Graceful quorum reconfiguration in a robust emulation of shared memory

Providing shared-memory abstraction in message-passing systems often simplifies the development of distributed algorithms and allows for the reuse of shared-memory algorithms in the message-passing setting. A robust emulation of atomic single-writer/multi-reader registers in message-passing systems was developed by Attiya, Bar-Noy and Dolev (1995). This emulation was extended by Lynch and Shvartsman (1995) to multi-writer/multi-reader registers using reconfigurable quorum systems. In this work we present a new atomic multi-writer/multi-reader register service that includes a fault-tolerant reconfiguration service. This new emulation has a substantially improved performance and fault-tolerance characteristics. We introduce the concept of intermediate quorum configurations and show how they can be used by readers/writers during reconfiguration. The result is that the quorum reconfigurations are graceful: readers and writers no longer 'busy-wait' during reconfigurations, but are able to complete their operations. An additional advance is that the reconfigurer is eliminated as the single point of failure. When the reconfigurer fails, readers and writers continue using intermediate configurations. In finite executions, read and write operations terminate in bounded time using bounded number of messages (the bounds depend on the 'currency' of the configuration at the invoker of the operation). Finally, the service places no restrictions on the installed quorum configuration: a previously installed quorum system can be replaced by an arbitrary new quorum system. Our algorithms are specified using I/O Automata; the safety proofs use the partial order techniques and invariants, and the performance is assessed using operational reasoning.