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31 janvier 2011 1 31 /01 /janvier /2011 04:29

database is a system intended to organize, store, and retrieve large amounts of data easily.It consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents  (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation and maintenance, and search and other access  (Dell Studio XPS 1640 Battery)  .

Architecture

Database architecture consists of three levels, externalconceptual and internal. Clearly separating the three levels was a major feature of the relational database model that dominates 21st century databases        (Dell Vostro 1710 Battery)  .

The external level defines how users understand the organization of the data. A single database can have any number of views at the external level. The internal level defines how the data is physically stored and processed by the computing system. Internal architecture is concerned with cost, performance, scalability and other operational matters  Dell KM958 battery .

The conceptual is a level of indirection between internal and external. It provides a common view of the database that is uncomplicated by details of how the data is stored or managed, and that can unify the various external views into a coherent whole  Dell Studio 1555 battery .

Database management systems

A database management system (DBMS) consists of software that operates databases, providing storage, access, security, backup and other facilities    Sony VGP-BPS13 battery .

Database management systems can be categorized according to the database model that they support, such as relational or XML, the type(s) of computer they support, such as a server cluster or a mobile phone, the query language(s) that access the database, such as SQL or XQuery, performance trade-offs, such as maximum scale or maximum speed or others   Sony VGP-BPS13/B battery .

Some DBMS cover more than one entry in these categories, e.g., supporting multiple query languages. Examples of some commonly used DBMS are MySQL, PostgreSQL, Microsoft Access, SQL Server, FileMaker,Oracle,Sybase, dBASE, Clipper,FoxPro etc   Sony VGP-BPS13/S battery .

Almost every database software comes with an Open Database Connectivity (ODBC) driver that allows the database to integrate with other databases.

Components of DBMS

Most DBMS as of 2009 implement a relational model          Sony VGP-BPS13A/B battery .

Other DBMS systems, such as Object DBMS, offer specific features for more specialized requirements. Their components are similar, but not identical           Sony VGP-BPS13B/B battery .

RDBMS components

  • Sublanguages— Relational DBMS (RDBMS) include Data Definition Language (DDL) for defining the structure of the database, Data Control Language (DCL) for defining security/access controls, and Data Manipulation Language (DML) for querying and updating data      Sony VGP-BPL9 battery .
  • Interface drivers—These drivers are code libraries that provide methods to prepare statements, execute statements, fetch results, etc. Examples include ODBC, JDBC, MySQL/PHP,FireBird/Python      Sony VGP-BPS13B/B battery .
  • SQL engine—This component interprets and executes the DDL, DCL, and DML statements. It includes three major components (compiler, optimizer, and executor).
  • Transaction engine—Ensures that multiple SQL statements either succeed or fail as a group, according to application dictates      Sony VGP-BPL15 battery .
  • Relational engine—Relational objects such as Table, Index, and Referential integrity constraints are implemented in this component.
  • Storage engine—This component stores and retrieves data from secondary storage, as well as managing transaction commit and rollback, backup and recovery, etc     Dell Inspiron E1505 battery .

ODBMS components

Object DBMS (ODBMS) has transaction and storage components that are analogous to those in an RDBMS. Some DBMS handle DDL, DML and update tasks differently. Instead of using sublanguages, they provide APIs for these purposes   Dell Latitude E6400 battery .

They typically include a sublanguage and accompanying engine for processing queries with interpretive statements analogous to but not the same as SQL. Example object query languages are OQL, LINQ, JDOQL, JPAQL and others. The query engine returns collections of objects instead of relational rows      HP Pavilion dv6000 Battery .

Types

Operational database

These databases store detailed data about the operations of an organization. They are typically organized by subject matter, process relatively high volumes of updates using transactions. Essentially every major organization on earth uses such databases   SONY VAIO VGN-FZ Battery .

Examples include customer databases that record contact, credit, and demographic information about a business' customers, personnel databases that hold information such as salary, benefits, skills data about employees, Enterprise resource planning that record details about product components     SONY VAIO VGN-FZ18 Battery ,

parts inventory, and financial databases that keep track of the organization's money, accounting and financial dealings.

Data warehouse

Data warehouses archive modern data from operational databases and often from external sources such as market research firms  SONY VAIO VGN-FZ21E Battery .

