On the Robustness of (Semi) Fast Quorum-Based Implementations of Atomic Shared Memory

This paper studies a trade-off between fault-tolerance and latency in implementations of atomic read/write objects in message-passing systems. In particular, considering

fast

or

semifast

quorum-based

implementations, that is, implementations where

all

or respectively

most

read and write operations complete in a single communication round-trip, it is shown that such implementations are

not robust

due to the fact that they necessarily require a quorum system with a common intersection between its quorums.

To trade speed for fault-tolerance, the notion of

weak-semifast

implementations is introduced. Here more than a single complete slow (two round-trip) read operation is allowed for each write operation (semifast implementations allow only one such slow read). A quorum-based algorithm is given next and it is formally shown that it constitutes a weak-semifast implementation of atomic registers. The algorithm uses the notion of

Quorum Views

to facilitate the characterization of all possible object timestamp distributions that a read operation may witness during its first communication round-trip. Noteworthy is that the algorithm allows fast read operations even if they are concurrent with other read and write operations. Finally, experimental results were gathered by simulating the algorithm using the NS-2 network simulator. The results show that under realistic conditions, less than 13% of read operations are slow, thus the overwhelming majority of operations take a single communication round-trip.