When designing distributed web services, there are three properties that are commonly desired: consistency, availability, and partition tolerance. It is impossible to achieve all three. In this note, we prove this conjecture in the asynchronous network model, and then discuss solutions to this dilemma in the partially synchronous model.

No abstract

No abstract

Almost twelve years ago, in 2000, Eric Brewer introduced the idea that there is a fundamental trade-off between consistency, availability, and partition tolerance. This trade-off, which has become known as the CAP Theorem, has been widely discussed ever since. In this paper, we review the CAP Theorem and situate it within the broader context of distributed computing theory. We then discuss the practical implications of the CAP Theorem, and explore some general techniques for coping with the inherent trade-offs that it implies.

This paper presents a new algorithm implementing reconfigurable atomic read/write memory for highly dynamic environments. The original RAMBO algorithm, recently developed by Lynch and Shvartsman [15,16], guarantees atomicity for arbitrary patterns of asynchrony, message loss, and node crashes. RAMBO II implements a different approach to establishing new configurations: instead of operating sequentially, the new algorithm reconfigures aggressively, transferring information from old configurations to new configurations in parallel. This improvement substantially reduces the time to establish a new configuration and to remove obsolete configurations. This, in turn, substantially increases fault tolerance and reduces the latency of read/write operations when the network is unstable or reconfiguration is bursty. This paper presents RAMBO II, a correctness proof, and a conditional analysis of its performance. Preliminary empirical studies illustrate the advantages of the new algorithm.

Abstract. We present a new approach, the GeoQuorums approach, for implementing atomic read/write shared memory in ad hoc networks. Our approach is based on abstract nodes associated with certain geographic locations. We assume the existence of focal points, geographic areas that are normally "populated" by mobile hosts. For example, a focal point may be a road junction, a scenic observation point, or a water resource in the desert. Mobile hosts that happen to populate a focal point participate in implementing shared atomic put/get objects, using a replicated state machine approach. These objects are then used to implement atomic read/write operations. The GeoQuorums algorithm defines certain intersecting sets of focal points, known as quorums. The quorum systems are used to maintain the consistency of the shared memory. We present a mechanism for changing quorum systems on the fly, thus improving efficiency. Overall, the new GeoQuorums algorithm efficiently implements read and write operations in a highly dynamic, mobile network.

Most people believe that renaming is easy: simply choose a name at random; if more than one process selects the same name, then try again. We highlight the issues that occur when trying to implement such a scheme and shed new light on the read-write complexity of randomized renaming in an asynchronous environment. At the heart of our new perspective stands an adaptive implementation of a randomized test-and-set object, that has poly-logarithmic step complexity per operation, with high probability. Interestingly, our implementation is anonymous, as it does not require process identifiers. Based on this implementation, we present two new randomized renaming algorithms. The first ensures a tight namespace of n names using O(n log 4 n) total steps, with high probability. This improves on the best previously known algorithm by almost a quadratic factor. The second algorithm achieves a namespace of size k(1 +) using O(k log 4 k/ log 2 (1 +)) total steps, both with high probability, where k is the total contention in the execution. It is the first adaptive randomized renaming algorithm, and it improves on existing deterministic solutions by providing a smaller namespace, and by significantly lowering complexity.

How efficiently can a malicious device disrupt a single-hop wireless network? Imagine two honest players attempting to exchange information in the presence of a malicious adversary that can disrupt communication by jamming or overwriting messages. Assume the adversary has a broadcast budget of β-unknown to the honest players. We show that communication can be delayed for 2β + Θ(lg |V |) rounds, where V is the set of values that the honest players may transmit. We then derivevia reduction to this 3-player game-round complexity lower bounds for several classical n-player problems: 2β + Ω(lg |V |) for reliable broadcast, 2β + Ω(log n) for leader election, and 2β + Ω(k lg |V |/k) for static k-selection. We also consider an extension of our adversary model that includes up to t crash failures. Here we show a bound of 2β + Θ(t) rounds for binary consensus. We provide tight, or nearly tight, upper bounds for all four problems. From these results we can derive bounds on the efficiency of malicious disruption, stated in terms of two new metrics: jamming gain (the ratio of rounds delayed to adversarial broadcasts) and disruption-free complexity (the rounds required to terminate in the special case of no disruption). Two key conclusions of this study: (1) all the problems considered feature semantic vulnerabilities that allow an adversary to disrupt termination more efficiently than simple jamming (i.e., all have a jamming gain greater than 1); and (2) for all the problems considered, the round complexity grows linearly with the number of bits to be communicated (i.e., all have a Ω(lg |V |) or Ω(lg n) disruption-free complexity.)