Often operational data undergoes transformation on its way into the warehouse, getting summarized, anonymized, reclassified, etc. The warehouse becomes the central source of data for use by managers and other end-users who may not have access to operational data      SONY VAIO VGN-FW21E Battery .

For example, sales data might be aggregated to weekly totals and converted from internal product codes to use UPC codes so that it can be compared with ACNielsen data.Some basic and essential components of data warehousing include retrieving and analyzing data, transforming,loading and managing data so as to make it available for further use  SONY VAIO VGN-NR11S/S Battery .

Operations in a data warehouse are typically concerned with bulk data manipulation, and as such, it is unusual and inefficient to target individual rows for update, insert or delete. Bulk native loaders for input data and bulk SQL passes for aggregation are the norm  SONY VAIO VGN-NR11M/S Battery .

Analytical database

Analysts may do their work directly against a data warehouse or create a separate analytic database for Online Analytical Processing. For example, a company might extract sales records for analyzing the effectiveness of advertising and other sales promotions at an aggregate level       SONY VAIO VGN-NR11Z/S Battery .

Distributed database

These are databases of local work-groups and departments at regional offices, branch offices, manufacturing plants and other work sites. These databases can include segments of both common operational and common user databases, as well as data generated and used only at a user’s own site       SONY VAIO VGN-NR11Z/T Battery .

End-user database

These databases consist of data developed by individual end-users. Examples of these are collections of documents in spreadsheets, word processing and downloaded files, even managing their personal baseball card collection   Sony VAIO VGN-FZ21E Battery .

External database

These databases contain data collected for use across multiple organizations, either freely or via subscription. The Internet Movie Database is one example      Sony VAIO VGN-FW21E Battery .

Hypermedia databases

The World wide web can be thought of as a database, albeit one spread across millions of independent computing systems. Web browsers "process" this data one page at a time, while web crawlersand other software provide the equivalent of database indexes to support search and other activities    Sony VAIO VGN-NR11S/S Battery .

Models

Post-relational database models

Products offering a more general data model than the relational model are sometimes classified as post-relational.Alternate terms include "hybrid database", "Object-enhanced RDBMS" and others   Sony VAIO VGN-NR11Z/S Battery .

The data model in such products incorporates relations but is not constrained by E.F. Codd's Information Principle, which requires that all information in the database must be cast explicitly in terms of values in relations and in no other way Sony VAIO VGN-NR11M/S Battery .

Some of these extensions to the relational model integrate concepts from technologies that pre-date the relational model. For example, they allow representation of a directed graph with trees on thenodes.

Some post-relational products extend relational systems with non-relational features       Sony VAIO VGN-NR11Z/T Battery .

Others arrived in much the same place by adding relational features to pre-relational systems. Paradoxically, this allows products that are historically pre-relational, such as PICK and MUMPS, to make a plausible claim to be post-relational      SONY VAIO VGN-FZ180E Battery .

Object database models

In recent years, the object-oriented paradigm has been applied in areas such as engineering and spatial databases, telecommunications and in various scientific domains. The conglomeration of object oriented programming and database technology led to this new kind of database       SONY VAIO VGN-FZ220E Battery .

These databases attempt to bring the database world and the application-programming world closer together, in particular by ensuring that the database uses the same type system as the application program. This aims to avoid the overhead (sometimes referred to as the impedance mismatch)        SONY VAIO VGN-FZ340E Battery

of converting information between its representation in the database (for example as rows in tables) and its representation in the application program (typically as objects). At the same time, object databases attempt to introduce key ideas of object programming, such as encapsulation and polymorphism, into the world of databases     SONY VAIO VGN-FZ430E Battery .

A variety of these ways have been tried for storing objects in a database. Some products have approached the problem from the application-programming side, by making the objects manipulated by the program persistent. This also typically requires the addition of some kind of query language      SONY VAIO VGN-FZ460E Battery ,

since conventional programming languages do not provide language-level functionality for finding objects based on their information content. Others have attacked the problem from the database end, by defining an object-oriented data model for the database       SONY VAIO VGN-FZ480E Battery ,

and defining a databaseprogramming language that allows full programming capabilities as well as traditional query facilities..

Storage structures

Databases may store relational tables/indexes in memory or on hard disk in one of many forms        SONY VAIO VGN-FZ4000 Battery:

The most commonly used are B+ trees and ISAM.

