Consensus (computer Science)
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Consensus (computer Science)
A fundamental problem in distributed computing and multi-agent systems is to achieve overall system reliability in the presence of a number of faulty processes. This often requires coordinating processes to reach consensus, or agree on some data value that is needed during computation. Example applications of consensus include agreeing on what transactions to commit to a database in which order, state machine replication, and atomic broadcasts. Real-world applications often requiring consensus include cloud computing, clock synchronization, PageRank, opinion formation, smart power grids, state estimation, control of UAVs (and multiple robots/agents in general), load balancing, blockchain, and others. Problem description The consensus problem requires agreement among a number of processes (or agents) for a single data value. Some of the processes (agents) may fail or be unreliable in other ways, so consensus protocols must be fault tolerant or resilient. The processes must some ...
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Atomic Broadcast
In fault-tolerant distributed computing, an atomic broadcast or total order broadcast is a broadcast where all correct processes in a system of multiple processes receive the same set of messages in the same order; that is, the same sequence of messages. The broadcast is termed " atomic" because it either eventually completes correctly at all participants, or all participants abort without side effects. Atomic broadcasts are an important distributed computing primitive. Properties The following properties are usually required from an atomic broadcast protocol: # Validity: if a correct participant broadcasts a message, then all correct participants will eventually receive it. # Uniform Agreement: if one correct participant receives a message, then all correct participants will eventually receive that message. # Uniform Integrity: a message is received by each participant at most once, and only if it was previously broadcast. # Uniform Total Order: the messages are totally order ...
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Distributed Computing
A distributed system is a system whose components are located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. Distributed computing is a field of computer science that studies distributed systems. The components of a distributed system interact with one another in order to achieve a common goal. Three significant challenges of distributed systems are: maintaining concurrency of components, overcoming the lack of a global clock, and managing the independent failure of components. When a component of one system fails, the entire system does not fail. Examples of distributed systems vary from SOA-based systems to massively multiplayer online games to peer-to-peer applications. A computer program that runs within a distributed system is called a distributed program, and ''distributed programming'' is the process of writing such programs. There are many different types of implementations for ...
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Raft (algorithm)
Raft is a consensus algorithm designed as an alternative to the Paxos family of algorithms. It was meant to be more understandable than Paxos by means of separation of logic, but it is also formally proven safe and offers some additional features. Raft offers a generic way to distribute a state machine across a cluster of computing systems, ensuring that each node in the cluster agrees upon the same series of state transitions. It has a number of open-source reference implementations, with full-specification implementations in Go, C++, Java, and Scala. It is named after Reliable, Replicated, Redundant, And Fault-Tolerant. Raft is not a Byzantine fault tolerant algorithm: the nodes trust the elected leader. Basics Raft achieves consensus via an elected leader. A server in a raft cluster is either a ''leader'' or a ''follower'', and can be a ''candidate'' in the precise case of an election (leader unavailable). The leader is responsible for log replication to the followers. ...
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Proof Of Space
Proof of space (PoS) is a type of consensus algorithm achieved by demonstrating one's legitimate interest in a service (such as sending an email) by allocating a non-trivial amount of memory or disk space to solve a challenge presented by the service provider. The concept was formulated in 2013 by Dziembowski ''et al.'' and (with a different formulation) by Ateniese ''et al.''. Proofs of space are very similar to proofs of work (PoW), except that instead of computation, storage is used to earn cryptocurrency. Proof-of-space is different from memory-hard functions in that the bottleneck is not in the number of memory access events, but in the amount of memory required. After the release of Bitcoin, alternatives to its PoW mining mechanism were researched, and PoS was studied in the context of cryptocurrencies. Proofs of space are seen as a fairer and greener alternative by blockchain enthusiasts due to the general-purpose nature of storage and the lower energy cost required by s ...
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Proof Of Stake
Proof-of-stake (PoS) protocols are a class of consensus mechanisms for blockchains that work by selecting validators in proportion to their quantity of holdings in the associated cryptocurrency. This is done to avoid the computational cost of proof-of-work schemes. The first functioning use of PoS for cryptocurrency was Peercoin in 2012. Description For a blockchain transaction to be recognized, it must be appended to the blockchain. In the proof of stake blockchain the appending entities are named ''minters'' or (in the proof of work blockchains this task is carried out by the miners); in most protocols, the validators receive a reward for doing so. For the blockchain to remain secure, it must have a mechanism to prevent a malicious user or group from taking over a majority of validation. PoS accomplishes this by requiring that validators have some quantity of blockchain tokens, requiring potential attackers to acquire a large fraction of the tokens on the blockchain to m ...
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Cryptographic Hash Function
A cryptographic hash function (CHF) is a hash algorithm (a map of an arbitrary binary string to a binary string with fixed size of n bits) that has special properties desirable for cryptography: * the probability of a particular n-bit output result (hash value) for a random input string ("message") is 2^ (like for any good hash), so the hash value can be used as a representative of the message; * finding an input string that matches a given hash value (a ''pre-image'') is unfeasible, unless the value is selected from a known pre-calculated dictionary (" rainbow table"). The ''resistance'' to such search is quantified as security strength, a cryptographic hash with n bits of hash value is expected to have a ''preimage resistance'' strength of n bits. A ''second preimage'' resistance strength, with the same expectations, refers to a similar problem of finding a second message that matches the given hash value when one message is already known; * finding any pair of different messa ...
