computer virtual memory

8/3/2019 Computer Virtual Memory

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In computing, virtual memory is a memory managementtechnique developed formultitaskingkernels. This technique virtualizes a computer architecture's various hardware memory devices(such as RAM modules anddisk storage drives), allowing aprogramto be designed as though:

there is only one hardware memory device and this "virtual" device acts like a RAM

module. the program has, by default, sole access to this virtual RAM module as the basis for a

contiguous working memory (an address space).

Systems that employ virtual memory:

use hardware memory more efficiently than systems without virtual memory.[citation needed]

make the programming of applications easier by:o hiding fragmentation.o delegating to the kernel the burden of managing the memory hierarchy; there is no

need for the program to handle overlays explicitly.

o obviating the need to relocate program code or to access memory withrelativeaddressing.

Memory virtualizationis a generalization of the concept of virtual memory.

Virtual memory is an integral part of acomputer architecture; all implementations (excluding[dubiousdiscuss]

emulatorsand virtual machines) require hardware support, typically in the form of a memorymanagement unit built into the CPU. Consequently, older operating systems (such asDOS[1] of the 1980sor those for the mainframes of the 1960s) generally have no virtual memory functionality[dubiousdiscuss],though notable exceptions include theAtlas,B5000, IBM System/360 Model 67, IBM System/370mainframe systems of the early 1970s, and theApple Lisa project circa 1980.

Embedded systems and other special-purpose computer systems that require very fast and/or veryconsistent response times may opt not to use virtual memory due to decreaseddeterminism; virtualmemory systems trigger unpredictable interrupts that may produce unwanted "jitter" duringI/Ooperations. This is because embedded hardware costs are often kept low by implementing all suchoperations with software (a technique calledbit-banging) rather than with dedicated hardware. In anycase, embedded systems usually have little use for multitasking features or complicated memoryhierarchies.
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In computer engineering, a cache (pronounced/k/kash in US and /ke/kaysh inAust/NZ) is a component that transparently stores data so that future requests for that data can beserved faster. The data that is stored within a cache might be values that have been computedearlier or duplicates of original values that are stored elsewhere. If requested data is contained inthe cache (cache hit), this request can be served by simply reading the cache, which iscomparatively faster. Otherwise (cache miss), the data has to be recomputed or fetched from itsoriginal storage location, which is comparatively slower. Hence, the more requests can be servedfrom the cache the faster the overall system performance is.

As opposed to a buffer, which is managed explicitly by a client[citation needed], a cache stores datatransparently: This means that a client who is requesting data from a system is not aware that thecache exists, which is the origin of the name cache (fromFrench "cacher", to conceal).

To be cost efficient and to enable an efficient use of data, caches are relatively small.Nevertheless, caches have proven themselves in many areas of computing because accesspatterns in typical computer applicationshave locality of reference. References exhibit temporallocality if data is requested again that has been recently requested already. References exhibitspatial localityif data is requested that is physically stored close to data that has been requestedalready.

CPU cache

Main article: CPU cache

Small memories on or close to the CPU can operate faster than the much larger main memory.Most CPUs since the 1980s have used one or more caches, and modern high-end embedded,desktop and servermicroprocessors may have as many as half a dozen, each specialized for aspecific function. Examples of caches with a specific function are the D-cache and I-cache (datacache and instruction cache).

[edit] Disk cache

Main article:Page cache
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While CPU caches are generally managed entirely by hardware, a variety of software managesother caches. Thepage cacheinmain memory, which is an example of disk cache, is managedby the operating system kernel.

While the hard drive's hardware disk bufferis sometimes misleadingly referred to as "disk

cache", its main functions are write sequencing and read prefetching. Repeated cache hits arerelatively rare, due to the small size of the buffer in comparison to the drive's capacity. However,high-end disk controllers often have their own on-board cache of hard diskdata blocks.