Object databases use a range of storage mechanisms. Some use virtual memory-mapped files to make the native language (C++, Java etc.) objects persistent     SONY VAIO VGN-FZ31B Battery .

This can be highly efficient but it can make multi-language access more difficult. Others disassemble objects into fixed- and varying-length components that are then clustered in fixed sized blocks on disk and reassembled into the appropriate format on either the client or server address space       SONY VAIO VGN-FZ31J Battery .

Another popular technique involves storing the objects in tuples (much like a relational database) which the database server then reassembles into objects for the client.

Other techniques include clustering by category (such as grouping data by month, or location), storing pre-computed query results, known as materialized views, partitioning data by range (e.g., a data range) or by hash   SONY VAIO VGN-FZ31M Battery .

Memory management and storage topology can be important design choices for database designers as well. Just as normalization is used to reduce storage requirements and improve database designs, conversely denormalization is often used to reduce join complexity and reduce query execution time     SONY VAIO VGN-FZ31Z Battery .

Indexing

Indexing is a technique for improving database performance. The many types of index share the common property that they eliminate the need to examine every entry when running a query. In large databases, this can reduce query time/cost by orders of magnitude      SONY VAIO VGN-FZ38M Battery .

The simplest form of index is a sorted list of values that can be searched using a binary search with an adjacent reference to the location of the entry, analogous to the index in the back of a book. The same data can have multiple indexes (an employee database could be indexed by last name and hire date    SONY VGP-BPS8 Battery .

Indexes affect performance, but not results. Database designers can add or remove indexes without changing application logic, reducing maintenance costs as the database grows and database usage evolves       SONY VGP-BPS13 Battery .

Given a particular query, the DBMS' query optimizer is responsible for devising the most efficient strategy for finding matching data. The optimizer decides which index or indexes to use, how to combine data from different parts of the database, how to provide data in the order requested, etc     SONY VGP-BPS13/S Battery .

Indexes can speed up data access, but they consume space in the database, and must be updated each time the data is altered. Indexes therefore can speed data access but slow data maintenance. These two properties determine whether a given index is worth the cost     SONY VGP-BPS13A/B Battery .

Transactions

As every software system, a DBMS operates in a faulty computing environment and prone to failures of many kinds. A failure can corrupt the respective database unless special measures are taken to prevent this       SONY VGP-BPS13B/B Battery .

A DBMS achieves certain levels of fault tolerance by encapsulating in database transactions units of work (executed programs) performed upon the respective database      SONY VGP-BPS13A/S Battery .

The ACID rules

Most DBMS provide some form of support for transactions, which allow multiple data items to be updated in a consistent fashion, such that updates that are part of a transaction succeed or fail in unison. The so-called ACID rules, summarized here, characterize this behavior      SONY VGP-BPS13AS Battery :

  • Atomicity: Either all the data changes in a transaction must happen, or none of them. The transaction must be completed, or else it must be undone (rolled back).
  • Consistency: Every transaction must preserve the declared consistency rules for the database  Dell Inspiron 1320n Battery .
  • Isolation: Two concurrent transactions cannot interfere with one another. Intermediate results within one transaction must remain invisible to other transactions. The most extreme form of isolation isserializability, meaning that transactions that take place concurrently could instead be performed in some series, without affecting the ultimate result Dell Inspiron 1464 Battery.
  • Durability: Completed transactions cannot be aborted later or their results discarded. They must persist through (for instance) DBMS restarts.

In practice, many DBMSs allow the selective relaxation of these rules to balance perfect behavior with optimum performance  Dell Inspiron 1564 Battery .

Concurrency control and locking

Concurrency control is essential for the correctness of transactions executed concurrently in a DBMS, which is the common execution mode for performance reasons. The main concern and goal of concurrency control is isolation  Dell Inspiron 1764 Battery.

Isolation

Isolation refers to the ability of one transaction to see the results of other transactions. Greater isolation typically reduces performance and/or concurrency, leading DBMSs to provide administrative options to reduce isolation   Dell Studio 1450 Battery .

For example, in a database that analyzes trends rather than looking at low-level detail, increased performance might justify allowing readers to see uncommitted changes ("dirty reads".)

