17868570
Condition-Based Protocols for Set Agreement Problems
A condition C is a set of input vectors to a problem. A class of conditions that allow to solve k-set agreement in spite of f crashes in an asynchronous system is identified. A k-set agreement protocol that is always safe is described: it is guaranteed to terminate when the input vector belongs to C and it always decides on at most k different values, even if the input vector does not belong to C. While there are simple solutions when f < k, it is known that the k-set agreement problem has no solution when f ? k. Thus, the paper identifies classes of conditions that allow to solve this problem even when f ? k. The paper gives evidence that these are the only conditions that allow to solve set agreement, by proving the wait-free case. Two natural concrete conditions that belong to such a class are described. Finally, a more efficient k-set agreement protocol with only linear complexity (does not use snapshots), for any C that allows to solve consensus, when k ? f/(n - f) + 1 is presented. This shows how to trade fault-tolerance for agreement precision using the condition based approach.
38731127
The Synchronous Condition-Based Consensus Hierarchy
In the context of a system made up of n processes where at most t can crash, the condition-based approach studies restrictions on the inputs of a distributed problem, called conditions, that make it solvable, or easier to solve (in case it is solvable without restricting its inputs). Previous work studied conditions for consensus and other agreement problems, mostly for asynchronous systems. This paper considers the condition-based approach for consensus in synchronous systems, and establishes a bridge between the asynchronous and synchronous models, with a hierarchy \( {\cal S}_t^{[-t]}\subset\cdots\subset {\cal S}_t^{[0]}\subset\cdots\subset {\cal S}_t^{[t]} \) where \({\cal S}_t^{[t]}\) includes all conditions (and in particular the trivial one made up of all possible input vectors). For a condition \(C \in{\cal S}_t^{[d]}\), –t ≤ d ≤ t, we have:
35647117
Early-Stopping k-Set Agreement in Synchronous Systems Prone to Any Number of Process Crashes
The k-set agreement problem is a generalization of the consensus problem: each process proposes a value, and each non-faulty process has to decide a value such that a decided value is a proposed value, and no more than k different values are decided.

This paper presents a surprisingly simple protocol that solves the k-set agreement problem in synchronous systems prone to up to t<n processes can crash (where n is the total number of processes). The proposed protocol is the first early stopping k-set agreement protocol that does not impose a constraint on t. It allows the processes to decide and stop by min $(\lfloor f/k \rfloor +2,\lfloor t/k \rfloor+1)$ rounds where f is the number of actual crashes (0≤ f ≤ t). In addition to its conceptual simplicity, the protocol has an additional noteworthy feature, namely, it is particularly efficient in common case scenarios. This comes from the fact that it is based on a mechanism that allows the processes to take into account the actual pattern of failures and not only their number, thereby allowing the processes to decide in much less than ${\lfloor f/k \rfloor +2}$ coding from an InlineMediaObject here! rounds in a lot of cases.
3212723
In Search of the Holy Grail: Looking for the Weakest Failure Detector for Wait-Free Set Agreement
Asynchronous failure detector-based set agreement algorithms proposed so far assume that all the processes participate in the algorithm. This means that (at least) the processes that do not crash propose a value and consequently execute the algorithm. It follows that these algorithms can block forever (preventing the correct processes from terminating) when there are correct processes that do not participate in the algorithm. This paper investigates the wait-free set agreement problem, i.e., the case where the correct participating processes have to decide a value whatever the behavior of the other processes (i.e., the processes that crash and the processes that are correct but do not participate in the algorithm). The paper presents a wait-free set agreement algorithm. This algorithm is based on a leader failure detector class that takes into account the notion of participating processes. Interestingly, this algorithm enjoys a first class property, namely, design simplicity.
7058731
Approximate real-time clocks for scheduled events
This paper presents a simple protocol that provides application processes with an approximate real-time notion. This time notion is very versatile. At one extreme, it behaves at least as virtual time (being thereby consistent with causality). At the other extreme, it behaves as real-time. More precisely, with respect to approximate real-time all scheduled events are always executed at their scheduled time. If local computations took no time, approximate real-time would always be real-time. The system facilitates the rescheduling of events to bring real-time and approximate time closer together if local computations take too much time. A time-slotted leader election protocol using the proposed approximate time notion is described.
5355347
An introduction to the renaming problem
The aim of this paper is to provide a brief introduction to the renaming problem for unfamiliar readers. In the renaming problem the processes have to acquire new names from a small bounded space despite possible process crashes and asynchrony. The problem is first introduced. Then two solutions are presented. One considers the shared memory model, while the second considers the message-passing model.
46587799
Strongly Terminating Early-Stopping k-Set Agreement in Synchronous Systems with General Omission Failures
The k-set agreement problem is a generalization of the consensus problem: considering a system made up of n processes where each process proposes a value, each non-faulty process has to decide a value such that a decided value is a proposed value, and no more than k different values are decided. It has recently be shown that, in the crash failure model, min$(\lfloor \frac{f}{k} \rfloor+2,\lfloor \frac{t}{k} \rfloor +1)$ is a lower bound on the number of rounds for the non-faulty processes to decide (where t is an upper bound on the number of process crashes, and f, 0 ≤f ≤t, the actual number of crashes) 
 
