Implementing Shared Registers in Asynchronous Message-Passing Systems

A distributed system is comprised of a collection of n processes which communicate with one another. Two means of interprocess communication have been heavily studied. Message-passing systems model computer networks where each process can send information over message channels to other processes. In shared-memory systems, processes communicate less directly by accessing information in shared data structures. Distributed algorithms are often easier to design for shared-memory systems because of their similarity to single-process system architectures. However, many real distributed systems are constructed as message-passing systems. Thus, a key problem in distributed computing is the implementation of shared memory in message-passing systems. Such implementations are also called simulations or emulations of shared memory. The most fundamental type of shared data structure to implement is a (read-write) register, which stores a value, taken from some domain D. It is initially assigned a value from D and can be accessed by two kinds of operations, read and write(v), where v ∈ D. A register may be either single-writer, meaning only one process is allowed to write it, or multi-writer, meaning any process may write to it. Similarly, it may be either single-reader or multi-reader. Attiya and Welch [4] give a survey of how to build multi-writer, multi-reader registers from single-writer, single-reader ones. If reads and writes are performed one at a time, they have the following effects: a read returns the value stored in the register to the invoking process, and a write(v) changes the value stored in the register to v and returns an acknowledgement, indicating that the operation is complete. When many processes apply operations concurrently, there are several ways to specify a register's behaviour [14]. A single-writer register is regular if each read returns either the argument of the write that completed most recently before the read began or the argument of some write operation that runs concurrently with the read. (If there is no write that completes before the read begins, the read may return either the initial value of the register or the value of a concurrent write operation.) A register is atomic (or linearizable) if each operation appears to take place instantaneously. More precisely, for any concurrent execution, there is a total order of the operations such that each read returns the value written by the last write that precedes it in the order (or the initial value of the register, if there is no such write). Moreover, …