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The Power of Abstraction
The Power of Abstraction Barbara Liskov March 2013 MIT CSAIL Software is Complex Systems are big and they do complicated things and they may be distributed and/or concurrent Addressing Complexity Algorithms, data structures, protocols Addressing Complexity Algorithms, data structures, protocols Programming methodology Programming languages This Talk Programming methodology as it developed Programming languages Programming languages today The Situation in 1970 The software crisis! Programming Methodology How should programs be designed? How should programs be structured? The Landscape E. W. Dijkstra. Go To Statement Considered Harmful. Cacm, Mar. 1968 The Landscape N. Wirth. Program Development by Stepwise Refinement. Cacm, April 1971 The Landscape D. L. Parnas. Information Distribution Aspects of Design Methodology. IFIP Congress, 1971 “The connections between modules are the assumptions which the modules make about each other.” Modularity A program is a collection of modules Modularity A program is a collection of modules Each module has an interface, described by a specification Modularity A program is a collection of modules Each has an interface, described by a specification A module’s implementation is correct if it meets the specification A using module depends only on the specification Modularity A program is a collection of modules Each has an interface, described by a specification A module’s implementation is correct if it meets the specification A using module depends only on the specification E.g. a sort routine sort(a) Benefits of Modularity Local reasoning Modifiability Independent development The Situation in 1970 Procedures were the only type of module Not powerful enough, e.g., a file system Not used very much Complicated connections Partitions B. -
MIT Turing Laureates Propose Creation of School of Computing an Open Letter to President Rafael Reif
9/26/2017 The Tech OPINION LETTER TO THE EDITOR MIT Turing laureates propose creation of School of Computing An open letter to President Rafael Reif By MIT Turing Laureates | Sep. 20, 2017 Facebook Dear Rafael, Twitter There comes a time, in the course of scientic evolution, when a discipline is ready to emerge from the womb of its parent disciplines and take its own place in the world. For Reddit computer science, or more accurately, for the eld of computing, this moment is now. Print Born from a combination of mathematics and electrical engineering, with the original intent of speeding up calculations, computer science has grown to encompass all information processing and most communications and now to provide an alternative evolutionary path to intelligence. Computer science is rapidly becoming an essential part of most academic disciplines, and students are voting with their feet. One third of MIT undergraduates are majoring in computer science. This trend is unlikely to slow down anytime soon. We, the 7 active MIT Turing Award winners, therefore write this open letter to recommend that you consider the bold step of establishing a School of Computing at MIT. The new school, a brother to the Schools of Engineering and Science, will allow the eld of computing, with its many https://thetech.com/2017/09/20/turing-laureates-open-letter-to-reif 1/4 9/26/2017 The Tech facets and sub-elds, to grow and interact naturally with the Institute’s scientic and engineering environment. The Tech Submit Campus Life Stories Today the study of computation is housed primarily in the EECS department within the School of Engineering, but departments are limited in their ability to hire and grow. -
Arxiv:1909.05204V3 [Cs.DC] 6 Feb 2020
Cogsworth: Byzantine View Synchronization Oded Naor, Technion and Calibra Mathieu Baudet, Calibra Dahlia Malkhi, Calibra Alexander Spiegelman, VMware Research Most methods for Byzantine fault tolerance (BFT) in the partial synchrony setting divide the local state of the nodes into views, and the transition from one view to the next dictates a leader change. In order to provide liveness, all honest nodes need to stay in the same view for a sufficiently long time. This requires view synchronization, a requisite of BFT that we extract and formally define here. Existing approaches for Byzantine view synchronization incur quadratic communication (in n, the number of parties). A cascade of O(n) view changes may thus result in O(n3) communication complexity. This paper presents a new Byzantine view synchronization algorithm named Cogsworth, that has optimistically linear communication complexity and constant latency. Faced with benign failures, Cogsworth has expected linear communication and constant latency. The result here serves as an important step towards reaching solutions that have overall quadratic communication, the known lower bound on Byzantine fault tolerant consensus. Cogsworth is particularly useful for a family of BFT protocols that already exhibit linear communication under various circumstances, but suffer quadratic overhead due to view synchro- nization. 1. INTRODUCTION Logical synchronization is a requisite for progress to be made in asynchronous state machine repli- cation (SMR). Previous Byzantine fault tolerant (BFT) synchronization mechanisms incur quadratic message complexities, frequently dominating over the linear cost of the consensus cores of BFT so- lutions. In this work, we define the view synchronization problem and provide the first solution in the Byzantine setting, whose latency is bounded and communication cost is linear, under a broad set of scenarios. -
