<p>The software industry is facing a great challenge. While systems get more complex and distributed across the world, users are becoming more dependent on their availability. As systems increase in size and complexity so does the risk that some part will fail. Unfortunately, it has proven hard to tackle faults in distributed systems without a rigorous approach. Therefore, it is crucial that the scientific community can provide answers to how distributed computer systems can continue functioning despite faults.</p><p>Our contribution in this thesis is regarding a special class of faults which occurs whennetwork links fail in such a way that parts of the network become isolated, such faults are termed network partitions. We consider the problem of how systems that have integrity constraints on data can continue operating in presence of a network partition. Such a system must act optimistically while the network is split and then perform a some kind of reconciliation to restore consistency afterwards.</p><p>We have formally described four reconciliation algorithms and proven them correct. The novelty of these algorithms lies in the fact that they can restore consistency after network partitions in a system with integrity constraints and that one of the protocols allows the system to provide service during the reconciliation. We have implemented and evaluated the algorithms using simulation and as part of a partition-tolerant CORBA middleware. The results indicate that it pays off to act optimistically and that it is worthwhile to provide service during reconciliation.</p>

The software industry is facing a great challenge. While systems get more complex and distributed across the world, users are becoming more dependent on their availability. As systems increase in size and complexity so does the risk that some part will fail. Unfortunately, it has proven hard to tackle faults in distributed systems without a rigorous approach. Therefore, it is crucial that the scientific community can provide answers to how distributed computer systems can continue functioning despite faults. Our contribution in this thesis is regarding a special class of faults which occurs whennetwork links fail in such a way that parts of the network become isolated, such faults are termed network partitions. We consider the problem of how systems that have integrity constraints on data can continue operating in presence of a network partition. Such a system must act optimistically while the network is split and then perform a some kind of reconciliation to restore consistency afterwards. We have formally described four reconciliation algorithms and proven them correct. The novelty of these algorithms lies in the fact that they can restore consistency after network partitions in a system with integrity constraints and that one of the protocols allows the system to provide service during the reconciliation. We have implemented and evaluated the algorithms using simulation and as part of a partition-tolerant CORBA middleware. The results indicate that it pays off to act optimistically and that it is worthwhile to provide service during reconciliation.

<p>Structured overlay networks form a major classof peer-to-peer systems, which are touted for their abilitiesto scale, tolerate failures, and self-manage. Any long livedInternet-scale distributed system is destined to facenetwork partitions. Although the problem of network partitionsand mergers is highly related to fault-tolerance andself-management in large-scale systems, it has hardly beenstudied in the context of structured peer-to-peer systems.These systems have mainly been studied under churn (frequentjoins/failures), which as a side effect solves the problemof network partitions, as it is similar to massive nodefailures. Yet, the crucial aspect of network mergers has beenignored. In fact, it has been claimed that ring-based structuredoverlay networks, which constitute the majority of thestructured overlays, are intrinsically ill-suited for mergingrings. In this thesis, we present a number of research papers representing our work on handling network partitions and mergers in structured overlay networks. The contribution of this thesis is threefold. First, we provide a solution for merging ring-based structured overlays. Our solution is tuneable, by a {\em fanout} parameter, to achieve a trade-off between message and time complexity. Second, we provide a network size estimation algorithm for ring-based structured overlays. We believe that an estimate of the current network size can be used for tuning overlay parameters that change according to the network size, for instance the fanout parameter in our merger solution.Third, we extend our work from fixing routing anomalies to achieving data consistency. We argue that decreasing lookup inconsistencies on the routing level aids in achieving data consistency in applications built on top of overlays. We study the frequency of occurence of lookup inconsistencies and discuss solutions to decrease the affect of lookup inconsistencies.</p>

Structured overlay networks form a major classof peer-to-peer systems, which are touted for their abilitiesto scale, tolerate failures, and self-manage. Any long livedInternet-scale distributed system is destined to facenetwork partitions. Although the problem of network partitionsand mergers is highly related to fault-tolerance andself-management in large-scale systems, it has hardly beenstudied in the context of structured peer-to-peer systems.These systems have mainly been studied under churn (frequentjoins/failures), which as a side effect solves the problemof network partitions, as it is similar to massive nodefailures. Yet, the crucial aspect of network mergers has beenignored. In fact, it has been claimed that ring-based structuredoverlay networks, which constitute the majority of thestructured overlays, are intrinsically ill-suited for mergingrings. In this thesis, we present a number of research papers representing our work on handling network partitions and mergers in structured overlay networks. The contribution of this thesis is threefold. First, we provide a solution for merging ring-based structured overlays. Our solution is tuneable, by a {\em fanout} parameter, to achieve a trade-off between message and time complexity. Second, we provide a network size estimation algorithm for ring-based structured overlays. We believe that an estimate of the current network size can be used for tuning overlay parameters that change according to the network size, for instance the fanout parameter in our merger solution.Third, we extend our work from fixing routing anomalies to achieving data consistency. We argue that decreasing lookup inconsistencies on the routing level aids in achieving data consistency in applications built on top of overlays. We study the frequency of occurence of lookup inconsistencies and discuss solutions to decrease the affect of lookup inconsistencies.