Title:  Dynamic Software Updating
Abstract:
 
Many important applications must run continuously and without
interruption, yet must be changed to fix bugs or upgrade functionality.
To date, no existing, general-purpose methodology for dynamic updating
achieves a practical balance between flexibility, robustness, low
overhead, and ease of use.

We present a new approach for imperative languages that provides
type-safe dynamic updating of  native code in an extremely flexible
manner (code, data, and types may be updated, at programmer-determined
times) and permits the use of automated tools to aid the programmer in
the updating process.  Our system is based around dynamic patches, which
contain both the updated code and the code needed to transition from the
old version to the new.  A novel aspect of our patches is that they
consist of verifiable native code (or VNC, e.g. Typed Assembly Language
or Proof-Carrying Code), which is native code accompanied by annotations
that allow on-line verification of the code's safety.  We discuss how
patches are generated mostly automatically, how they are applied using
our own novel dynamic-linking technology for VNC systems, and how code
is compiled to make it updateable.

To concretely illustrate and validate our system, we have implemented a
sizeable application: a dynamically updateable web server, called
FlashEd.  We discuss our experience building and maintaining FlashEd.
Performance experiments show that updateable FlashEd runs roughly 2%
slower than a static one under various workloads.