Generalized Symmetry Breaking Tasks and Nondeterminism in Concurrent Objects.

Processes in a concurrent system need to coordinate using an underlying shared memory or a message-passing system in order to solve agreement tasks such as, for example, consensus or set agreement. However, coordination is often needed to break the symmetry of processes that are initially in the same state-for example, to get exclusive access to a shared resource, to get distinct names, or to elect a leader. This paper introduces and studies the family of generalized symmetry breaking (GSB) tasks, which includes election, renaming, and many other symmetry breaking tasks, and studies how nondeterminism properties of objects solving tasks affects the computability power of GSB tasks. The aim is to develop the understanding of symmetry breaking tasks and their relation with agreement tasks and to study nondeterminism properties of objects solving tasks and how these properties affect the computability power of symmetry breaking tasks. Among various results characterizing the family of GSB tasks, it is shown that perfect renaming, i.e., (n; n)-renaming, is universal for all GSB tasks. The paper also shows that there is a large family of GSB tasks, which includes perfect renaming, that is strictly more powerful than (n, n 1)-set agreement. Some of these tasks are equivalent to perfect renaming, while others lie strictly between perfect renaming and (n; n + 1)-renaming. Results comparing renaming and set agreement are proved, and the results in this paper complement known results. This paper sheds new light on the relations linking set agreement and symmetry breaking. The proofs are based on combinatorial topology techniques and new ideas about different notions of nondeterminism that can be associated with shared objects.