A common way to achieve isolation is by locking      Dell Studio 1457 Battery .

When a transaction modifies a resource, the DBMS stops other transactions from also modifying it, typically by locking it. Locks also provide one method of ensuring that data does not change while a transaction is reading it or even that it doesn't change until a transaction that once read it has completed      Dell Latitude D610 Battery .

Lock types

Locks can be shared or exclusive, and can lock out readers and/or writers. Locks can be created implicitly by the DBMS when a transaction performs an operation, or explicitly at the transaction's request.

Shared locks allow multiple transactions to lock the same resource. The lock persists until all such transactions complete  Toshiba NB100 Battery .

Exclusive locks are held by a single transaction and prevent other transactions from locking the same resource.

Read locks are usually shared, and prevent other transactions from modifying the resource. Write locks are exclusive, and prevent other transactions from modifying the resource    Toshiba Satellite M65 battery .

On some systems, write locks also prevent other transactions from reading the resource.

The DBMS implicitly locks data when it is updated, and may also do so when it is read. Transactions explicitly lock data to ensure that they can complete without complications. Explicit locks may be useful for some administrative tasks   Toshiba Satellite M60 battery .

Locking can significantly affect database performance, especially with large and complex transactions in highly concurrent environments.

Lock granularity

Locks can be coarse, covering an entire database, fine-grained, covering a single data item, or intermediate covering a collection of data such as all the rows in a RDBMS table       Dell Latitude D830 Battery .

Deadlocks

Deadlocks occur when two transactions each require data that the other has already locked exclusively. Deadlock detection is performed by the DBMS, which then aborts one of the transactions and allows the other to complete     Dell Latitude D620 Battery .

Replication

Database replication involves maintaining multiple copies of a database on different computers, to allow more users to access it, or to allow a secondary site to immediately take over if the primary site stops working     Dell Inspiron Mini 10 Battery .

Some DBMS piggyback replication on top of their transaction logging facility, applying the primary's log to the secondary in near real-time. Database clustering is a related concept for handling larger databases and user communities by employing a cluster of multiple computers to host a single database that can use replication as part of its approach  Sony VGN-FW11S Battery .

Security

Database security denotes the system, processes, and procedures that protect a database from unauthorized activity.

DBMSs usually enforce security through access control, auditing, and encryption      Sony VGN-FW11M Battery :

  • Access control manages who can connect to the database via authentication and what they can do via authorization.
  • Auditing records information about database activity: who, what, when, and possibly where.
  • Encryption protects data at the lowest possible level by storing and possibly transmitting data in an unreadable form Sony VGN-FW139E/H battery .
  • The DBMS encrypts data when it is added to the database and decrypts it when returning query results. This process can occur on the client side of a network connection to prevent unauthorized access at the point of use        Dell Latitude E5400 Battery .

Confidentiality

Law and regulation governs the release of information from some databases, protecting medical history, driving records, telephone logs, etc         Dell Latitude E4200 Battery .

In the United Kingdom, database privacy regulation falls under the Office of the Information Commissioner. Organizations based in the United Kingdom and holding personal data in digital format such as databases must register with the Office     Dell Inspiron 300M Battery .

real-time database is a processing system designed to handle workloads whose state is constantly changing (Buchmann). This differs from traditional databases containing persistent data, mostly unaffected by time. For example, a stock market changes very rapidly and is dynamic      Dell Vostro A840 Battery .

The graphs of the different markets appear to be very unstable and yet a database has to keep track of current values for all of the markets of the New York Stock Exchange (Kanitkar). Real-time processing means that a transaction is processed fast enough for the result to come back and be acted on right away (Capron)       Dell Studio 1737 battery .

Real-time databases are useful for accounting, banking, law, medical records, multi-media, process control, reservation systems, and scientific data analysis (Snodgrass). As computers increase in power and can store more data, they are integrating themselves into our society and are employed in many applications      Dell Inspiron E1505 battery .

Overview

Real-time databases are traditional databases that use an extension to give the additional power to yield reliable responses. They use timing constraints that represent a certain range of values for which the data are valid        Dell RM791 battery .

This range is called temporal validity. A conventional database cannot work under these circumstances because the inconsistencies between the real world objects and the data that represents them are too severe for simple modifications. An effective system needs to be able to handle time-sensitive queries, return only temporally valid data, and support priority scheduling  Dell XPS M1530 battery .

