Composable Database System Research has yielded components such as Apache Arrow for Storage, Meta's Velox for processing and Apache Calcite for query planning. What is lacking, however, is a design for a general, efficient and easy-to-use architecture to connect them. We propose such an architecture. Our proposal is based on the ideas of partial query evaluation and a carefully designed, unified exchange format for query plans and data. We implement the architecture in a system called BOSS that combines the Apache Arrow, the GPU-accelerated compute kernel ArrayFire and the CPU-oriented Velox kernel into a fully-featured relational Data Management System (DMS). We demonstrate that the architecture is general enough to incorporate practically any DMS component, easy-to-use and virtually overhead-free. Based on the architecture, BOSS achieves significant performance improvement over the CPU-only Velox kernel and even outperforms the highly-optimized GPU-only DMS HeavyDB for some queries.