Microservice Architecture (MSA) is a service-based architectural style with a strong emphasis on high cohesion and loose coupling. It is commonly regarded as a descendant of Service-oriented Architecture (SOA) and thus might draw on existing findings of SOA research. This paper presents a metamodel for Model-driven Development (MDD) of MSA, which is deduced from existing SOA modeling approaches, but also incorporates MSA-specific modeling concepts. It is divided into the three viewpoints Data, Service and Operation, each of which encapsulates concepts related to a certain aspect of MSA. The metamodel aims to support DevOps-based MSA development and automatic transformation of metamodel instances into MSA implementations.