A Formal Characterization of Epsilon Serializability

Epsilon Serializability (ESR) is a generalization of classic serializability (SR). ESR allows some limited amount of inconsistency in transaction processing (TP), through an interface called epsilon-transactions (ETs). For example, some query ETs may view inconsistent data due to non-SR interleaving with concurrent updates. In this paper, we restrict our attention to the situation where query-only ETs run concurrently with consistent update transactions that are SR without the ETs. This paper presents a formal characterization of ESR and ETs. Using the ACTA framework, the first part of this characterization formally expresses the inter-transaction conflicts that are recognized by ESR and, through that, defines ESR, analogous to the manner in which conflict-based serializability is defined. The second part of the paper is devoted to deriving expressions for: (1) the inconsistency in the values of data -- arising from ongoing updates, (2) the inconsistency of the results of a query ““ arising from the inconsistency of the data read in order to process the query, and (3) the inconsistency exported by an update ET - arising from ongoing queries reading uncommitted data produced by the update ET. These expressions are used to determine the preconditions that ET operations have to satisfy in order to maintain the limits on the inconsistency in the data read by query ETs, the inconsistency exported by update ETs, and the inconsistency in the results of queries. This determination suggests possible mechanisms that can be used to realize ESR

Adaptive Fault Tolerance and Graceful Degradation Under Dynamic Hard Real-time Scheduling

Static redundancy allocation is inappropriate in hard realtime systems that operate in variable and dynamic environments, (e.g., radar tracking, avionics). Adaptive Fault Tolerance (AFT) can assure adequate reliability of critical modules, under temporal and resources constraints, by allocating just as much redundancy to less critical modules as can be afforded, thus gracefully reducing their resource requirement. In this paper, we propose a mechanism for supporting adaptive fault tolerance in a real-time system. Adaptation is achieved by choosing a suitable redundancy strategy for a dynamically arriving computation to assure required reliability and to maximize the potential for fault tolerance while ensuring that deadlines are met. The proposed approach is evaluated using a real-life workload simulating radar tracking software in AWACS early warning aircraft. The results demonstrate that our technique outperforms static fault tolerance strategies in terms of tasks meeting their timing constraints. Further, we show that the gain in this timing-centric performance metric does not reduce the fault tolerance of the executing tasks below a predefined minimum level. Overall, the evaluation indicates that the proposed ideas result in a system that dynamically provides QOS guarantees along the fault-tolerance dimension

Maintaining Mutual Consistency for Cached Web Objects

Existing web proxy caches employ cache consistency mechanisms to ensure that locally cached data is consistent with that at the server. In this paper, we argue that techniques for maintaining consistency of individual objects are not sufficient—a proxy should employ additional mechanisms to ensure that related web objects are mutually consistent with one another. We formally define the notion of mutual consistency and the semantics provided by a mutual consistency mechanism to end-users. We then present techniques for maintaining mutual consistency in the temporal and value domains. A novel aspect of our techniques is that they can adapt to the variations in the rate of change of the source data, resulting in judicious use of proxy and network resources. We evaluate our approaches using real-world web traces and show that (i) careful tuning can result in substantial savings in the network overhead incurred without any substantial loss in fidelity of the consistency guarantees, and (ii) the incremental cost of providing mutual consistency guarantees over mechanisms to provide individual consistency guarantees is small

Verifying Completeness of Relational Query Results in Data Publishing

10.1145/1066157.1066204Proceedings of the ACM SIGMOD International Conference on Management of Data407-41

The Atomic Manifesto: a Story in Four Quarks

This report summarizes the viewpoints and insights gathered in the Dagstuhl Seminar on Atomicity in System Design and Execution, which was attended by 32 people from four different scientific communities: database and transaction processing systems, fault tolerance and dependable systems, formal methods for system design and correctness reasoning, and hardware architecture and programming languages. Each community presents its position in interpreting the notion of atomicity and the existing state of the art, and each community identifies scientific challenges that should be addressed in future work. In addition, the report discusses common themes across communities and strategic research problems that require multiple communities to team up for a viable solution. The general theme of how to specify, implement, compose, and reason about extended and relaxed notions of atomicity is viewed as a key piece in coping with the pressing issue of building and maintaining highly dependable systems that comprise many components with complex interaction patterns