In a virtualized infrastructure where physical resources are shared, a single physical server failure will terminate several virtual servers and crippling the virtual infrastructures which contained those virtual servers. In the worst case, more failures may cascade from overloading the remaining servers. To guarantee some level of reliability, each virtual infrastructure, at instantiation, should be augmented with backup virtual nodes and links that have sufficient capacities. This ensures that, when physical failures occur, sufficient computing resources are available and the virtual network topology is preserved. However, in doing so, the utilization of the physical infrastructure may be greatly reduced. This can be circumvented if backup resources are pooled and shared across multiple virtual infrastructures, and intelligently embedded in the physical infrastructure. These techniques can reduce the physical footprint of virtual backups while guaranteeing reliability.
Analysis on the reliability of overlay networks in terms of connectivity in the overlays has been developed @cite_9 . It achieves a good estimation of connectivity in the embedded overlay networks through a Monte Carlo simulation-based algorithm. Unfortunately, it is not applicable to our problem as we are concerned with critical virtual nodes and embedding them, as well as the whole infrastructure with reliability guarantees.
@cite_9
2017999041 2049693545 2022378993 2027480718
Reliability and availability have always been important characteristics of systems but become even more critical and complex issues on networks. Due to the high complexity of accurate calculation methods, simulation methods for network reliability and availability evaluation have been a popular area of research, and received significant attention. In this paper the network topology model, the failure driven simulation method and the reliability and availability statistical algorithms are put forward, and then the network reliability and availability simulation flow based on the Monte Carlo method was designed. This simulation method can be used to calculate the reliability and availability of a network whose nodes and links may be failure. Two repair policies, immediate repair and delay repair, are provided to choose. Finally, the backbone of CERNET (China Education and Research Network) is taken as an example and the simulation results are analyzed. We investigate the problem of designing a scalable overlay network to support decentralized topic-based pub sub communication. We introduce a new optimization problem, called Minimum Topic-Connected Overlay (Min-TCO), that captures the tradeoff between the scalability of the overlay (in terms of the nodes' fanout) and the message forwarding overhead incurred by the communicating parties. Roughly, the Min-TCO problem is as follows: Given a collection of nodes and their subscriptions, connect the nodes using the minimum possible number of edges so that for each topic t, a message published on t could reach all the nodes interested in t by being forwarded by onlythe nodes interested in t. We show that the decision version of Min-TCO is NP-complete, and present a polynomial algorithm that approximates the optimal solution within a logarithmic factor with respect to the number of edges in theconstructed overlay. We further prove that this approximation ratio is almost tight by showing that no polynomial algorithm can approximate Min-TCO within a constant factor (unless P=NP). We show experimentally that on typical inputs, the fanout of the overlay constructed by our approximation algorithm is significantly lower thanthat of the overlays built by the existing algorithms, and that its running time is just a small fraction of the analytical worst case bound. As Min-TCO can be shown to capture several important aspects of most known overlay-based pub sub implementations, our study sheds light on the inherent limitations of the existing systems as well asprovides an insight into the best possible feasible solution. Finally, we introduce a flexible framework that generalizes Min-TCO and formalizes most similar overlay design problems that occur in scalable pub sub systems. We also briefly discuss several examples of such problems, and show some results with respect to their complexity. This paper studies the problem of answering aggregation queries, satisfying the interval validity semantics, in a distributed system prone to continuous arrival and departure of participants. The interval validity semantics states that the query answer must be calculated considering contributions of at least all processes that remained in the distributed system for the whole query duration. Satisfying this semantics in systems experiencing unbounded churn is impossible due to the lack of connectivity and path stability between processes. This paper presents a novel architecture, namely Virtual Tree, for building and maintaining a structured overlay network with guaranteed connectivity and path stability in settings characterized by bounded churn rate. The architecture includes a simple query answering algorithm that provides interval valid answers. The overlay network generated by the Virtual Tree architecture is a tree-shaped topology with virtual nodes constituted by clusters of processes and virtual links constituted by multiple communication links connecting processes located in adjacent virtual nodes. We formally prove a bound on the churn rate for interval valid queries in a distributed system where communication latencies are bounded by a constant unknown by processes. Finally, we carry out an extensive experimental evaluation that shows the degree of robustness of the overlay network generated by the virtual tree architecture under different churn rates. We consider network reliability in layered networks where the lower layer experiences random link failures. In layered networks, each failure at the lower layer may lead to multiple failures at the upper layer. We generalize the classical polynomial expression for network reliability to the multilayer setting. Using random sampling techniques, we develop polynomial-time approximation algorithms for the failure polynomial. Our approach gives an approximate expression for reliability as a function of the link failure probability, eliminating the need to resample for different values of the failure probability. Furthermore, it gives insight on how the routings of the logical topology on the physical topology impact network reliability. We show that maximizing the min cut of the (layered) network maximizes reliability in the low-failure-probability regime. Based on this observation, we develop algorithms for routing the logical topology to maximize reliability.