FOCS 2005 Program SUNDAY October 23, 2005
FOCS 2005 Program SUNDAY October 23, 2005 Talks in Grand Ballroom, 17th floor Session 1: 8:50am – 10:10am Chair: Eva´ Tardos 8:50 Agnostically Learning Halfspaces Adam Kalai, Adam Klivans, Yishay Mansour and Rocco Servedio 9:10 Noise stability of functions with low influences: invari- ance and optimality The 46th Annual IEEE Symposium on Elchanan Mossel, Ryan O’Donnell and Krzysztof Foundations of Computer Science Oleszkiewicz October 22-25, 2005 Omni William Penn Hotel, 9:30 Every decision tree has an influential variable Pittsburgh, PA Ryan O’Donnell, Michael Saks, Oded Schramm and Rocco Servedio Sponsored by the IEEE Computer Society Technical Committee on Mathematical Foundations of Computing 9:50 Lower Bounds for the Noisy Broadcast Problem In cooperation with ACM SIGACT Navin Goyal, Guy Kindler and Michael Saks Break 10:10am – 10:30am FOCS ’05 gratefully acknowledges financial support from Microsoft Research, Yahoo! Research, and the CMU Aladdin center Session 2: 10:30am – 12:10pm Chair: Satish Rao SATURDAY October 22, 2005 10:30 The Unique Games Conjecture, Integrality Gap for Cut Problems and Embeddability of Negative Type Metrics Tutorials held at CMU University Center into `1 [Best paper award] Reception at Omni William Penn Hotel, Monongahela Room, Subhash Khot and Nisheeth Vishnoi 17th floor 10:50 The Closest Substring problem with small distances Tutorial 1: 1:30pm – 3:30pm Daniel Marx (McConomy Auditorium) Chair: Irit Dinur 11:10 Fitting tree metrics: Hierarchical clustering and Phy- logeny Subhash Khot Nir Ailon and Moses Charikar On the Unique Games Conjecture 11:30 Metric Embeddings with Relaxed Guarantees Break 3:30pm – 4:00pm Ittai Abraham, Yair Bartal, T-H. -
Kein Folientitel
182.703: Problems in Distributed Computing (Part 3) WS 2019 Ulrich Schmid Institute of Computer Engineering, TU Vienna Embedded Computing Systems Group E191-02 [email protected] Content (Part 3) The Role of Synchrony Conditions Failure Detectors Real-Time Clocks Partially Synchronous Models Models supporting lock-step round simulations Weaker partially synchronous models Dynamic distributed systems U. Schmid 182.703 PRDC 2 The Role of Synchrony Conditions U. Schmid 182.703 PRDC 3 Recall Distributed Agreement (Consensus) Yes No Yes NoYes? None? meet All meet Yes No No U. Schmid 182.703 PRDC 4 Recall Consensus Impossibility (FLP) Fischer, Lynch und Paterson [FLP85]: “There is no deterministic algorithm for solving consensus in an asynchronous distributed system in the presence of a single crash failure.” Key problem: Distinguish slow from dead! U. Schmid 182.703 PRDC 5 Consensus Solvability in ParSync [DDS87] (I) Dolev, Dwork and Stockmeyer investigated consensus solvability in Partially Synchronous Systems (ParSync), varying 5 „synchrony handles“ : • Processors synchronous / asynchronous • Communication synchronous / asynchronous • Message order synchronous (system-wide consistent) / asynchronous (out-of-order) • Send steps broadcast / unicast • Computing steps atomic rec+send / separate rec, send U. Schmid 182.703 PRDC 6 Consensus Solvability in ParSync [DDS87] (II) Wait-free consensus possible Consensus impossible s/r Consensus possible s+r for f=1 ucast bcast async sync Global message order message Global async sync Communication U. Schmid 182.703 PRDC 7 The Role of Synchrony Conditions Enable failure detection Enforce event ordering • Distinguish „old“ from „new“ • Ruling out existence of stale (in-transit) information • Distinguish slow from dead • Creating non-overlapping „phases of operation“ (rounds) U. -
42 Paxos Made Moderately Complex
Paxos Made Moderately Complex ROBBERT VAN RENESSE and DENIZ ALTINBUKEN, Cornell University This article explains the full reconfigurable multidecree Paxos (or multi-Paxos) protocol. Paxos is by no means a simple protocol, even though it is based on relatively simple invariants. We provide pseudocode and explain it guided by invariants. We initially avoid optimizations that complicate comprehension. Next we discuss liveness, list various optimizations that make the protocol practical, and present variants of the protocol. Categories and Subject Descriptors: C.2.4 [Computer-Communication Networks]: Distributed Syst- ems—Network operating systems; D.4.5 [Operating Systems]: Reliability—Fault-tolerance General Terms: Design, Reliability Additional Key Words and Phrases: Replicated state machines, consensus, voting ACM Reference Format: Robbert van Renesse and Deniz Altinbuken. 2015. Paxos made moderately complex. ACM Comput. Surv. 47, 3, Article 42 (February 2015), 36 pages. DOI: http://dx.doi.org/10.1145/2673577 1. INTRODUCTION Paxos [Lamport 1998] is a protocol for state machine replication in an asynchronous environment that admits crash failures. It is useful to consider the terms in this 42 sentence carefully: —A state machine consists of a collection of states, a collection of transitions between states, and a current state. A transition to a new current state happens in response to an issued operation and produces an output. Transitions from the current state to the same state are allowed and are used to model read-only operations. In a deterministic state machine, for any state and operation, the transition enabled by the operation is unique and the output is a function only of the state and the operation. -
