  Elasticity is a key property of cloud computing. However, elasticity is
offered today at the granularity of virtual machines, which take tens of
seconds to start. This is insufficient to react to load spikes and sudden
failures in latency sensitive applications, leading users to resort to
expensive overprovisioning. Function-as-a-Service (FaaS) provides significantly
higher elasticity than VMs, but comes coupled with an event-triggered
programming model and a constrained execution environment that makes them
unsuitable for off-the-shelf applications. Previous work tries to overcome
these obstacles but often requires re-architecting the applications. In this
paper, we show how off-the-shelf applications can transparently benefit from
ephemeral elasticity with FaaS. We built Boxer, an interposition layer spanning
VMs and AWS Lambda, that intercepts application execution and emulates the
network-of-hosts environment that applications expect when deployed in a
conventional VM/container environment. The ephemeral elasticity of Boxer
enables significant performance and cost savings for off-the-shelf applications
with, e.g., recovery times over 5x faster than EC2 instances and absorbing load
spikes comparable to overprovisioned EC2 VM instances.
