A Graph Transformation-Based Approach for the Validation of Checkpointing Algorithms in Distributed Systems

International audience—Autonomic Computing Systems are oriented to pre-vente the human intervention and to enable distributed systems to manage themselves. One of their challenges is the efficient monitoring at runtime oriented to collect information from which the system can automatically repair itself in case of failure. Quasi-Synchronous Checkpointing is a well-known technique, which allows processes to recover in spite of failures. Based on this technique, several checkpointing algorithms have been developed. According to the checkpoint properties detected and ensured, they are classified into: Strictly Z-Path Free (SZPF), Z-Path Free (ZPF) and Z-Cycle Free (ZCF). In the literature, the simulation has been the method adopted for the performance evaluation of checkpointing algorithms. However, few works have been designed to validate their correctness. In this paper, we propose a validation approach based on graph transformation oriented to automatically detect the previous mentioned checkpointing properties. To achieve this, we take the vector clocks resulting from the algorithm execution, and we model it into a causal graph. Then, we design and use transformation rules oriented to verify if in such a causal graph, the algorithm is exempt from non desirable patterns, such as Z-paths or Z-cycles, according to the case

The Minimal Dependency Relation for Causal Event Ordering in Distributed Computing

International audienceSeveral algorithms of different domains in distributed systems are designed over the principle of the Happened-Before Relation (HBR). One common as-pect among them is that they intend to be efficient in their implementation by identifying and ensuring the necessary and sufficient dependency constraints. In this pursuit, some previous works talk about the use of a transitive reduction of the causality. However, none of these works formally prove in a broad manner that such transitive reduction is the minimal expression of the HBR. In this pa-per, a formal study of the minimal binary relation (transitive reduction) of the HBR is presented, which is called the Immediate Dependency Relation (IDR). The study shows that since the transitive closure of the HBR is antisymmetric and finite, it implies that the IDR is unique. This is important because it means that all of the works that deal with a minimal expression of the HBR discuss the same minimal binary relation. In addition, an extension to the IDR to iden-tify causal immediate dependencies only among a subset of relevant events is presented. Finally, as case of study, the extension of the IDR is applied to the causal delivery of messages

Temporal Alignment Model for Data Streams in Wireless Sensor Networks Based on Causal Dependencies

International audienceNew applications based on wireless sensor networks (WSN), such as person-locator services, harvest a large amount ofdata streams that are simultaneously generated by multiple distributed sources. Specifically, in a WSN this paradigm of datageneration/transmission is known as event-streaming. In order to be useful, all the collected data must be aligned so that it canbe fused at a later phase. To perform such alignment, the sensors need to agree on common temporal references. Unfortunately,this agreement is difficult to achieve mainly due to the lack of perfectly synchronized physical clocks and the asynchronous natureof the execution. Some solutions tackle the issue of the temporal alignment; however, they demand extra resources to the networkdeployment since they try to impose global references by using a centralized scheme. In this paper,we propose a temporal alignmentmodel for data streams that identifies temporal relationships and which does not require the use of synchronized clocks, globalreferences, centralized schemes, or additional synchronization signals. The identification of temporal relationships without theuse of synchronized clocks is achieved by translating temporal dependencies based on a time-line to causal dependencies amongstreams. Finally, we show the viability and the effectiveness of the model by simulating it over a sensor network with multihopcommunication

An Efficient Δ-Causal Distributed Algorithm for Synchronous Cooperative Systems in Unreliable Networks

Abstract. In cooperative systems causal ordering delivery has been used to resolve problems of coherency of type producer-consumer. Causal order delivery is important for distributed systems since it allows an asynchronous execution to participants. When time delivery constraints are considered, ensuring causal delivery becomes more complex, as is the case for synchronous cooperative systems, such as Telemedicine and Teleimmersion. In these systems, the messages (units of data of continuous and discrete media) have an associated lifetime that determines the period of useful time in which the messages must be delivered. On the other hand, generally in these systems there is no time for retransmit them when messages are lost. Causal order with time constraints has previously been addressed, and it is called causal−Δ order. In this paper, we present an efficient causal−Δ distributed algorithm for unreliable networks that is characterized by the use of a forward error correction (FEC) scheme and a distributed method to calculate the message lifetime based on relative time points (i.e. no global time is used). We show the efficiency of our causal−Δ algorithm in terms of the control information attached per message

An Efficient Causal Group Communication Protocol for Free Scale Peer-to-Peer Networks

International audienceIn peer-to-peer (P2P) overlay networks, a group of n (≥2) peer processes have to cooperate with each other. Each peer sends messages to every peer and receives messages from every peer in a group. In group communications, each message sent by a peer is required to be causally delivered to every peer. Most of the protocols designed to ensure causal message order are designed for networks with a plain architecture. These protocols can be adapted to use in free scale and hierarchical topologies; however, the amount of control information is O(n), where n is the number of peers in the system. Some protocols are designed for a free scale or hierarchical networks, but in general they force the whole system to accomplish the same order viewed by a super peer. In this paper, we present a protocol that is specifically designed to work with a free scale peer-to-peer network. By using the information about the network's architecture and by representing message dependencies on a bit level, the proposed protocol ensures causal message ordering without enforcing super peers order. The designed protocol is simulated and compared with the Immediate Dependency Relation and the Dependency Sequences protocols to show its lower overhead

A survey on distributed NFV multi-domain orchestration from an algorithmic functional perspective

