Causal consistency is an attractive consistency model for replicated data
stores. It is provably the strongest model that tolerates partitions, it avoids
the long latencies associated with strong consistency, and, especially when
using read-only transactions, it prevents many of the anomalies of weaker
consistency models. Recent work has shown that causal consistency allows
"latency-optimal" read-only transactions, that are nonblocking, single-version
and single-round in terms of communication. On the surface, this latency
optimality is very appealing, as the vast majority of applications are assumed
to have read-dominated workloads.
In this paper, we show that such "latency-optimal" read-only transactions
induce an extra overhead on writes, the extra overhead is so high that
performance is actually jeopardized, even in read-dominated workloads. We show
this result from a practical and a theoretical angle.
First, we present a protocol that implements "almost laten- cy-optimal" ROTs
but does not impose on the writes any of the overhead of latency-optimal
protocols. In this protocol, ROTs are nonblocking, one version and can be
configured to use either two or one and a half rounds of client-server
communication. We experimentally show that this protocol not only provides
better throughput, as expected, but also surprisingly better latencies for all
but the lowest loads and most read-heavy workloads.
Then, we prove that the extra overhead imposed on writes by latency-optimal
read-only transactions is inherent, i.e., it is not an artifact of the design
we consider, and cannot be avoided by any implementation of latency-optimal
read-only transactions. We show in particular that this overhead grows linearly
with the number of clients
