  Microservices are a popular architectural style adopted by the industry when
it comes to deploying software that requires scalability, maintainability, and
agile development. There is an increasing demand for improving the
sustainability of microservice systems in the industry. This rapid review
gathers 22 peer-reviewed studies and synthesizes architectural tactics that
improve the environmental sustainability of microservices from them. We list 6
tactics that are presented in an actionable way and categorized according to
their sustainability aspects and context. The sustainability aspects include
energy efficiency, carbon efficiency, and resource efficiency, among which
resource efficiency is the most researched one while energy efficiency and
carbon efficiency are still in the early stage of study. The context
categorization, including serverless platforms, decentralized networks, etc.,
helps to identify the tactics that we can use in a specific setting.
Additionally, we present how the evidence of optimization after adopting these
tactics is presented, like the measurement unit and statistical methods, and
how experiments are generally set up so that this review is both instructive
for our future study and our industrial practitioners' interest. We further
study the insufficiencies of the current study and hope to provide insight for
other researchers and the industry.
