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Quorum (Distributed Systems)
A quorum is the minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation in a distributed system. A quorum-based technique is implemented to enforce consistent operation in a distributed system. Quorum-based techniques in distributed database systems Quorum-based voting can be used as a replica control method, as well as a commit method to ensure transaction atomicity in the presence of network partitioning. Quorum-based voting in commit protocols In a distributed database system, a transaction could execute its operations at multiple sites. Since atomicity requires every distributed transaction to be atomic, the transaction must have the same fate ( commit or abort) at every site. In case of network partitioning, sites are partitioned and the partitions may not be able to communicate with each other. This is where a quorum-based technique comes in. The fundamental idea is that a transaction is executed if the majo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Distributed System
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 t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Replication (computer Science)
Replication in computing involves sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility. Terminology Replication in computing can refer to: * ''Data replication'', where the same data is stored on multiple storage devices * ''Computation replication'', where the same computing task is executed many times. Computational tasks may be: ** ''Replicated in space'', where tasks are executed on separate devices ** ''Replicated in time'', where tasks are executed repeatedly on a single device Replication in space or in time is often linked to scheduling algorithms. Access to a replicated entity is typically uniform with access to a single non-replicated entity. The replication itself should be transparent to an external user. In a failure scenario, a failover of replicas should be hidden as much as possible with respect to quality of service. Computer scientis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Transaction
A database transaction symbolizes a unit of work, performed within a database management system (or similar system) against a database, that is treated in a coherent and reliable way independent of other transactions. A transaction generally represents any change in a database. Transactions in a database environment have two main purposes: # To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure. For example: when execution prematurely and unexpectedly stops (completely or partially) in which case many operations upon a database remain uncompleted, with unclear status. # To provide isolation between programs accessing a database concurrently. If this isolation is not provided, the programs' outcomes are possibly erroneous. In a database management system, a transaction is a single unit of logic or work, sometimes made up of multiple operations. Any logical calculation done in a consistent mode in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Atomicity (database Systems)
In database systems, atomicity (; from grc, ἄτομος, átomos, undividable) is one of the ACID (''Atomicity, Consistency, Isolation, Durability'') transaction properties. An atomic transaction is an ''indivisible'' and ''irreducible'' series of database operations such that either ''all'' occurs, or ''nothing'' occurs. A guarantee of atomicity prevents updates to the database occurring only partially, which can cause greater problems than rejecting the whole series outright. As a consequence, the transaction cannot be observed to be in progress by another database client. At one moment in time, it has not yet happened, and at the next it has already occurred in whole (or nothing happened if the transaction was cancelled in progress). An example of an atomic transaction is a monetary transfer from bank account A to account B. It consists of two operations, withdrawing the money from account A and saving it to account B. Performing these operations in an atomic transaction ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Network Partitioning
A network partition is a division of a computer network into relatively independent subnets, either by design, to optimize them separately, or due to the failure of network devices. Distributed software must be designed to be partition-tolerant, that is, even after the network is partitioned, it still works correctly. For example, in a network with multiple subnets where nodes A and B are located in one subnet and nodes C and D are in another, a partition occurs if the network switch device between the two subnets fails. In that case nodes A and B can no longer communicate with nodes C and D, but all nodes A-D work the same as before. As a CAP trade-off The CAP Theorem is based on three trade-offs: consistency, availability, and partition tolerance. Partition tolerance, in this context, means the ability of a data processing system to continue processing data even if a network partition causes communication errors between subsystems. External links Partition of the Large Netw ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Commit (data Management)
In computer science and data management, a commit is the making of a set of tentative changes permanent, marking the end of a transaction and providing ''Durability'' to ACID transactions. A ''commit'' is an act of committing. The record of commits is called the commit log. Data management A COMMIT statement in SQL ends a transaction within a relational database management system (RDBMS) and makes all changes visible to other users. The general format is to issue a BEGIN WORK (or BEGIN TRANSACTION, depending on the database vendor) statement, one or more SQL statements, and then the COMMIT statement. Alternatively, a ROLLBACK statement can be issued, which undoes all the work performed since BEGIN WORK was issued. A COMMIT statement will also release any existing savepoints that may be in use. In terms of transactions, the opposite of commit is to discard the tentative changes of a transaction, a rollback. See also * Commit (version control) * Atomic commit * Two-phase co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rollback (data Management)
In database technologies, a rollback is an operation which returns the database to some previous state. Rollbacks are important for database integrity, because they mean that the database can be restored to a clean copy even after erroneous operations are performed. They are crucial for recovering from database server crashes; by rolling back any transaction which was active at the time of the crash, the database is restored to a consistent state. The rollback feature is usually implemented with a transaction log, but can also be implemented via multiversion concurrency control. Cascading rollback A cascading rollback occurs in database systems when a transaction (T1) causes a failure and a rollback must be performed. Other transactions dependent on T1's actions must also be rollbacked due to T1's failure, thus causing a cascading effect. That is, one transaction's failure causes many to fail. Practical database recovery techniques guarantee cascadeless rollback, therefore a ca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Serializability
In concurrency control of databases, Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987)''Concurrency Control and Recovery in Database Systems''(free PDF download), Addison Wesley Publishing Company, Gerhard Weikum, Gottfried Vossen (2001)''Transactional Information Systems'' Elsevier, transaction processing (transaction management), and various transactional applications (e.g., transactional memoryMaurice Herlihy and J. Eliot B. Moss. ''Transactional memory: architectural support for lock-free data structures.'' Proceedings of the 20th annual international symposium on Computer architecture (ISCA '93). Volume 21, Issue 2, May 1993. and software transactional memory), both centralized and distributed, a transaction schedule is serializable if its outcome (e.g., the resulting database state) is equal to the outcome of its transactions executed serially, i.e. without overlapping in time. Transactions are normally executed concurrently (they overlap), since this is the most ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
CAP Theorem
In theoretical computer science, the CAP theorem, also named Brewer's theorem after computer scientist Eric Brewer, states that any distributed data store can provide only two of the following three guarantees:Seth Gilbert and Nancy Lynch"Brewer's conjecture and the feasibility of consistent, available, partition-tolerant web services" ''ACM SIGACT News'', Volume 33 Issue 2 (2002), pg. 51–59. . ; Consistency: Every read receives the most recent write or an error. ; Availability: Every request receives a (non-error) response, without the guarantee that it contains the most recent write. ; Partition tolerance: The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes. When a network partition failure happens, it must be decided whether to do one of the following: * cancel the operation and thus decrease the availability but ensure consistency * proceed with the operation and thus provide availability but risk i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Management Systems
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spans formal techniques and practical considerations, including data modeling, efficient data representation and storage, query languages, security and privacy of sensitive data, and distributed computing issues, including supporting concurrent access and fault tolerance. A database management system (DBMS) is the software that interacts with end users, applications, and the database itself to capture and analyze the data. The DBMS software additionally encompasses the core facilities provided to administer the database. The sum total of the database, the DBMS and the associated applications can be referred to as a database system. Often the term "database" is also used loosely to refer to any of the DBMS, the database system or an application ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