Finally, fast local hard disk can also cache information held on even slower data storage devices,such as remote servers (web cache) or local tape drivesoroptical jukeboxes. Such a scheme isthe main concept ofhierarchical storage management.

[edit] Web cache

Main article: Web cache

Web browsers andweb proxy servers employ web caches to store previous responses from webservers, such as web pages. Web caches reduce the amount of information that needs to betransmitted across the network, as information previously stored in the cache can often be re-used. This reduces bandwidth and processing requirements of the web server, and helps toimprove responsiveness for users of the web.

Web browsers employ a built-in web cache, but someinternet service providers or organizationsalso use a caching proxy server, which is a web cache that is shared among all users of thatnetwork.

Another form of cache is P2P caching, where the files most sought for bypeer-to-peerapplications are stored in anISP cache to accelerate P2P transfers.

There are three kinds of web caches: 1)Client Caching, 2)Proxy Caches, 3)Gateway caches.

[edit] Other caches

The BIND DNS daemon caches a mapping of domain names to IP addresses, as does a resolverlibrary.

Write-through operation is common when operating over unreliable networks (like an Ethernet

LAN), because of the enormous complexity of the coherency protocol required between multiplewrite-back caches when communication is unreliable. For instance, web page caches and client-sidenetwork file system caches (like those inNFS orSMB) are typically read-only or write-through specifically to keep the network protocol simple and reliable.

Search enginesalso frequently make web pagesthey have indexed available from their cache.For example, Google provides a "Cached" link next to each search result. This can prove usefulwhen web pages from a web serverare temporarily or permanently inaccessible.
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Another type of caching is storing computed results that will likely be needed again, ormemoization. ccache, a program that caches the output of the compilation to speed up thesecond-time compilation, exemplifies this type.

Database cachingcan substantially improve the throughput ofdatabaseapplications, for example

in the processing ofindexes, data dictionaries, and frequently used subsets of data.Distributed caching[1] uses caches spread across different networked hosts.

[edit] The difference between buffer and cache

The terms "buffer" and "cache" are not mutually exclusive and the functions are frequentlycombined; however, there is a difference in intent.

Abufferis a temporary memory location, that is traditionally used because CPU instructionscannot directly address data stored in peripheral devices. Thus, addressable memory is used as

intermediate stage. Additionally such a buffer may be feasible when a large block of data isassembled or disassembled (as required by a storage device), or when data may be delivered in adifferent order than that in which it is produced. Also a whole buffer of data is usuallytransferred sequentially (for example to hard disk), so buffering itself sometimes increasestransfer performance or reduce the variation or jitter of the transfer's latency as opposed tocaching where the intent is to reduce the latency. These benefits are present even if the buffereddata are written to thebufferonce and read from the buffer once.

A cache also increases transfer performance. A part of the increase similarly comes from thepossibility that multiple small transfers will combine into one large block. But the mainperformance-gain occurs because there is a good chance that the same datum will be read from

cache multiple times, or that written data will soon be read. A cache's sole purpose is to reduceaccesses to the underlying slower storage. Cache is also usually anabstraction layerthat isdesigned to be invisible from the perspective of neighbouring layers.

Cache (pronounced cash) memory is extremely fast memory that is built into acomputerscentral processing unit

(CPU), or located next to it on a separate chip. The CPU uses cache memory to store instructions that are

repeatedly required to run programs, improving overall system speed. The advantage of cache memory is that the

CPU does not have to use the motherboards system bus for data transfer. Whenever data must be passed through

the system bus, the data transfer speed slows to the motherboards capability. The CPU can process data much

faster by avoiding the bottleneck created by the system bus.

Cpu Cache on wiseGEEK:

When looking at new computers check out the amounts of L1, L2 and L3 cache. All else being equal,a system with more CPUcache will perform better, and synchronous cache is faster thanasynchronous.

