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As distributed applications evolve, incremental customization of middleware services is often required; these customizations should be unpluggable, modular, and efficient. This is difficult to achieve because the customizations depend on both application-specific needs and the services provided. Although middleware allows programmers to separate application-specific functionality from lower-level details, traditional methods of customization do not allow efficient modularization. Currently, making even minor changes to customize middleware is complicated by the lack of locality. Programmers may have to compromise between the two extremes: to interpose a simple, well-localized layer of functionality between the application and middleware, or to make a large number of small, poorly localized, invasive changes to all execution points which interact with middleware services. Although the invasive approach allows a more efficient customization, it is harder to ensure consistency, more tedious to implement, and exceedingly difficult to unplug. Thus, a common approach is to add an extra layer for systemic concerns such as robustness, caching, filtering, and security. Aspect-oriented programming (AOP) offers a potential alternative between the interposition and invasive approaches by providing modular support for the implementation of crosscutting concerns. AOP enables the implementation of efficient customizations in a structured and unpluggable manner. We demonstrate this approach by comparing traditional and AOP customizations of fault tolerance in a distributed file system model, JNFS. Our results show that using AOP can reduce the amount of invasive code to almost zero, improve efficiency by leveraging the existing application behaviour, and facilitate incremental customization and extension of middleware services.