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Process-control systems usually deal with changeable environments so that high adaptability of these systems is required. After a process-control system has been implemented and deployed, it is not an easy task to adapt the system to satisfy new requirements. Considering this problem, we propose the use of relational program architectures to design process-control systems for high adaptability. With the relational program architecture, a process-control system is designed to be the composition of several Independently Developable End-user Assessable Logical (IDEAL) components through pre-defined composition patterns. Each IDEAL component is associated with several end-user visible properties and can be solved and validated in its restricted "view" of the system, i.e., it can be designed and implemented independently, and can be tested or verified by the end-user independently. In addition, the system level properties (safety, stability, and reliability) can be inferred from the individual IDEAL components mathematically. The relational program architecture can provide not only the ultra-high dependability assurance of a process-control system, but can also make the system highly adaptable. We propose several principles, by which we can accurately determine which components of the system need to be adapted and how to adapt them after the system's requirement has changed. These principles can be evolved to automated tool support.