  Elastic scaling is one of the central benefits provided by serverless
platforms, and requires that they scale resource up and down in response to
changing workloads. Serverless platforms scale-down resources by terminating
previously launched instances (which are containers or processes). The
serverless programming model ensures that terminating instances is safe
assuming all application code running on the instance has either completed or
timed out. Safety thus depends on the serverless platform's correctly
determining that application processing is complete.
  In this paper, we start with the observation that current serverless
platforms do not account for pending asynchronous I/O operations when
determining whether application processing is complete. These platforms are
thus unsafe when executing programs that use asynchronous I/O, and incorrectly
deciding that application processing has terminated can result in data
inconsistency when these platforms are used. We show that the reason for this
problem is that current serverless semantics couple termination and response
generation in serverless applications. We address this problem by proposing an
extension to current semantics that decouples response generation and
termination, and demonstrate the efficacy and benefits of our proposal by
extending OpenWhisk, an open source serverless platform.
