10.9  Error Handling

The external solver’s optimisation can abort without completely solving the problem, because of some error, or some resource limit was reached. Eplex classifies these into the following cases, with default ways of handling them:

suboptimal

This means that a solution was found but it may be suboptimal. The default behaviour is to print a warning and succeed.

unbounded

This means that the problem is unbounded. The default behaviour is to bind Cost to infinity (positive or negative depending on the optimisation direction), print a warning and succeed. CAUTION: No solution values are computed when the problem is unbounded, so unless the problem was set up with the solution(no) option, an error will occur when trying to continue as if the optimisation had succeeded.

infeasible

This means that the problem is infeasible. Note that it is possible for a problem that is on the boundary between feasible and infeasible to be returned as feasible or infeasible, depending on the solver and solving method used. The default behaviour is to fail.

unknown

This means that due to the solution method chosen, it is unknown whether the problem is unbounded or infeasible. The default behaviour is to print a warning and fail (even though this may be logically wrong!).

abort

Some other error condition occurred during optimisation. The default behaviour is to print an error and abort.

The default behaviours can be overridden for each problem by giving a user defined goal to handle each case during problem setup in eplex_solver_setup/4 (lp_setup/4, lp_demon_setup/5, or later with eplex_set/2 or lp_set/3) as an option. If given, the user defined goal will be executed instead. The user defined handler could for instance change parameter settings and call lp_solve again.

Redefining the default behaviour for infeasible problems allows the infeasible problem state to be examined, e.g. by extracting the IIS (Irreducible Infeasible Subsystem) from it with eplex_get_iis/4. It is recommended that the user-defined handler should fail after examining the state to preserve the correct logical behaviour.

The default behaviour is implemented by raising the events eplex_suboptimal, eplex_unbounded, eplex_infeasible, eplex_unknown and eplex_abort for the different abort cases. These events can themselves be redefined to change the default behaviours. However, as this changes the behaviour globally, it is not recommended.

Note that in the user defined handlers, it may be possible to extract some useful bound information on the optimal objective value using the best_bound and worst_bound options of lp_get/3.