Dynamic software updating

Many important applications must run continuously and without interruption, yet must be changed to fix bugs or upgrade functionality. No prior 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 C-like 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 on dynamic patches that both contain 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 (e.g. Proof-Carrying Code [17] or Typed Assembly Language [16]), 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 dynamic-linking technology, and how code is compiled to make it updateable. To concretely illustrate our system, we have implemented a dynamically-updateable web server, FlashEd. We discuss our experience building and maintaining FlashEd. Performance experiments show that for FlashEd, the overhead due to updating is typically less than 1%.