Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition in Software

Encoder Code 128 Code Set C in Software Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition

The McGraw Hill Companies, 2001

Summary

The various concurrency-control schemes used in a centralized system can be modi ed for use in a distributed environment In the case of locking protocols, the only change that needs to be incorporated is in the way that the lock manager is implemented There are a variety of different approaches here One or more central coordinators may be used If, instead, a distributed lock-manager approach is taken, replicated data must be treated specially Protocols for handling replicated data include the primary-copy, majority, biased, and quorum-consensus protocols These have different tradeoffs in terms of cost and ability to work in the presence of failures In the case of timestamping and validation schemes, the only needed change is to develop a mechanism for generating unique global timestamps Many database systems support lazy replication, where updates are propagated to replicas outside the scope of the transaction that performed the update Such facilities must be used with great care, since they may result in nonserializable executions Deadlock detection in a distributed lock-manager environment requires cooperation between multiple sites, since there may be global deadlocks even when there are no local deadlocks To provide high availability, a distributed database must detect failures, recon gure itself so that computation may continue, and recover when a processor or a link is repaired The task is greatly complicated by the fact that it is hard to distinguish between network partitions or site failures The majority protocol can be extended by using version numbers to permit transaction processing to proceed even in the presence of failures While the protocol has a signi cant overhead, it works regardless of the type of failure Less-expensive protocols are available to deal with site failures, but they assume network partitioning does not occur Some of the distributed algorithms require the use of a coordinator To provide high availability, the system must maintain a backup copy that is ready to assume responsibility if the coordinator fails Another approach is to choose the new coordinator after the coordinator has failed The algorithms that determine which site should act as a coordinator are called election algorithms Queries on a distributed database may need to access multiple sites Several optimization techniques are available to choose which sites need to be accessed Based on fragmentation and replication, the techniques can use semijoin techniques to reduce data transfer Heterogeneous distributed databases allow sites to have their own schemas and database system code A multidatabase system provides an environment in which new database applications can access data from a variety of preexisting databases located in various heterogeneous hardware and software environments The local database systems may employ different logical mod-

The McGraw Hill Companies, 2001

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els and data-de nition and data-manipulation languages, and may differ in their concurrency-control and transaction-management mechanisms A multidatabase system creates the illusion of logical database integration, without requiring physical database integration Directory systems can be viewed as a specialized form of database, where information is organized in a hierarchical fashion similar to the way les are organized in a le system Directories are accessed by standardized directory access protocols such as LDAP Directories can be distributed across multiple sites to provide autonomy to individual sites Directories can contain referrals to other directories, which help build an integrated view whereby a query is sent to a single directory, and it is transparently executed at all relevant directories

Homogeneous distributed database Heterogeneous distributed database Data replication Primary copy Data fragmentation Horizontal fragmentation Vertical fragmentation Data transparency Fragmentation transparency Replication transparency Location transparency Name server Aliases Distributed transactions Local transactions Global transactions Transaction manager Transaction coordinator System failure modes Network partition Commit protocols Two-phase commit protocol (2PC) Ready state In-doubt transactions Blocking problem Three-phase commit protocol

(3PC)

Persistent messaging Concurrency control Single lock-manager Distributed lock-manager Protocols for replicas Primary copy Majority protocol Biased protocol Quorum consensus protocol Timestamping Master slave replication Multimaster (update-anywhere) replication Transaction-consistent snapshot Lazy propagation Deadlock handling Local wait-for graph Global wait-for graph False cycles Availability Robustness