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Paxos (computer science) - Wikipedia
Paxos is a family of protocols for solving consensus in a network of unreliable or fallible processors. Consensus is the process of agreeing on one result among a group of participants. This problem becomes difficult when the participants or their communications may experience failures. Consensus protocols are the basis for the state machine replication approach to distributed computing, as suggested by Leslie Lamport [2] and surveyed by Fred Schneider.
Ad-hoc techniques may leave important cases of failures unresolved. The principled approach proposed by Lamport et al. ensures all cases are handled safely. The Paxos protocol was first submitted in and named after a fictional legislative consensus system used on the Paxos island in Greece, where Lamport wrote that the parliament had to function "even though legislators continually wandered in and out of the parliamentary Chamber".
The Paxos family of protocols includes a spectrum of trade-offs paper presentation in computer science the number of processors, number of message paper presentation in computer science before learning the agreed value, paper presentation in computer science, the activity level of individual participants, number of messages sent, and types of failures.
Although no deterministic fault-tolerant consensus protocol can guarantee progress in an asynchronous network a result proved in a paper by FischerLynch and Paterson [6]Paxos guarantees safety consistencyand the conditions that could prevent it from making progress are difficult to provoke.
Paxos is usually used where durability is required for example, to replicate a file or a databasein which the amount of durable state could be large. The protocol attempts to make progress even during periods when some bounded number of paper presentation in computer science are unresponsive. There is also a mechanism to drop a permanently failed replica or to add a new replica. The topic predates the protocol. InLynchDwork and Stockmeyer had demonstrated [7] the solvability of consensus in a broad family of "partially synchronous" systems.
Paxos has strong similarities to a protocol used for agreement in "viewstamped replication", first published by Oki and Liskov inpaper presentation in computer science, in the context of distributed transactions. Reconfigurable state machines have strong ties to prior work on reliable group multicast protocols that support dynamic group membership, paper presentation in computer science, for example Birman 's work in and on the virtually synchronous gbcast [9] protocol, paper presentation in computer science.
However, gbcast is unusual in supporting durability and addressing partitioning failures. Most reliable multicast protocols lack these properties, which are required for implementations of the state machine replication model. This point is elaborated in a paper by LamportMalkhi and Zhou. Paxos protocols are members of a theoretical class of solutions to a problem formalized as uniform agreement with crash failures.
Lower bounds for this problem have been proved by Keidar and Shraer. This protocol matches the Keidar and Shraer optimality bounds, and maps efficiently to modern remote DMA RDMA datacenter hardware but uses TCP if RDMA is not available. In order to simplify the presentation of Paxos, the following assumptions and definitions are made explicit. Techniques to broaden the applicability are known in the literature, and are not covered in this article.
However, using reconfiguration, a protocol may be employed which survives any number of total failures as long as no more than F fail simultaneously. For Paxos protocols, these reconfigurations can be handled as separate configurations. Paxos describes the actions of the processors by their roles in the protocol: client, acceptor, proposer, learner, and leader. In typical implementations, a single processor may play one or more roles at the same time.
Quorums express the safety or consistency properties of Paxos by ensuring at least some surviving processor retains knowledge of the results. Quorums are defined as subsets of the set of Acceptors such that any two subsets that is, any two Quorums share at least one member. Typically, a Quorum is any majority of participating Acceptors. For example, given the set of Acceptors {A,B,C,D}, a majority Quorum would be any three Acceptors: {A,B,C}, paper presentation in computer science, {A,C,D}, {A,B,D}, {B,C,D}.
More generally, arbitrary positive weights can be assigned to Acceptors; in that case, a Quorum can be defined as any subset of Acceptors with the summary weight greater than half of the total weight of all Acceptors. Each attempt to define an agreed value v is performed with proposals which may or may not be accepted by Acceptors. Each proposal is uniquely numbered for a given Proposer. So, e. The value corresponding to a numbered proposal can be computed as part of running the Paxos protocol, but need not be.
In order to guarantee safety also called "consistency"Paxos defines three properties and ensures the first two are always held, regardless of the pattern of failures:.
Note that Paxos is not guaranteed to terminate, and thus does not have the liveness property. This is supported by the Fischer Lynch Paterson impossibility result FLP [6] which states that a consistency protocol can only have two of safetylivenessand fault tolerance, paper presentation in computer science.
As Paxos's point is to ensure fault tolerance and it guarantees safety, it cannot also guarantee liveness. In most deployments of Paxos, each participating process acts in three roles; Proposer, Acceptor and Learner. In Paxos, clients send commands to a leader. During normal operation, the leader receives a client's command, assigns it a new command number i, and then begins the ith instance of the consensus algorithm by sending messages to a paper presentation in computer science of acceptor processes.
