The evolution of complex distributed software systems often requires intricate composition operations in order to adapt or a dd functionalities, to react to unan ticipated changes, or to apply performance improvements that cannot be modul arized in terms of existing services and components. These evolutions often need controlled access to selected parts of the implementation, e.g., to manage exceptional situations and crossc utting within services and their compositions. However, existing composition techniques typi cally support only interface-level (black-box) composition or arbitrary access to the implementation (gray-box or white-box composition). In this paper, we present a structured approach to the composition of complex software systems that require invasive modi fications. Concretely, we provide three contributions: (i) we present a small kernel composition language for structured gray-box composition using invasive distributed patterns; (ii) we motivate that gray-box composition approaches should be defined and evaluated in terms of the flexibility and control they provide, a notion of degrees of invasiveness is introduced to help assess this trade-off; (iii) we apply our appr oach to a new case study of evolution and evaluate it in the context of two previous studies involving two real-world software systems : benchmarking of grid algorithms with NASGrid and transactional replication with JBoss Cache. As a main result, we show that gray-box composition using invasive distributed patterns allows the declarative and modular de finition of evolutions of real-world a pplications that n eed moderate to high degrees of invasive modifications.