 	char *error_msg;
 	char *OK_msg;
 	int sticky_conn;
@@ -170,6 +171,7 @@ class Query_Processor_Output {
 		timeout=-1;
 		retries=-1;
 		delay=-1;
+		sticky_backend=-1;
 		sticky_conn=-1;
 		multiplex=-1;
 		gtid_from_hostgroup=-1;

lib/MyHGC.cpp

@@ -362,6 +362,36 @@ MySrvC *MyHGC::get_random_MySrvC(char * gtid_uuid, uint64_t gtid_trxid, int max_
 		k++;
 		New_sum=0;
 
+		// This is promoting the stickyness value in the query hint to be a session variable
+		if (sess->qpo->sticky_backend != -1 && 0 <= sess->qpo->sticky_backend && sess->qpo->sticky_backend <= 2) {
+			proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "promoting sticky_backend with value %d\n", sess->qpo->sticky_backend);
+			sess->sticky_backend = sess->qpo->sticky_backend;
+		}
+
+		if (sess->sticky_backend > 0) {
+			if (auto my_srv_hint = sess->sticky_backend_hint.find(this->hid); my_srv_hint != sess->sticky_backend_hint.end()) {
+				proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "found prefered sticky backend\n");
+				for (j=0; j<num_candidates; j++) {
+					mysrvc = mysrvcCandidates[j];
+					if(mysrvc == my_srv_hint->second) {
+						proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "matched prefered sticky backend\n");
+						return mysrvc;
+					}
+				}
+
+				//Abort if we can't find the backend in candidates and we are in strict mode.
+				if (sess->sticky_backend==2) {
+					proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "didn't match prefered sticky backend in strict mode\n");
+					return NULL;
+				}
+
+				// We are clearing the server because .insert doesn't replace...
+				proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "clearing prefered sticky backend in strict mode\n");
+				sess->sticky_backend_hint.erase(this->hid);
+			}
+		}
+
+
 		for (j=0; j<num_candidates; j++) {
 			mysrvc = mysrvcCandidates[j];
 			New_sum+=mysrvc->weight;
@@ -373,6 +403,14 @@ MySrvC *MyHGC::get_random_MySrvC(char * gtid_uuid, uint64_t gtid_trxid, int max_
 #ifdef TEST_AURORA
 				array_mysrvc_cands += num_candidates;
 #endif // TEST_AURORA
+				if (sess->sticky_backend > 0) {
+					proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "insert prefered sticky backend\n");
+					sess->sticky_backend_hint.insert({this->hid, mysrvc});
+				} else {
+					proxy_debug(PROXY_DEBUG_MYSQL_CONNPOOL, 7, "erase prefered sticky backend\n");
+					sess->sticky_backend_hint.erase(this->hid);
+				}
+
 				return mysrvc;
 			}
 		}

lib/MySQL_Session.cpp

@@ -728,6 +728,12 @@ MySQL_Session::MySQL_Session() {
 	use_ssl = false;
 	change_user_auth_switch = false;
 
+	// Session-level status of the sticky_backend
+	// 0 means non-sticky
+	// 1 means sticky, fallback to new server in case of server failure
+	// 2 means strictly sticky, throw an error in case of server failure
+	sticky_backend=0;
+
 	//gtid_trxid = 0;
 	gtid_hid = -1;
 	memset(gtid_buf,0,sizeof(gtid_buf));

lib/Query_Processor.cpp

@@ -2600,6 +2600,12 @@ bool Query_Processor::query_parser_first_comment(Query_Processor_Output *qpo, ch
 		remove_spaces(value);
 		if (strlen(key)) {
 			char c=value[0];
+			if (!strcasecmp(key,"sticky_backend")) {
+				if (c >= '0' && c <= '9') { // it is a digit
+					int t=atoi(value);
+					qpo->sticky_backend=t;
+				}
+			}
 			if (!strcasecmp(key,"cache_ttl")) {
 				if (c >= '0' && c <= '9') { // it is a digit
 					int t=atoi(value);