Jerzy Brzeziński

Deadlock Models and a General Algorithm for Distributed Deadlock Detection

This paper deals with the problem of deadlock detection in asynchronous message passing systems in a system model that covers unspecified receptions and non-FIFO channels. It presents a hierarchy of deadlock models and deadlock detection problems. It abstracts deadlocks by a general deadlock model that has the same modeling power as the OR-AND model; however, it has much concise expressive power. An abstract general definition of deadlocks in distributed systems is presented that defines deadlocks independently of the underlying deadlock model. This formulation can be used to design a single distributed deadlock detection algorithm which uniformly addresses all deadlocks in the context of various request models such as AND, OR, AND-OR, and k-out-of-n requests. A simple generalized deadlock detection algorithm that uses a circulating token is presented to illustrate the concept. The algorithm is formally described and proven correct. Moreover, possible refinements of the basic solution concerning improvements of token routing and parallel implementation are outlined and evaluated. Extensions to individual and global termination issues are also addressed. Since the proposed deadlock detection algorithm is designed around the abstract definition of deadlocks, it has some very favorable features.

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.

D-reserve: distributed reliable service environment

Nowadays, a major paradigm of large scale distributed processing is service-oriented computing. To improve the availability and reliability of the SOA-based systems and applications, a Reliable Service Environment ReServE, providing an external support of web services recovery, has been proposed. The functionality of ReServE can be enhanced by distributing its architecture. However, this causes several problems, which have to be solved. In this paper we address these problems and propose their solution.

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.

Healthcare Integration Platform

In this paper we tackle the issue of exchanging and integrating medical information originating from different health care information systems. We propose a solution — Healthcare Integration Platform (HIP) — which utilises some of the concepts contained in the IHE profiles [1] combined with the existing EHR standards in order to maintain a high level of interoperability. We confirm the value of our solution by presenting a working prototype based on our concepts. The prototype relies on the Service Oriented Architecture paradigm using RESTful web services.

ROsWeL Workflow Language: A Declarative, Resource-oriented Approach

Well defined business processes are a crucial success factor for deploying SOA/SOKU architectures. In this paper, the declarative business process description language—ROsWeL—which supports applications compatible with ROA, is discussed. ROsWeL provides a declarative, reliable and semi-automatic composition of RESTful web services, enriched by the knowledge representation. The paper discusses benefits of ROsWeL, and presents an example of a simple workow that captures essential ROsWeL features.

FaDe: RESTful service for failure detection in SOA environment

FaDe service is a novel proposal of failure detection service based on REST paradigm for SOA environment. It is fully distributed service. Internal communication between FaDe nodes can be realized using two communication protocols (gossip, Kademlia). It makes the nodes cooperation efficient and provide good level of scalability. The failure monitoring is based on accrual failure detector which provides flexibility in the context of different client expectations considering the speed and accuracy of detections.

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.

Consistency requirements of distributed shared memory for Dijkstra's mutual exclusion algorithm

As is well known any algorithm correct in an asynchronous shared memory setting (physically shared memory) can be directly applied in distributed shared memory (DSM) systems provided that the latter guarantees strong consistency (atomic or sequential) of replicas. Generally however, in DSM systems, weaker consistency models (causal, processor, PRAM, etc.) are often considered to improve the performance. A weakening consistency model may however imply the incorrectness of the algorithm. So we face a consistency requirement problem, the problem of finding the weakest consistency model of DSM that is sufficient and necessary for algorithm correctness. We consider a reliable DSM environment, and present a complex consistency model comprising three elementary models: sequential consistency, coherence and PRAM consistency. This complex model is then applied to Dijsktras (1965) algorithm for mutual exclusion of n processes, one of the first solutions to a fundamental problem in both centralised and distributed operating systems. In the resulting algorithm, coherence and PRAM consistency are associated with some write operations performed at shared memory locations. As concurrent execution of write operations with weaker consistency models is more efficient when compared to the execution of strongly consistent operations, the proposed solution reduces synchronisation delay (mutual exclusion overhead) and thereby increases system throughput. The presented model is proven to be sufficient for algorithm correctness. Moreover, the algorithm is shown to be optimal in the sense that further relaxation of any write operations semantics violates progress (liveness) or safety of the algorithm.