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Proof Of Work
Proof of work (PoW) is a form of cryptographic proof in which one party (the ''prover'') proves to others (the ''verifiers'') that a certain amount of a specific computational effort has been expended. Verifiers can subsequently confirm this expenditure with minimal effort on their part. The concept was invented by Moni Naor and Cynthia Dwork in 1993 as a way to deter denial-of-service attacks and other service abuses such as spam on a network by requiring some work from a service requester, usually meaning processing time by a computer. The term "proof of work" was first coined and formalized in a 1999 paper by Markus Jakobsson and Ari Juels. Proof of work was later popularized by Bitcoin as a foundation for consensus in a permissionless decentralized network, in which miners compete to append blocks and mint new currency, each miner experiencing a success probability proportional to the computational effort expended. PoW and PoS ( proof of stake) remain the two best known ...
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Bitcoin
Bitcoin (abbreviation: BTC; sign: ₿) is a decentralized digital currency that can be transferred on the peer-to-peer bitcoin network. Bitcoin transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. The cryptocurrency was invented in 2008 by an unknown person or group of people using the name Satoshi Nakamoto. The currency began use in 2009, when its implementation was released as open-source software. The word "''bitcoin''" was defined in a white paper published on October 31, 2008. It is a compound of the words ''bit'' and ''coin''. The legality of bitcoin varies by region. Nine countries have fully banned bitcoin use, while a further fifteen have implicitly banned it. A few governments have used bitcoin in some capacity. El Salvador has adopted Bitcoin as legal tender, although use by merchants remains low. Ukraine has accepted cryptocurrency donations to fund the resistance to the 2022 Rus ...
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Barriers To Entry
In theories of competition in economics, a barrier to entry, or an economic barrier to entry, is a fixed cost that must be incurred by a new entrant, regardless of production or sales activities, into a market that incumbents do not have or have not had to incur. Because barriers to entry protect incumbent firms and restrict competition in a market, they can contribute to distortionary prices and are therefore most important when discussing antitrust policy. Barriers to entry often cause or aid the existence of monopolies and oligopolies, or give companies market power. Barriers of entry also have an importance in industries. First of all it is important to identify that some exist naturally, such as brand loyalty. Governments can also create barriers to entry to meet consumer protection laws, protecting the public. In other cases it can also be due to inherent scarcity of public resources needed to enter a market. Definitions Various conflicting definitions of "barrier to en ...
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Sybil Attack
Sibyls were oracular women believed to possess prophetic powers in ancient Greece. Sybil or Sibyl may also refer to: Films * ''Sybil'' (1921 film) * ''Sybil'' (1976 film), a film starring Sally Field * ''Sybil'' (2007 film), a remake of the 1976 film starring Tammy Blanchard and Jessica Lange * ''Sibyl'' (2019 film), a French comedy-drama film Literature * ''Sybil'' (novel) or ''The Two Nations'', an 1845 novel by Benjamin Disraeli * ''Sybil'' (Schreiber book), a book by Flora Rheta Schreiber about Shirley Ardell Mason, an alleged sufferer from multiple personality disorder * ''Sybil'', a 1952 novel by Louis Auchincloss * ''The Sybil'' or ''Sibyllan'', a 1956 Swedish novel by Pär Lagerkvist * ''The Sybil'', an American dress reform periodical founded by Lydia Sayer Hasbrouck Music * ''Sybil'' (album), a 1989 album by American singer Sybil * ''Sybil'' (operetta) adaptation of ''Szibill'' by Victor Jacobi *Sibyl Vane (band), indie rock band from Pau, France created in 200 ...
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Randomized Algorithm
A randomized algorithm is an algorithm that employs a degree of randomness as part of its logic or procedure. The algorithm typically uses uniformly random bits as an auxiliary input to guide its behavior, in the hope of achieving good performance in the "average case" over all possible choices of random determined by the random bits; thus either the running time, or the output (or both) are random variables. One has to distinguish between algorithms that use the random input so that they always terminate with the correct answer, but where the expected running time is finite ( Las Vegas algorithms, for example Quicksort), and algorithms which have a chance of producing an incorrect result ( Monte Carlo algorithms, for example the Monte Carlo algorithm for the MFAS problem) or fail to produce a result either by signaling a failure or failing to terminate. In some cases, probabilistic algorithms are the only practical means of solving a problem. In common practice, randomized a ...
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Denial-of-service Attack
In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to a network. Denial of service is typically accomplished by flooding the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled. In a distributed denial-of-service attack (DDoS attack), the incoming traffic flooding the victim originates from many different sources. More sophisticated strategies are required to mitigate this type of attack, as simply attempting to block a single source is insufficient because there are multiple sources. A DoS or DDoS attack is analogous to a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter, thus disrupting trade. Criminal perpetrators of DoS attacks ...
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