International audienceUnder the Network Function Virtualization multidomain orchestration approach, many service providers jointly manage the lifecycle of network services composed by Virtual Network Functions (VNFs). Many orchestration algorithms have been published focusing on different tasks for the lifecycle of network services. Nowadays, some general-purpose and architectural orchestration surveys have been published. However, currently, no survey classifies and evaluates distributed multidomain orchestration algorithms. In this paper, we focus on multi-domain orchestration works from an algorithm perspective by considering the cooperative and competitive approaches. Algorithms are classified and evaluated. We propose a taxonomy for works that consider different lifecycle tasks of network services. Classification and evaluation are done by identifying key requirements present in competitive and cooperative scenarios. This allows us to have a perspective on the current state of multi-domain orchestration algorithms. We also identify future research directions based on such a current perspective

Autonomic Web Services Based on Asynchronous Checkpointing

International audienceThe evolution of business software technologies is constant and is becoming increasingly complex which leads to a great probability of software/hardware failures. Business processes are built based on web services as they allow the creation of complex business functionalities. To attack the problem of failures presented by the use of web services, organizations are extrapolating the autonomic computing paradigm to their business processes as it enables them to detect, diagnose, and repair problems improving dependability. Sophisticated solutions that increase system dependability exist, however, those approaches have drawbacks; for example, they affect system performance, have high implementation costs, and or they may jeopardize the scalability of the system. To facilitate evolution to self-management, systems must implement the monitoring, analyzing, planning, and execution (MAPE) control loop. An open challenge for MAPE loop is to carry out in an efficient manner the diagnosis and decision-making processes, recollecting data from which the system can detect, diagnose, and repair potential problems. Also, dealt by systems dependability, specifically as fault tolerant mechanisms. One useful tool for this purpose is the communication induced checkpointing (CiC). We use CiC in attacking the dependability problem of using web services in a distributed and efficient manner. First, we present an approach for web services compositions that supports fault tolerance based on the CiC mechanism. Second, we present an algorithm aimed at web services compositions based on an autonomic computing and checkpointing mechanism. Experimental results support the feasibility of this concept proposal

Synchronization Protocol for Real Time Multimedia in Mobile Distributed Systems

International audienceMobile distributed systems (MDS) deal multimedia data transmissions among geographically distributed mobile sources. In this environment, the preservation of temporal dependencies among exchanged real-time mul-timedia data is mainly affected by the asynchronous transmissions, the constantly topology changes, unpredictable delays and the lack of global references as memory and perfectly synchronized clocks. Albeit a few works are oriented to fulfill intermedia temporal dependencies, the vast majority of them do not completely support the constraints and characteristics of MDS. This paper presents a causal protocol oriented to satisfy logical and temporal dependencies among real-time multimedia data in a MDS. One of the main aspects of our protocol is the translation of temporal constraints to causal dependencies of the multimedia data using logical mapping, avoiding the use of global references. With the simulations results we demonstrate that our protocol is effective diminishing the synchronization error. Furthermore, the protocol is efficient as far as processing and storage costs at the mobile hosts, and in the overhead attached per message with a reduced usage of bandwidth across the wired and wireless channels in comparison to the Real-time Transport Protocol (RTP)

A Mechanism for the Causal Ordered Set Representation in Large-Scale Distributed Systems

International audienceDistributed systems have undergone a very fast evolution these last years. Large-scale distributed systems have become an integral part of everyday life with the development of new large-scale applications, consisting of thousands of computers and supporting millions of users. Examples include global Internet services, cloud computing systems, " big data " analytics platforms, peer-to-peer systems, wireless sensor networks and so on. The recent research addresses questions related to the way one may design, build, operate and maintain large-scale distributed systems. An other question related to such area, is how to represent causal dependencies in such systems in a minimal way. In general, causal dependencies can be established according to the Happened-Before Relation (HBR), which was introduced by Lamport. The HBR is a strict partial order, and therefore, one main problem linked to it is the combinatorial state explosion. The Immediate Dependency Relation (IDR) and the Causal Order Set Abstraction (CAOS) present a solution for such a problem. In this paper, we propose a mechanism which uses the concepts HBR, IDR, CAOS to model a large-scale distributed system execution in the form of the minimal graph (IDR graph) and the compact graph (CAOS graph). This mechanism is implemented in C++. The results of its execution are given here. The resultant causal graphs can be used for different purposes, such as for the design of more efficient algorithms, validation, verification, and/or the debugging of the existing ones, among others

Message ordering framework for collaborative web services-based environments

International audienceWeb service paradigm is becoming a very powerful architecture for organizations when integrating heterogeneous applications. These provide functionality and form the basis for complex distributed business processes. Open standards make suitable Web Services interoperable for distributed environments. Collaboration between organizations is crucial in this context since it allows Web users to share knowledge, ideas, and modify information. Sharing information in a collaborative manner can minimize time spent in problem resolution. Message ordering is critical in this context, information presented to each user must be consistent way to preserve data integrity. For this purpose, causal ordering protocols are essential while exchanging information, however, their implementation is expensive to set up in distributed systems. Ongoing studies try to reduce the overhead imposed by the information carried out by each message, the optimal way of reducing such overhead is implementing the Immediate Dependency Relationship (IDR). In this paper, we present a framework for such message ordering, relative in collaborative environments, maintaining low overhead and computational cost based on the IDR