Time-Driven Orphan Elimination

An orphan in a transaction system is an activity executing on behalf of an aborted transaction. This paper proposes a new method for managing orphans created by crashes and by aborts. The method prevents orphans from observing inconsistent states, and ensures that orphans are detected and eliminated in a timely manner. A major advantage of this method is simplicity: it is easy to understand, to implement, and to prove correct. The method is based on timeouts using clocks local to each site. The method is failsafe: although it performs best when clocks are closely synchronized and message delays are predictable, unsynchronized clocks and lost messages cannot produce inconsistencies or protect orphans from eventual elimination. This research was sponsored in part by the Defense Advanced Research Projects Agency (DOD), ARPA Order No. 3597, monitored by the Air Force Avionics Laboratory Under Contract F33615-81K-1539, and in part by the USAF Rome Air Development Center under contract number F30602-84C-0063 and the U.S. Naval Ocean Systems Center under contract number N66001-83-C-0305. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Defense Advanced Research Projects Agency, RADC, NOSC, or the US Government.