Scaling out database management systems (DBMSs) requires distributed coordination, which can easily become a bottleneck. Recent work on speeding up distributed transactions has addressed this problem by proposing scale-out techniques that are deeply integrated with the concurrency control mechanism of the DBMS. This paper explores the design of modular coordination layers, which encapsulate all scale-out logic and can be applied to scale out any unmodified single-server DBMS. It proposes Gyro, a modular coordination layer that runs on top of a collection of single-server DBMS instances and interacts with them only through their client interface. Gyro distributes the load by ensuring that as many requests as possible are executed by only one DBMS instance. Our experiments show that modular distributed coordination is practically viable and that it can be much faster than traditional distributed transaction protocols using two-phase commit.