A common architectural pattern for complex online systems is a collection of ``microservices'' communicating via RPC interfaces. This arrangement provides autonomy for teams owning services to develop, deploy, and operate their services without centralized coordination. But this autonomy comes at a cost in leverage—the fixed costs of running a service are high, and the desire to avoid them can lead to poor design choices.

In this paper we describe Strato, a system built at Twitter that runs many microservices hosted in a single service platform—trading a little autonomy for a lot of leverage. Hosted microservices are written in a Scala-like DSL that supports transparent concurrency and direct access to Thrift data (the common currency of services at Twitter). The DSL compiles to a concurrency library written in Scala with an arrows-based interface. We detail the design and implementation of the DSL and concurrency library, and evaluate the effectiveness of Strato in improving leverage at Twitter.