% $Id: fault-model.tex 4841 2006-12-11 21:50:41Z sutambe $

\subsection{The Fault Model}

Enterprise DRE systems are quite often built over heterogenous platforms
which consists of several layers of hardware as well as software. We
consider the fact that faults may occur at any of these multi-layer
systems. For example, node failures, operating system failure, ORB failures,
container failures, component failures etc. We dont' consider multiple
cascaded failures.

\textbf{Fault Recovery Granularity}

Quite often critical functionality of component based eDRE systems
is spread across multiple components. As a result,
multiple distributed components form a unit of critical functionality.
for highly available systems it is desirable that all the components in the 
critical path be protected against failure. When an isolated failure 
actually occurs in one of the critical components, to bring back the
system in its fault free state of operation, it is often not
sufficient to restart just the failed component service. This is because
failure recovery takes finite amount of time and therefore, by the time
the broken functionality comes back to life, the system may lose
some critical system events. Therefore, it is highly desirable in such
situations to failover on another identical replica of the protected 
operational string although the failure is isolated in a single component.
Thus, the fault recovery granularity can be much larger than the system
elements affected by the single failure. 

