Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for securely executing customized computing workflows for selecting digital components are described. In one aspect, a method includes receiving a request for a digital component for presentation by a client device. In response to receiving the request, a pool of candidate digital components is retrieved from a repository of digital components, and each stage of a multi-stage workflow for selecting a digital component from the pool of digital components is executed in a sequence defined by the multi-stage workflow, including a filtering stage, a selection stage, and a rendering stage. After the rendering stage has been executed, a selected and packaged digital component is transmitted for receipt by the client device.