A Universal Construction for Concurrent Objects

A concurrent object is an object that can be concurrently accessed by several processes. A wait-free implementation of an object is such that any operation issued by a non-faulty process terminates in a finite number of its own steps, whatever the behavior of the other processes (that can be very slow or even have crashed). An object type is universal if objects of that type, together with atomic registers, allows implementing any concurrent object defined by a sequential specification. A universal construction is a wait-free algorithm, based only on atomic registers and universal objects, that, given any sequential object type T, provides the processes with a wait-free concurrent object of the type T. In a famous paper (titled "Wait-free synchronization") Herlihy has shown that consensus objects are universal, and has presented a consensus-based universal construction. We present here a new universal construction. That construction, that is built incrementally, is particularly simple. While, in addition to consensus objects, Herlihy's universal construction uses low-level objects such as pointers, the design of the construction presented here is based on the simple and well-known state machine replication paradigm. Its proof is also simple and consequently allows to better understand not only the power of consensus objects but also the subtleties of wait-free computations and the way the consensus objects allow coping with both process failures and non-determinism. In that sense, this paper has a pedagogical flavor.