A General Characterization of Indulgence
A General Characterization of Indulgence R. Guerraoui1;2 N. Lynch2 (1) School of Computer and Communication Sciences, EPFL (2) Computer Science and Arti¯cial Intelligence Laboratory, MIT Abstract. An indulgent algorithm is a distributed algorithm that, be- sides tolerating process failures, also tolerates arbitrarily long periods of instability, with an unbounded number of timing and scheduling failures. In particular, no process can take any irrevocable action based on the op- erational status, correct or failed, of other processes. This paper presents an intuitive and general characterization of indulgence. The characteri- zation can be viewed as a simple application of Murphy's law to partial runs of a distributed algorithm, in a computing model that encompasses various communication and resilience schemes. We use our characteriza- tion to establish several results about the inherent power and limitations of indulgent algorithms. 1 Introduction Indulgence The idea of indulgence is motivated by the di±culty for any process in a dis- tributed system to accurately ¯gure out, at any point of its computation, any information about which, and in what order, processes will take steps after that point. For instance, a process can usually not know if other processes have failed and stopped operating or are simply slow to signal their activity and will in- deed perform further computational steps. More generally, a process can hardly exclude any future interleaving of the processes. This uncertainty is at the heart of many impossibilities and lower bounds in distributed computing, e.g., [9], and it has been expressed in various forms and assuming speci¯c computation models,e.g., [7, 4, 19]. -
Velos: One-Sided Paxos for RDMA Applications
Velos: One-sided Paxos for RDMA applications Rachid Guerraoui Antoine Murat Athanasios Xygkis EPFL EPFL EPFL Abstract These solutions primarily focus on increasing the through- put and lowering the latency of common case executions, Modern data centers are becoming increasingly equipped thus achieving consensus in the order of a few microseconds. with RDMA-capable NICs. These devices enable distributed Nevertheless, outside of their common case, these systems systems to rely on algorithms designed for shared memory. suffer from orders of magnitude higher failover times, rang- RDMA allows consensus to terminate within a few microsec- ing from 1ms [2] to tens or hundreds of ms [23, 25]. ond in failure-free scenarios, yet, RDMA-optimized algo- In this work we introduce a leader-based consensus algo- rithms still use expensive two-sided operations in case of fail- rithm based on the well-known Paxos [16] that is efficient ure. In this work, we present a new leader-based algorithm both during the common-case as well as during failover. Our for consensus based on Paxos that relies solely on one-sided algorithm provides comparative performance to the state-of- RDMA verbs. Our algorithm decides in a single one-sided . s RDMA operation in the common case, and changes leader the-art system Mu, i.e., it achieves consensus in under 1 9µ , making it only twice as slow as Mu. At the same time, our also in a single one-sided RDMA operation in case of fail- algorithm is 13 times faster than Mu in the event of a leader ure. -
Safe and Efficient Sharing of Persistent Objects in Thor
This paper appears in Proceedings of the 1996 ACM SIGMOD Int. Conf. on Management of Data, Montreal, Canada, June 1996. Note: the pages are numbered 318±329, as in the proceedings. Safe and Ef®cient Sharing of Persistent Objects in Thor B.Liskov, A.Adya, M.Castro, M.Day , S.Ghemawat , R.Gruber , U.Maheshwari, A.C.Myers, L.Shrira Laboratory for Computer Science, Massachusetts Institute of Technology, 545 Technology Square, Cambridge, MA 02139 liskov,adya,castro, , , ,umesh,andru,liuba @lcs.mit.edu Abstract Thor is an object-oriented database system designed for use in a are likely to be accessible despite failures. Thor supports heterogeneous distributed environment. It provides highly-reliable heterogeneity at the levels of the machine, network, operating and highly-available persistent storage for objects, and supports system, and especially the programming language. Programs safe sharing of these objects by applications written in different written in different programming languages can easily share programming languages. objects between different applications, or components of Safe heterogeneous sharing of long-lived objects requires the same application. Furthermore, even when client code encapsulation: the system must guarantee that applications interact is written in unsafe languages (such as C or C++), Thor with objects only by invoking methods. Although safety concerns are important, most object-oriented databases forgo safety to avoid guarantees the integrity of the persistent store. paying the associated performance costs. This paper describes the interface and implementation of This paper gives an overview of Thor's design and implementa- Thor and focuses on a novel aspect of Thor in each area. -
Arxiv:2106.11534V1 [Cs.DL] 22 Jun 2021 2 Nanjing University of Science and Technology, Nanjing, China 3 University of Southampton, Southampton, U.K
