  When configuring a software product line, finding a good trade-off between
multiple orthogonal quality concerns is a challenging multi-objective
optimisation problem. State-of-the-art solutions based on search-based
techniques create invalid configurations in intermediate steps, requiring
additional repair actions that reduce the efficiency of the search. In this
work, we introduce consistency-preserving configuration operators
(CPCOs)--genetic operators that maintain valid configurations throughout the
entire search. CPCOs bundle coherent sets of changes: the activation or
deactivation of a particular feature together with other (de)activations that
are needed to preserve validity. In our evaluation, our instantiation of the
IBEA algorithm with CPCOs outperforms two state-of-the-art tools for optimal
product line configuration in terms of both speed and solution quality. The
improvements are especially pronounced in large product lines with thousands of
features.