To enter the data in the records, often a sensor or an input device monitors the state of the physical system and updates the database with new information to reflect the physical system more accurately (Abbot). When designing a real-time database system, one should consider how to represent valid time, how facts are associated with real-time system   Dell XPS M2010 battery .

Also, consider how to represent attribute values in the database so that process transactions and data consistency have no violations.

When designing a system, it is important to consider what the system should do when deadlines are not met   Acer Aspire One battery.

For example, an air-traffic control system constantly monitors hundreds of aircraft and makes decisions about incoming flight paths and determines the order in which aircraft should land based on data such as fuel, altitude, and speed. If any of this information is late, the result could be devastating     Toshiba Satellite P10 Battery .

To address issues of obsolete data, the timestamp can support transactions by providing clear time references.

Preserving data consistency

Although the real-time database system may seem like a simple system, problems arise during overload when two or more database transactions require access to the same portion of the database       SONY VGN-FZ210CE Battery .

A transaction is usually the result of an execution of a program that accesses or changes the contents of a database (Singhal). A transaction is different from a stream because a stream only allows read-only operations, and transactions can do both read and write operations       Dell Precision M70 Battery .

This means in a stream, multiple users can read from the same piece of data, but they cannot both modify it (Abbot). A database must let only one transaction operate at a time to preserve data consistency. For example, if two students demand to take the remaining spot for a section of a class and they hit submit at the same time, only one student should be able to register for it (Abbot)  Toshiba Satellite L305 Battery .

Real-time databases can process these requests utilizing scheduling algorithms for concurrency control, prioritizing both students’ requests in some way. Throughout this article, we assume that the system has a single processor, a disk based database, and a main memory pool(Haritsa)        Toshiba Satellite T4900 Battery .

In real-time databases, deadlines are formed and different kinds of systems respond differently to data that does not meet its deadline. In a real-time system, each transaction uses a timestamp to schedule the transactions (Abbot)   Toshiba PA3399U-2BRS battery .

A priority mapper unit assigns a level of importance to each transaction upon its arrival in the database system that is dependent on how the system views times and other priorities. The timestamp method on relies on the arrival time in the system. Researchers indicate that for most studies, transactions are sporadic with unpredictable arrival times   Toshiba Satellite A200 Battery .

For example, the system gives an earlier request deadline to a higher priority and a later deadline to a lower priority (Haritsa). Below is a comparison of different scheduling algorithms.

Earliest Deadline

PT = DT — The value of a transaction is not important. An example is a group of people calling to order a product Toshiba Satellite 1200 Battery .

Highest Value

PT = 1/VT — The deadline is not important. Some transactions should get to CPU based on criticalness, not fairness. This is an example of least slack that can wait the least amount of time. If the telephone switchboards were overloaded, people who call 911 should get priority (Snodgrass)     Toshiba Satellite M300 Battery .

Value inflated deadline

PT = DT/VT — Gives equal weight to deadline and values based on scheduling. An example is registering for classes where the student selects a block of classes that he wishes to take and presses submit     SONY VGP-BPS13A/Q Battery .

In this scenario, higher priorities often take up precedence. A school registration system probably uses this technique when the server receives two registration transactions. If one student had 22 credits and the other had 100 credits, the person with 100 credits would take priority (Value based scheduling)         SONY VGP-BPS13A/Q Battery .

Timing constraints and deadlines

A system that correctly perceives the serialization and timing constraints associated with transactions with soft or firm deadlines, takes advantage of absolute consistency (Lee). Another way of making sure that data is absolute is using relative constraints  SONY VGP-BPS13B/Q Battery .

Relative constraints ensure transactions enter into the system at the same time as the rest of the group that the data transaction is associated with. Using the mechanisms of absolute and relative constraints greatly ensures the accuracy of data   SONY VGP-BPS13B/Q Battery .

An additional way of dealing with conflict resolution in a real-time database system besides deadlines is a wait policy method. This process helps ensure the latest information in time critical systems. The policy avoids conflict by asking all non-requesting blocks to wait until the most essential block of data is processed (Abbot)   SONY VGP-BPS13/Q Battery .

