Global state predicates in rough real-time

Distributed systems are characterized by the fact that the constituent processes have neither common memory nor a common system clock. These processes communicate solely via message passing. While providing a number of benefits such as increased reliability, increased computational power, and geographic dispersion, this architecture significantly complicates many of the tasks of software development and verification, including evaluation of the program state. In the case of distributed systems, the program state is comprised of the local states of the constituent processes, as well as the state of the channels between processes, and is called the global state.;With no common system clock, many distributed system protocols rely on the global ordering of local process events imposed by the message passing that occurs between processes. This leads to a partial global ordering of local process events, which can then be used to determine which process states could (or could not) have occurred simultaneously.;Traditional predicate evaluation protocols evaluate predicates on the global state of a distributed computation using consistent global states. This evaluation is complicated by the fact that the event ordering imposed by message passing is only partial. A complete history of the global states that occurred during an execution cannot always be constructed. This introduces inefficiency into predicate detection protocols and prohibits detection of certain predicates.;This dissertation explores the use of this rough global time base for global state predicate evaluation within distributed systems. By structuring the evaluation on the assumption that a global time base exists, we can develop simple and efficient protocols for both stable and unstable predicate evaluation. Further, we can evaluate certain predicates which are not easily evaluated using consistent global states. We demonstrate these advantages by developing protocols for detection of distributed termination, distributed deadlock detection, and detection of certain unstable predicates as they occur. as the global time base is rough, we can only detect unstable predicates which remain true for a sufficient duration. We additionally develop several formalizations which assist the protocol developer in dealing with the fact that the global time base is not perfect. We demonstrate the application of these formalizations within the protocols that we develop

Causal synchrony in the design of distributed programs

The outcome of any computation is determined by the order of the events in the computation and the state of the component variables of the computation at those events. The level of knowledge that can be obtained about event order and process state influences protocol design and operation. In a centralized system, the presence of a physical clock makes it easy to determine event order. It is a more difficult task in a distributed system because there is normally no global time. Hence, there is no common time reference to be used for ordering events. as a consequence, distributed protocols are often designed without explicit reference to event order. Instead they are based on some approximation of global state. Because global state is also difficult to identify in a distributed system, the resulting protocols are not as efficient or clear as they could be.;We subscribe to Lamport\u27s proposition that the relevant temporal ordering of any two events is determined by their causal relationship and that knowledge of the causal order can be a powerful tool in protocol design. Mattern\u27s vector time can be used to identify the causal order, thereby providing the common frame of reference needed to order events in a distributed computation. In this dissertation we present a consistent methodology for analysis and design of distributed protocols that is based on the causal order and vector time. Using it we can specify conditions which must be met for a protocol to be correct, we can define the axiomatic protocol specifications, and we can structure reasoning about the correctness of the specified protocol. Employing causality as a unifying concept clarifies protocol specifications and correctness arguments because it enables them to be defined purely in terms of local states and local events.;We have successfully applied this methodology to the problems of distributed termination detection, distributed deadlock detection and resolution, and optimistic recovery. In all cases, the causally synchronous protocols we have presented are efficient and demonstrably correct