Connect timeout after ProxySQL start, during storm of backend connections (3.x only, not reproducible on 2.x)
Team,
Note: The issue described below is reproducible every time ProxySQL starts and has been observed only with ProxySQL 3.x, including the latest tested version, ProxySQL 3.0.11. The issue is not reproducible with ProxySQL 2.x.
Due to this behavior, the issue is a concern and may create challenges when upgrading from ProxySQL 2.x to ProxySQL 3.x.
Issue : Basically, the bug is triggered when ProxySQL has to create bunch of new connections to the backend and that backend has TLS enabled. If TLS is disabled between ProxySQL and MySQL backend, such storms aren't dangerous.
Here're steps to reproduce this bug.
Required Files : proxysql-bug-docker-compose.tgz
- Create ProxySQL+MySQL+sysbech setup with docker compose.
docker compose -f docker-compose-mysql-8.4.yml up
- Log in to sysbench container and run workload using sysbench
docker exec -it sysbench /bin/bash
[root@c7b4454d5ebb sysbench]# ./connection-only.sh- ProxySQL logs will be filled with "Connect timeout" errors. Backend might be shunned. Like Below :
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 828686us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 828686us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 828686us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 828686us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 MySrvC.cpp:129:connect_error(): [ERROR] Shunning server mysql-server:3306 with 5 errors/sec. Shunning for 10 seconds
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 MySrvC.cpp:129:connect_error(): [ERROR] Shunning server mysql-server:3306 with 5 errors/sec. Shunning for 10 seconds
proxysql | 2026-09-21 10:42:57 mysql_connection.cpp:1441:handler(): [ERROR] Connect timeout on mysql-server:3306 : exceeded by 1512638us
proxysql | 2026-09-21 10:42:57 MyHGC.cpp:202:get_random_MySrvC(): [ERROR] Hostgroup 0 has no servers available! Checking servers shunned for more than 1 second
proxysql | 2026-09-21 10:42:57 MyHGC.cpp:202:get_random_MySrvC(): [ERROR] Hostgroup 0 has no servers available! Checking servers shunned for more than 1 second
proxysql | 2026-09-21 10:42:59 MyHGC.cpp:202:get_random_MySrvC(): [ERROR] Hostgroup 0 has no servers available! Checking servers shunned for more than 1 second
proxysql | 2026-09-21 10:42:59 MyHGC.cpp:202:get_random_MySrvC(): [ERROR] Hostgroup 0 has no servers available! Checking servers shunned for more than 1 secondWe also tried increasing mysql-connect_timeout_server to 5000 ms. Although this helps in some cases, the issue still occurs intermittently.
The main concern is why the same issue is not reproducible with ProxySQL 2.x using the same configuration, particularly with mysql-connect_timeout_server=3000, whereas with ProxySQL 3.x, the issue is reproducible after every ProxySQL restart.
This behavioral difference between ProxySQL 2.x and 3.x is the primary concern, especially when considering an upgrade from ProxySQL 2.x to ProxySQL 3.x.
Source: sysown/proxysql