CesiumSpray: a Precise and Accurate Global Time Service for Large-scale Systems

P. Veríssimo, L. Rodrigues and A. Casimiro

In Special Issue on the Challenge of Global Time in Large-Scale Distributed Real-Time Systems, Journal of Real-Time Systems, Vol 12, n. 3, November 1997, Kluwer Academic Publishers.


In large-scale systems, such as Internet-based distributed systems, classical clock-synchronization solutions become impractical or poorly performing, due to the number of nodes and/or the distance among them. We present a global time service for world-wide systems, based on an innovative clock synchronization scheme, named CesiumSpray. The service exhibits high precision and accuracy; it is virtually indefinitely scalable; and it is fault-tolerant. It is deterministic for real-time machinery in the local area, which makes it particularly well-suited for, though not limited to, large-scale real-time systems.

The main features of our clock synchronization scheme can be summarized as follows: hybrid external/internal synchronization protocol improves effectiveness of synchronization; heterogeneous failure semantics for clocks and processors improves previous lower bounds on processors; two-level hierarchy improves scalability. The root of the hierarchy is the GPS satellite constellation, which ``sprays'' its reference time over a set of nodes provided with GPS receivers, one per local network. The second level of the hierarchy performs internal synchronization, further ``spraying'' the external time inside the local network.

Also available extended report (gzip postscript).