One distinctive characteristic of software is its malleability and succeeding opportunity and also need for a constant change. However, in certain types of software, many agencies in the public sector are bound by the laws on competition and financed by governmental budgeting processes. Consequently, the acquiring agencies adapt their processes to what the existing systems allow due to complexities in making changes and modifications. In this paper, we study the relation of public sector software acquisition and delivery, and the continuously changing nature of software. Then, we analyse pain points of evolving software in public sector information systems, and propose technical artefacts to improve the status quo. Finally, to add flexibility to public sector information systems’ acquisition and evolution, we elaborate a novel architectural pattern called Mosaic architecture, based on macroservices, which allows independent subcontracting, development, and deployment. The approach is demonstrated with a real-world industrial study from a public service that has been sketched applying the macroservice principles.