  The demand for data analytics has been consistently increasing in the past
years at Twitter. In order to fulfill the requirements and provide a highly
scalable and available query experience, a large-scale in-house SQL system is
heavily relied on. Recently, we evolved the SQL system into a hybrid-cloud SQL
federation system, compliant with Twitter's Partly Cloudy strategy. The
hybrid-cloud SQL federation system is capable of processing queries across
Twitter's data centers and the public cloud, interacting with around 10PB of
data per day.
  In this paper, the design of the hybrid-cloud SQL federation system is
presented, which consists of query, cluster, and storage federations. We
identify challenges in a modern SQL system and demonstrate how our system
addresses them with some important design decisions. We also conduct
qualitative examinations and summarize instructive lessons learned from the
development and operation of such a SQL system.
