Enterprise applications are transactional, distributed multi-user applications. Their distributed structure can be modeled as a process topology that consists of distributed application processes, datastores, and client/server communication relationships. Many non-functional properties of an enterprise application, such as scalability, response time, and availability are directly influenced by the design of the underlying process topology. It is the thesis of this paper that demanding enterprise applications require custom process topologies to address application-specific requirements. Furthermore, topologies should easy to adapt to changing requirements without having to redesign large parts of the enterprise application. We identify key requirements for enterprise application middleware. Based on the requirements, we present an architecture with explicit support for custom and configurable distributed process topologies. With our architecture, the evolution of the distributed structure of an enterprise application is mainly a matter of reconfiguration.