پاورپوینت کامل Chapter 20: Database System Architectures 41 اسلاید در PowerPoint

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پاورپوینت کامل Chapter 20: Database System Architectures 41 اسلاید در PowerPoint

اسلاید ۴: A Centralized Computer System

اسلاید ۵: Client-Server SystemsServer systems satisfy requests generated at m client systems, whose general structure is shown below:

اسلاید ۶: Client-Server Systems (Cont.)Database functionality can be divided into:Back-end: manages access structures, query evaluation and optimization, concurrency control and recovery.Front-end: consists of tools such as forms, report-writers, and graphical user interface facilities.The interface between the front-end and the back-end is through SQL or through an application program interface.

اسلاید ۷: Client-Server Systems (Cont.)Advantages of replacing mainframes with networks of workstations or personal computers connected to back-end server machines:better functionality for the costflexibility in locating resources and expanding facilitiesbetter user interfaceseasier maintenance

اسلاید ۸: Server System ArchitectureServer systems can be broadly categorized into two kinds:transaction servers which are widely used in relational database systems, anddata servers, used in object-oriented database systems

اسلاید ۹: Transaction ServersAlso called query server systems or SQL server systemsClients send requests to the serverTransactions are executed at the serverResults are shipped back to the client.Requests are specified in SQL, and communicated to the server through a remote procedure call (RPC) mechanism.Transactional RPC allows many RPC calls to form a transaction.Open Database Connectivity (ODBC) is a C language application program interface standard from Microsoft for connecting to a server, sending SQL requests, and receiving results.JDBC standard is similar to ODBC, for Java

اسلاید ۱۰: Transaction Server Process StructureA typical transaction server consists of multiple processes accessing data in shared memory.Server processesThese receive user queries (transactions), execute them and send results backProcesses may be multithreaded, allowing a single process to execute several user queries concurrentlyTypically multiple multithreaded server processesLock manager processMore on this laterDatabase writer processOutput modified buffer blocks to disks continually

اسلاید ۱۱: Transaction Server Processes (Cont.)Log writer processServer processes simply add log records to log record bufferLog writer process outputs log records to stable storage. Checkpoint processPerforms periodic checkpointsProcess monitor processMonitors other processes, and takes recovery actions if any of the other processes failE.g. aborting any transactions being executed by a server process and restarting it

اسلاید ۱۲: Transaction System Processes (Cont.)

اسلاید ۱۳: Transaction System Processes (Cont.)Shared memory contains shared data Buffer poolLock tableLog bufferCached query plans (reused if same query submitted again)All database processes can access shared memoryTo ensure that no two processes are accessing the same data structure at the same time, databases systems implement mutual exclusion using eitherOperating system semaphoresAtomic instructions such as test-and-setTo avoid overhead of interprocess communication for lock request/grant, each database process operates directly on the lock table instead of sending requests to lock manager processLock manager process still used for deadlock detection

اسلاید ۱۴: Data ServersUsed in high-speed LANs, in cases whereThe clients are comparable in processing power to the serverThe tasks to be executed are compute intensive.Data are shipped to clients where processing is performed, and then shipped results back to the server.This architecture requires full back-end functionality at the clients.Used in many object-oriented database systems Issues:Page-Shipping versus Item-ShippingLockingData CachingLock Caching

اسلاید ۱۵: Data Servers (Cont.)Page-shipping versus item-shippingSmaller unit of shipping more messagesWorth prefetching related items along with requested itemPage shipping can be thought of as a form of prefetchingLockingOverhead of requesting and getting locks from server is high due to message delaysCan grant locks on requested and prefetched items; with page shipping, transaction is granted lock on whole page.Locks on a prefetched item can be P{called back} by the server, and returned by client transaction if the prefetched item has not been used. Locks on the page can be deescalated to locks on items in the page when there are lock conflicts. Locks on unused items can then be returned to server.