Design of multi-invariant data structures for robust shared accesses in multiprocessor systems

Multiprocessor systems are widely used in many application programs to enhance system reliability and performance. However, reliability does not come naturally with multiple processors. We develop a multi-invariant data structure approach to ensure efficient and robust access to shared data structures in multiprocessor systems. Essentially, the data structure is designed to satisfy two invariants, a strong invariant, and a weak invariant. The system operates at its peak performance when the strong invariant is true. The system will operate correctly even when only the weak invariant is true, though perhaps at a lower performance level. The design ensures that the weak invariant will always be true in spite of fail-stop processor failures during the execution. By allowing the system to converge to a state satisfying only the weak invariant, the overhead for incorporating fault tolerance can be reduced. We present the basic idea of multi-invariant data structures. We also develop design rules that systematically convert fault-intolerant data abstractions into corresponding fault-tolerant versions. In this transformation, we augment the data structure and access algorithms to ensure that the system always converges to the weak invariant, even in the presence of fail-stop processor failures. We also design methods for the detection of integrity violations and for restoring the strong invariant. Two data structures, namely binary search tree and double-linked list, are used to illustrate the concept of multi-invariant data structures.