By merging roles, the protocol "collapses" into an efficient client-master-replica style deployment, typical of the database community, paper presentation in computer science. A typical implementation's message flow is covered in the section Multi-Paxos. This protocol is the most basic of the Paxos family. Each "instance" or "execution" of the basic Paxos protocol decides on a single output value. The protocol proceeds over several rounds.
A successful round has 2 phases: phase 1 which is divided into parts a and b and phase 2 which is divided into parts a and b. See below the description of the phases. Remember that we assume an asynchronous model, so e.
a processor may be in one phase while another processor may be in another. This Accept message should be interpreted as a "request", as in "Accept this proposal, please! Note that an Acceptor can accept multiple proposals. This can happen when another Proposer, unaware of the new value being decided, starts a new round with a higher identification number n.
In that case, the Acceptor can promise and later accept the new proposed value even though it has accepted another one earlier. These proposals may even have different values in the presence of certain failures [ example needed ].
However, the Paxos protocol will guarantee that the Acceptors will ultimately agree on a single value. Notice that a Proposer in Paxos could propose "I am the leader," or, for example, "Proposer X is the leader" [20].
Because of the agreement and validity guarantees of Paxos, if accepted by a Quorum, then the Proposer is now known to be the leader to all other nodes. Paper presentation in computer science satisfies the needs of leader election [21] because there is a single node believing it is the leader and a single node known to be the leader at all times. Some cases show how the Basic Paxos protocol copes with the failure of certain redundant components of the distributed system.
Note that the values returned in the Promise message are "null" the first time a proposal is made since no Acceptor has accepted a value before in this round. In the diagram below, there is 1 Client, 1 Proposer, 3 Acceptors i.
the Quorum size is 3 and 2 Learners represented by the 2 vertical lines. This diagram represents the case of a first round, which is successful i. no process in the network fails. The simplest error cases are the failure of an Acceptor when a Quorum of Acceptors remains alive and failure of a redundant Learner. In these cases, the protocol requires no "recovery" i.
In the following diagram, one of the Acceptors in the Quorum fails, so the Quorum size becomes 2. In this case, the Basic Paxos protocol still succeeds. In the following case, one of the redundant Learners fails, paper presentation in computer science, but the Basic Paxos protocol still succeeds. In this case, a Proposer fails after proposing a value, but before the agreement is reached. Specifically, it fails in the middle of the Accept message, so only one Acceptor of the Quorum receives the value.
Meanwhile, a new Leader a Proposer is elected but this is not shown in detail. Note that there are 2 rounds in this case rounds proceed vertically, from the top to the bottom. The most complex case is when multiple Proposers believe themselves to be Leaders. For instance, the current leader may fail and later recover, but the other Proposers have already re-selected a new leader.
The recovered leader has not learned this yet and attempts to begin one round in conflict with the current leader. In the diagram below, 4 unsuccessful rounds are shown, but there could be more as suggested at the bottom of the diagram. A typical deployment of Paxos requires a continuous stream of agreed values acting as commands to a distributed state machine.
If each command is the result of a single instance of the Basic Paxos protocol, a paper presentation in computer science amount of overhead would result. If the leader is relatively stable, phase 1 becomes unnecessary. Thus, it is possible to skip phase 1 for future instances of the protocol with the same leader.
To achieve this, the round number I is included along with each value which is incremented in each round by the same Leader. Multi-Paxos reduces the failure-free message delay proposal to learning from 4 delays to 2 delays. In the following diagram, only paper presentation in computer science instance or "execution" of the basic Paxos protocol, with an initial Leader a Proposeris shown.
Note that a Multi-Paxos consists of several instances of the basic Paxos protocol. Note that the Leader should be stable, i. it should not crash or change. A common deployment of the Multi-Paxos consists in collapsing the role of the Proposers, paper presentation in computer science, Acceptors and Learners to "Servers". So, in the end, there are only "Clients" and "Servers". The following diagram represents the first "instance" of a basic Paxos protocol, when the roles of the Proposer, Acceptor and Learner are collapsed to a single role, called the "Server".
In the subsequent instances of the basic Paxos protocol, paper presentation in computer science, with the same leader as in the previous instances of the basic Paxos protocol, paper presentation in computer science, the phase 1 can be skipped. A number of optimisations can be performed to reduce the number of exchanged messages, to improve the performance of the protocol, etc.
A few of these optimisations are reported below. This reduction in processor requirements comes paper presentation in computer science the expense of liveness; if too many main processors fail in a short time, the system must halt until the auxiliary processors can reconfigure the system.
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Paxos is a family of protocols for solving consensus in a network of unreliable or fallible processors. Consensus is the process of agreeing on one result among a group of participants. This problem becomes difficult when the participants or their communications may experience failures This is a list of computer science Presentation Topics for students and professionals updated in These topics can be used for PowerPoint Paper presentation, Poster Presentation, classroom ppt presentation, seminars, seminars, webinars and conferences. etc We would like to show you a description here but the site won’t allow blogger.com more
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