Sequential Distributed Coordination

We give a broad characterization of distributed coordination, building on a basic agreement clause, then de ning a generic sequence clause and a generic termination clause. We instantiate these to obtain common problems. Our decomposition isolates communication costs due solely to ordering or termination, giving a clear way of comparing coordination problems. We further explore the notion of exempting processes from participating in the coordination, and show that any problem in which exempt processes must be made to appear as if they had crashed is equally hard irrespective of channel properties (lossy or reliable). While somewhat surprising, this result also exposes why coordination is di cult, and reasonable ways to weaken it for systems prone partitioning.