Noname manuscript No. (will be inserted by the editor) Turing Award elites revisited: patterns of productivity, collaboration, authorship and impact Yinyu Jin1 · Sha Yuan1∗ · Zhou Shao2, 4 · Wendy Hall3 · Jie Tang4 Received: date / Accepted: date Abstract The Turing Award is recognized as the most influential and presti- gious award in the field of computer science(CS). With the rise of the science of science (SciSci), a large amount of bibliographic data has been analyzed in an attempt to understand the hidden mechanism of scientific evolution. These include the analysis of the Nobel Prize, including physics, chemistry, medicine, etc. In this article, we extract and analyze the data of 72 Turing Award lau- reates from the complete bibliographic data, fill the gap in the lack of Turing Award analysis, and discover the development characteristics of computer sci- ence as an independent discipline. First, we show most Turing Award laureates have long-term and high-quality educational backgrounds, and more than 61% of them have a degree in mathematics, which indicates that mathematics has played a significant role in the development of computer science. Secondly, the data shows that not all scholars have high productivity and high h-index; that is, the number of publications and h-index is not the leading indicator for evaluating the Turing Award. Third, the average age of awardees has increased from 40 to around 70 in recent years. This may be because new breakthroughs take longer, and some new technologies need time to prove their influence. Besides, we have also found that in the past ten years, international collabo- ration has experienced explosive growth, showing a new paradigm in the form of collaboration. -
Distributed Computing Column 36 Distributed Computing: 2009 Edition
Distributed Computing Column 36 Distributed Computing: 2009 Edition Idit Keidar Dept. of Electrical Engineering, Technion Haifa, 32000, Israel [email protected] It’s now the season for colorful reviews of 2009. While you can read elsewhere about the year in sports, top-40 pop hit charts, people of the year, and so on, I throw my share into the mix with a (biased) review of this year’s major distributed computing events. Awards First, let’s look at awards. This year we learned that two women were recognized with ACM and IEEE prestigious awards for their achievements in, (among other things), distributed computing. Barbara Liskov won the 2008 ACM Turing Award for a range of contributions to practical and theoretical foundations of programming language and system design, some of which are in distributed computing. The award citation mentions her impact on distributed programming, decentralized information flow, replicated storage, and modular upgrading of distributed systems. I include below a short reflection, by Rodrigo Rodrigues, on Barbara Liskov’s Turing Award and legacy. Looking ahead to 2010, it was recently announced that Nancy Lynch is the recipient of the prestigious 2010 IEEE Emanuel R. Piore Award1 for her contributions to foundations of distributed and concurrent computing. The award is given for outstanding contributions in the field of information processing in relation to computer science. Leslie Lamport has won the same award in 2004 for his seminal contributions to the theory and practice of concurrent programming and fault-tolerant computing. The 2009 Edsger W. Dijkstra Prize in Distributed Computing was awarded to Joseph Halpern and Yoram Moses for “Knowledge and Common Knowledge in a Distributed Environment”. -
2011 Edsger W. Dijkstra Prize in Distributed Computing
2011 Edsger W. Dijkstra Prize in Distributed Computing We are proud to announce that the 2011 Edsger W. Dijkstra Prize in Distributed Computing is awarded to Hagit Attiya, Amotz Bar-Noy, and Danny Dolev, for their paper Sharing Memory Robustly in Message-Passing Systems which appeared in the Journal of the ACM (JACM) 42(1):124-142 (1995). This fundamental paper presents the first asynchronous fault-tolerant simulation of shared memory in a message passing system. In 1985 Fischer, Lynch, and Paterson (FLP) proved that consensus is not attainable in asynchronous message passing systems with failures. In the following years, Loui and Abu-Amara, and Herlihy proved that solving consensus in an asynchronous shared memory system is harder than implementing a read/write register. However, it was not known whether read/ write registers are implementable in asynchronous message-passing systems with failures. Hagit Attiya, Amotz Bar-Noy, and Danny Dolev’s paper (ABD) answered this fundamental question afrmatively. The ABD paper makes an important foundational contribution by allowing one to “view the shared-memory model as a higher-level language for designing algorithms in asynchronous distributed systems.” In a manner similar to that in which Turing Machines were proved equivalent to Random Access Memory, ABD proved that the implications of FLP are not idiosyncratic to a particular model. Impossibility results and lower bounds can be proved in the higher-level shared memory model, and then automatically translated to message passing. Besides FLP, this includes results such as the impossibility of k-set agreement in asynchronous systems as well as various weakest failure detector constructions.