Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for obtaining copies of resources at a canary server and from a hosting server. The resources include initial instructions for responding to requests from an external device. The canary server executes an update that modifies the initial instructions in the resources to create modified instructions. A request router determines a routing of a request for resources that render a webpage based on parameters in the request. The request is processed using the modified instructions rather than the initial instructions and in response to the request router determining that the canary server is a destination of the determined routing of the request. The system determines a reliability measure of the update when the request is processed at the canary server. The reliability measure identifies whether the update will trigger a fault during execution at production servers of the system.