MooseException
MooseException is a class that derives from the standard C++ exception class. Its purpose is to give developers an opportunity to terminate a solve, while informing MOOSE that it should perform the necessary steps to clean up the current stack, notify other threads/processors of the error and communicate with the solver that the current solve has failed. When applicable and possible, the solver will allow MOOSE to cut the time step and make another attempt at a solve.
This exception should be used directly in user code when non-fatal situations are encountered such as the inability to converge a local Newton solve in a material, or if a interpolated value from a variable or lookup table ends up out-of-range.
MOOSE is set up to catch a MooseException within several user defined callbacks that occur during the solver's computeResidual and computeJacobian callbacks. Note that these callbacks also include the evaluation of several other objects such as Materials, Constraints, and occasionally objects such as Postprocessors (when requested). This makes it possible to cut the time step and retry the solve by throwing a MooseException from most user-defined objects within MOOSE.
How it works
MOOSE Wraps several threaded sections in PARALLEL_TRY/PARALLEL_CATCH macros (See SystemBase.h). PARALLEL_TRY does nothing (but provide scope). PARALLEL_CATCH invokes the FEProblemBase::checkExceptionAndStopSolve() (See FEProblemBase.h) routine after the threaded section has terminated and all threads on the same processor agree on the error status. checkExceptionAndStopSolve() then does parallel communication of the exception to ensure that the same exception exists on all processors. If there is an exception, that method sets a variable that results in the solver being notified that the solution has diverged due to not-a-number (NaN) entries in the residual vector during the next residual evaluation. This in turn results in the time step being cut and the solution being attempted again, in the same manner that other failed solutions of a time step are handled.