Efficient Optimistic Concurrency Control Using Loosely Synchronized Clocks

Efficient Optimistic Concurrency Control Using Loosely Synchronized Clocks

This paper describes an efficient optimistic concurrency control
scheme for use in distributed database systems in which objects are
cached and manipulated at client machines while persistent storage
and transactional support are provided by servers. The scheme
provides both serializability and external consistency for committed
transactions; it uses loosely synchronized clocks to achieve global
serialization. It stores only a single version of each object, and
avoids maintaining any concurrency control information on a per-
object basis; instead, it tracks recent invalidations on a per-client
basis, an approach that has low in-memory space overhead and no
per-object disk overhead. In addition to its low space overheads,
the scheme also performs well. The paper presents a simulation
study that compares the scheme to adaptive callback locking, the
best concurrency control scheme for client-server object-oriented
database systems studied to date. The study shows that our
scheme outperforms adaptive callback locking for low to moderate
contention workloads, and scales better with the number of clients.
For high contention workloads, optimism can result in a high abort
rate; the scheme presented here is a first step toward a hybrid scheme
that we expect to perform well across the full range of workloads.