Digital factories comprise a multi-layered integration of various activities along the factories and product life-cycles. A central aspect of a digital factory is that of enabling the product lifecycle stakeholders to collaborate through the use of software solutions. The digital factory expands outside the company boundaries and allows to collaborate on business processes over the whole supply chain. This extended abstract, based on a recently published paper, discusses an interoperability architecture for digital factories. It analyses the key requirements for enabling a scalable factory architecture characterized by access to services, aggregation of data, and orchestration of production processes.