While studies in labs have found that data-deadline based policies do not improve performance significantly, the forced wait policy can improve performance by 50 percent (Porkka). The forced wait policy may involve waiting for higher priority transactions to process in order to prevent deadlock      SONY VGP-BPS13/Q Battery .

Another example of when data can be delayed is when a block of data is about to expire. The forced wait policy delays processing until the data is updated using new input data. The latter method helps increase the accuracy of the system and can cut down on the number of necessary processes that are aborted (Kang)       SONY VGP-BPS21A/B Battery .

Generally relying on wait policies is a not optimal (Kang).

It is necessary to discuss the formation of deadlines. Deadlines are the constraints for soon-to-be replaced data accessed by the transaction. Deadlines can be either observant or predictive (Kang)        SONY VGP-BPS21/S Battery .

In an observant deadline system, all unfinished transactions are examined and the processor determines whether any had met its deadline (Abbot). Problems arise in this method because of variations caused by seek time variations, buffer management and page faults (An Overview of Real-Time Database Systems)    SONY VGP-BPS21B Battery .

A more stable way of organizing deadlines is the predictive method. It builds a candidate schedule and determines if a transaction would miss its deadline under the schedule (Abbot).

The type of response to a missed deadline depends on whether the deadline is hard, soft, or firm       SONY VGP-BPS21A Battery .

Hard deadlines require that each data packet reach its destination before the packet has expired and if not, the process could be lost, causing a possible problem. Problems like these are not very common because omnipotence of the system is required before assigning deadlines to determine worst case      SONY VGP-BPS21 Battery .

This is very hard to do and if something unexpected happens to the system such as a minute hardware glitch, it could throw the data off. For soft or firm deadlines, missing a deadline can lead to a degraded performance but not a catastrophe (Haritsa). A soft deadline meets as many deadlines as possible      Sony VGP-BPS21A/B Battery .

However, no guarantee exists that the system can meet all deadlines. Should a transaction miss its deadline, the system has more flexibility and the transaction may increase in importance. Below is a description of these responses    Sony VGP-BPS21/S Battery :

Hard deadline: If not meeting deadlines creates problems, a hard deadline is best. It is periodic, meaning that it enters the database on a regular rhythmic pattern. An example is data gathered by a sensor. These are often used in life critical systems (Stankovic)  Sony VGP-BPS21A Battery .

Firm deadline: Firm deadlines appear to be similar to hard deadlines yet they differ from hard deadlines because firm deadlines measure how important it is to complete the transaction at some point after the transaction arrives. Sometimes completing a transaction after its deadline has expired may be harmful or not helpful, and both the firm and hard deadlines consider this  Sony VGP-BPS21 Battery .

An example of a firm deadline is an autopilot system (Snodgrass).

Soft deadline: If meeting time constrains is desirable but missing deadlines do not cause serious damage, a soft deadline may be best. It operates on an aperiodic or irregular schedule. In fact, the arrival of each time for each task is unknown  Sony Vaio PCG-5G2L Battery .

An example is an operator switchboard for a telephone(Stankovic).

Hard deadline processes abort transactions that have passed the deadline, improving the system by cleaning out clutter that needs to be processed      Sony Vaio PCG-5G3L Battery .

Processes can clear out not only the transactions with expired deadlines but also transactions with the longest deadlines, assuming that once they reach the processor they would be obsolete. This means other transactions should be of higher priority. In addition, a system can remove the least critical transactions     Sony Vaio PCG-5J1L Battery .

When I was pre-selecting classes on during a high traffic period, a field in the database can become so busy with registration requests that it was unavailable for a while and the result of my transaction was a display of the SQL query sent and a message that said that the data is currently unavailable           Sony Vaio PCG-5K2L Battery .

This error is caused by the checker, a mechanism that checks the condition of the rules, and the rule that occurred before it (Ramamritham).

The goal of scheduling periods and deadlines is to update transactions guaranteed to complete before their deadline in such a way that the workload is minimal      Sony Vaio PCG-5J2L Battery .

With large real-time databases, buffering functions can help improve performance tremendously. A buffer is part of the database that is stored in main memory to reduce transaction response time. In order to reduce disk input and output transactions, a certain number of buffers should be allocated (O'Neil)        Sony Vaio PCG-5K1L Battery .

