ProxySQL comes with a powerful feature called query rules. The main use of these rules at Shopify is to reroute, rewrite, or reject queries matching a specified regex. However, with great power comes great responsibility. These rules are powerful and can have unexpected consequences if used incorrectly. At Shopify’s scale, we’re running thousands of ProxySQL instances, so applying query rules to each one is a painful and time consuming process, especially during an incident. We’ve built a tool to help us address these challenges and make deploying new rules safe and scalable.