Cezary Sobaniec

From session causality to causal consistency

We discuss relationships between client-centric consistency models (known as session guarantees), and data-centric consistency models. The first group includes: read-your-writes guarantee, monotonic-writes guarantee, monotonic-reads guarantee and writes-follow-reads guarantee. The other group includes: atomic consistency, sequential consistency, causal consistency, processor consistency, PRAM consistency, weak consistency, release consistency, scope consistency and entry consistency. We use a consistent notation to present formal definitions of both kinds of consistency models in the context of replicated shared objects. Next, we prove a relationship between causal consistency model and client-centric consistency models. Apparently, causal consistency is similar to writes-follow-reads guarantee. We show that in fact causal consistency requires all common session guarantees, i.e. read-your-writes, monotonic-writes, monotonic-reads and writes-follow-reads to be preserved.

Session Guarantees to Achieve PRAM Consistency of Replicated Shared Objects

In this paper we discuss relationships between client-centric consistency models (known as session guarantees), and data-centric consistency models. Appropriate models are formally defined in the context of replicated shared objects using a consistent notation. We prove that PRAM consistency model may be achieved as a proper combination of different session guarantees.

Safety of a server-based version vector protocol implementing session guarantees

Session guarantees are used to manage replica consistency of a distributed system from the client perspective. This paper defines formally the guarantees, presents and proves safety of a protocol implementing session guarantees using server-based version vectors.

Version vector protocols implementing session guarantees

This paper analyses different protocols of session guarantees. Session guarantees (also known as client-centric consistency models) are one of the class of consistency models of replicated shared data, besides data-centric consistency models. The presentations comprises details of the data structures for the information maintained locally and passed to check consistency conditions, as well as the algorithms to process the information. The protocols are also discussed with respect to accuracy and data overhead.

Dependability Infrastructure for SOA Applications

This chapter describes two tools for improving dependability of SOA-based applications: ReSP (Reliable SOA Platform) and DyMST (Dynamic Management SOA Toolkit). ReSP is a set of modules to improve dependability in respect to availability and reliability, and to some extent safety. It is comprised of the mechanisms of reliable group communication, replication, recovery, and transaction processing. DyMST is a set of components for failure detection, monitoring and autonomic management, and distributed security policy enforcement. In order to show the dependability aspects of real applications and usage of these tools, two case studies from the medical healthcare domain are presented: Healthcare Integration Platform for the exchange of patients’ medical data among various healthcare units, and Medical Event, and Data Registering Platform for daily work improvement of medical staff.

Combining optimistic and pessimistic replication

This paper presents a concept of combining pessimistic and optimistic approach to replication. Optimistic replication allows for tentative system states, which increases availability and efficiency, but makes behaviour of the system less predictable, even if some operations seem completed. To enable more stable results, pessimistic and optimistic modes of operations are distinguished. Operations issued in the optimistic mode accept or produce tentative states, while operations issued in the pessimistic mode appear as completed in a stable state, termed committed. Orthogonally, to refine expectations of the results, modifications are specified as either synchronous or asynchronous, and reads as either synchronised or immediate.

On-demand server synchronization algorithms for session guarantees

Session guarantees define required properties of the system regarding consistency of replicas in a distributed system from a single, migrating clientpsilas point of view. This paper presents propositions of two new server synchronization algorithms. These algorithms solve two of the main problems of some earlier protocols: lack of serverpsilas history pruning and synchronous history update sending.

Replication infrastructure for RESTful web services

This paper describes a proposal for replication of RESTful web services. It focuses on the exploitation of specific features of RESTful web services in the context of replication. The features imposed by the REST architectural style that are crucial to replication include: resource orientation, explicit resource identification, and fixed interface with well understood semantics of operations. We propose a replication infrastructure for RESTful web services in the form of interceptors between consumers and providers. Interceptors in the case of HTTP protocol take the form of proxy servers. The concept has been materialized in a prototype implementation.

Safety of a session guarantees protocol using plausible clocks

Session guarantees is a group of consistency models used to manage replica consistency in a distributed system from the clients perspective. In this paper we present and prove safety of a novel protocol implementing session guarantees. The protocol uses server-based version vectors conceptually based on plausible clocks. The version vectors are constant-size and accept dynamic reconfigurations, which is the main advantage of this approach. The cost is reduced accuracy of representation of sets of operations, which, however, does not violate session guarantees.

Safety of an object-based version vector consistency protocol of session guarantees

Replication provides high performance and availability but introduces the problem of data consistency that arises when replicas are modified. Session guarantees may be used to manage replica consistency of a distributed system from a migrating clients perspective. This paper presents and proves safety of a new consistency protocol of session guarantees using object-based version vectors.