Sometimes multiversions are stored in buffers when the data block the transaction needs is currently in use. Later, the database has the data appended to it. Different strategies allocate buffers and must balance between taking an excessive amount of memory and having everything in one buffer that it has to search for       Sony Vaio PCG-5L1L Battery .

The goal is to eliminate search time and distribute the resources between buffer frames in order to access data quickly. A buffer manager is capable of allocating more memory, if necessary, to improve response time. The buffer manager can even determine whether a transaction that it has should advance        Sony Vaio PCG-6S2L Battery .

Buffering can improve speed in real-time systems (O'Neil)

Future database systems

Traditional databases are persistent but are incapable of dealing with dynamic data that constantly changes. Therefore, another system is needed         Sony Vaio PCG-6S3L Battery .

Real-time databases may be modified to improve accuracy and efficiency and to avoid conflict, by providing deadlines and wait periods to insure temporal consistency. Real-time database systems offer a way of monitoring a physical system and representing it in data streams to a database        Sony Vaio PCG-6V1L Battery .

A data stream, like memory, fades over time. In order to guarantee that the freshest and most accurate information is recorded there are a number of ways of checking transactions to make sure they are executed in the proper order. An online auction house provides an example of a rapidly changing database         Sony Vaio PCG-6W1L Battery .

This paper discusses the example of a school registration system.

Now database systems are faster than they were in the past. In the future, we can look forward to even faster database systems. Although we have faster systems now, an effort to reduce misses and tardy times will still be beneficial  Sony Vaio PCG-6W2L Battery.

The ability to process results in a timely and predictable manner will always be more important than fast processing. Fast processing that is misapplied is not helpful for real-time database systems. Transactions that run faster still sometimes block in such a way that they have to be aborted and restarted       Sony Vaio PCG-6W3L Battery .

In fact, faster processing hurts some real-time applications because increased speed brings more complexity and more of a chance for problems caused by a variance of speed. Faster processing makes it harder to determine which deadlines have been met successfully (Lam)        Sony Vaio PCG-7111L Battery .

With future database systems running even faster than ever, there is a need to do more studies so we can continue to have efficient systems (Lam).

The amount of research studying real-time database systems will increase because of commercial applications such as web based auction houses like e-bay        Sony Vaio PCG-7112L Battery .

More developing countries are expanding their phone systems, and the number of people with cell phones in the United States as well as other places in the world continues to grow. Also likely to spur real-time research is the exponentially increasing speed of the microprocessor        Sony Vaio PCG-7113L Battery .

This enables new technologies such as web-video conferencing and instant messenger conversations in sound and high-resolution video, which are reliant on real-time database systems. Studies of temporal consistency result in new protocols and timing constraints with the goal of handling real-time transactions more effectively (Haritsa)         Sony Vaio PCG-7133L Battery .

Database theory encapsulates a broad range of topics related to the study and research of the theoretical realm of databases and database management systems.

Theoretical aspects of data management include, among other areas, the foundations of query languages  Sony Vaio PCG-7Z2L Battery ,

computational complexity and expressive power of queries, finite model theory, database design theory, dependency theory, foundations of concurrency control and database recovery, deductive databases, temporal and spatial databases, real time databases, managing uncertain data andprobabilistic databases, and Web data   Sony Vaio PCG-8Y1L Battery .

Most research work has traditionally been based on the relational model, since this model is usually considered the simplest and most foundational model of interest. Corresponding results for other data models, such as object-oriented or semi-structured models, or, more recently, graph data models and XML, are often derivable from those for the relational model  Sony Vaio PCG-8Y2L Battery .

A central focus of database theory is on understanding the complexity and power of query languages and their connection to logic. Starting from relational algebra and first-order logic (which are equivalent by Codd's theorem) and the insight that important queries such as graph reachability are not expressible in this language    Sony Vaio PCG-8Z1L Battery ,

more powerful language based on logic programming and fixpoint logic such as datalog were studied. Another focus was on the foundations of query optimization and data integration. Here most work studied conjunctive queries, which admit query optimization even under constraints using the chase algorithm     Sony Vaio PCG-8Z2L Battery .

The main research conferences in the area are the ACM Symposium on Principles of Database Systems (PODS) and the International Conference on Database Theory (ICDT)        Sony VAIO PCG-5G2L Battery .

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