Continuous data objects imply that classical transaction serializability and recoverability may not be necessary for correctness. This is because collective temporal correctness of activities is optimized by partial orderings of occurence based on their time constraints, while logical correctness depends on partial orderings of occurence based on the logic of the algorithm for concurrency control or distributed agreement. These two partial orderings are rarely the same, and must be resolved satisfactorily from both standpoints. Thus, faults, errors, and failures are often clustered in physical space and time. Recovery Recovery concepts and techniques are based on redundancy in time or space.
Forward recovery is often essential.
Recovery based on space redundancy techniques is often prohibitive in real-time computing systems due to strong restrictions on computing system size, weight, power, and cost. But this is especially true in real-time systems. But this is not the focus of work on process group approaches to distributed system fault tolerance. Synchronous and Asynchronous Systems Research on the principles of distributed computing systems subject to failures is currently focused primarily on progress group approaches in either synchronous or asynchronous systems. Fault tolerance concepts and techniques for such systems are quite distinct from those for systems presumed to be synchronous.