1. A computer program product, tangibly embodied in a computer-readable medium, the computer program product being operable to cause data processing apparatus to perform operations comprising: receiving first data characterizing web service calls for an application and a placeholder destination for the web service calls, the placeholder destination being a placeholder for a system, the system to be identified for the web service calls during configuration of proxies for the web service calls, and the system not being identifiable during a design-time of the application; receiving second data characterizing an identification of the system to be mapped to the placeholder; and generating a configuration for proxy objects for the web service calls, the configuration comprising the system for their configuration instead of the placeholder destination.