This paper considers the k-set agreement problem in synchronous systems where up to t < n /2 processes can experience general omission failures (i.e., a process can crash or omit sending or receiving messages). It first introduces a new property, called strong termination. This property is on the processes that decide. It is satisfied if, not only every non-faulty process, but any process that neither crashes nor commits receive omission failures decides. The paper then presents a k-set agreement protocol that enjoys the following features. First, it is strongly terminating (to our knowledge, it is the first agreement protocol to satisfy this property, whatever the failure model considered). Then, it is early deciding and stopping in the sense that a process that either is non-faulty or commits only send omission failures decides and halts by round min$(\lfloor \frac{f}{k} \rfloor+2,\lfloor \frac{t}{k} \rfloor +1)$. To our knowledge, this is the first early deciding k-set agreement protocol for the general omission failure model. Moreover, the protocol provides also the following additional early stopping property: a process that commits receive omission failures (and does not crash) executes at most min$(\lceil \frac{f}{k} \rceil +2,\lfloor \frac{t}{k} \rfloor +1)$ rounds. It is worth noticing that the protocol allows each property (strong termination vs early deciding/stopping vs early stopping) not to be obtained at the detriment of the two others 
 
The combination of the fact that min$(\lfloor \frac{f}{k} \rfloor+2,\lfloor \frac{t}{k} \rfloor +1)$ is lower bound on the number of rounds in the crash failure model, and the very existence of the proposed protocol has two very interesting consequences. First, it shows that, although general omission failure model is more severe than the crash failure model, both models have the same lower bound for the non-faulty processes to decide. Second, it shows that, in the general omission failure model, that bound applies also the processes that commit only send omission failures
10131958
An optimal atomic broadcast protocol and an implementation framework
Atomic broadcast (where all processes deliver broadcast messages in the same order) is a very useful group communication primitive for building fault-tolerant distributed systems. This paper presents an atomic broadcast protocol that can be claimed to be optimal in terms of failure detection, resilience, and latency. The protocol requires only the weakest of the useful failure detectors for liveness, and permits up to (n-1)/2 processes to crash in a system of n processes; at most two communication steps and n broadcasts are needed in a run during which process crashes and failure-suspicions do not occur. We also introduce the notion of notifying broadcast which can reduce the message overhead further in 'nice' runs in which all processes are operational and communication delays do not exceed the bound assumed. If nice runs persist, the average message overhead is just one broadcast. That is, the protocol extracts no message overhead for providing crash-tolerance if process failures and unanticipated fluctuations in communication delays do not occur. We are currently implementing our protocol as a CORBA (Common Object Request Broker Architecture) component. All known ORBs use IIOP as the standard protocol for inter-process communication, which in turn uses TCP/IP (transmission control protocol/Internet protocol) as the common transport protocol. It turns out that the notifying broadcast is easy to implement on top of the TCP transport layer.