CPUcache caters to the needs of the microprocessor by anticipating data requests so that processinginstructions are provided without delay. CPUcache is faster than random access memory (RAM), andis designed to prevent bottlenecks in performance.
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L2 Cache on wiseGEEK:

To optimize overall performance, the best result is obtained by having the smallest, fastest cachemost immediate to the CPU itself, followed by a slightly larger pool of L2 cache, and an even largerpool of L3 cache. The idea is to keep the most frequently used instructions in L1, with L2 cacheholding the next most likely needed bits of data, and L3 following suit.

While L1 cache is built into CPUs today, it might also reside alongside the CPU on older PCs. L2cache can be built into the CPU or present on the motherboard, along with L3 cache.

L1 Cache on wiseGEEK:

The goal is to maximize hits and minimize misses that slow performance. While L1 cache is built intoCPUs today, it might also reside alongside the CPU on older PCs.

When this happens, the CPU can process the request without delay, drastically improving computerperformance. The CPU will check L1 cache first, followed by L2 and L3 cache. If it finds the neededbits of data, this is a cache hit, but if the cache doesnt anticipate the request, the CPU gets a cachemiss, and the data must be pulled from slower RAM or the hard drive which is slower still.

virtual memory

0diggsdigg

)(n.) An imaginarymemory area supported by someoperating systems(for example, Windowsbut not DOS) in

conjunction with thehardware. You can think of virtual memory as an alternate set of memoryaddresses. Programs

use these virtual addresses rather than real addresses to storeinstructions and data. When the program is actually

executed, the virtual addresses are convertedinto real memory addresses.

The purpose of virtual memory is to enlarge theaddress space, the set of addresses a program can utilize. For

example, virtual memory might contain twice as many addresses asmain memory. A program using all of virtual

memory, therefore, would not be able to fit in main memory all at once. Nevertheless, thecomputercould execute
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such a program by copying into main memory those portions of the program needed at any given point during

execution.

To facilitate copying virtual memory into real memory, the operating system divides virtual memory intopages, each

of which contains a fixed number of addresses. Each page is stored on adiskuntil it is needed. When the page is

needed, the operating system copies it from disk to main memory, translating the virtual addresses into real

addresses.

The process of translating virtual addresses into real addresses is called mapping. The copying of virtual pages from

disk to main memory is known aspagingorswapping.

- Virtual Memory is a feature of an operating system that enables a process to use a

memory (RAM) address space that is independent of other processes running in the

same system, and use a space that is larger than the actual amount of RAM

present, temporarily relegating some contents from RAM to a disk, with little or no

overhead.

An extranetis a computer networkthat allows controlled access from the outside, for specificbusiness or educational purposes. An extranet can be viewed as an extension of a company'sintranet that is extended to users outside the company, usually partners, vendors, and suppliers. Ithas also been described as a "state of mind" in which the Internet is perceived as a way to dobusiness with a selected set of other companies (business-to-business, B2B), in isolation from allother Internet users. In contrast,business-to-consumer(B2C) models involve known servers ofone or more companies, communicating with previously unknown consumer users. An extranetis like a DMZ in that it provides access to needed services for channel partners, without grantingaccess to an organization's entire network.

Enterprise applications

During the late 1990s and early 2000s, several industries started to use the term extranet todescribe central repositories of shared data made accessible via the web only to authorizedmembers of particular work groups. Some applications are offered on a Software as a Service(SaaS) basis by vendors functioning as Application service providers(ASPs).

Specially secured extranets are used to provide virtualdata room services to companies inseveral sectors (including law and accountancy).

For example, in the construction industry, project teams may access a project extranet to sharedrawings and documents, make comments, issue requests for information, etc. In 2003 in theUnited Kingdom, several of the leading vendors formed the Network forConstructionCollaboration TechnologyProviders (NCCTP) to promote the technologies and to establish dataexchange standards between the different data systems. The same type of construction-focusedtechnologies have also been developed in theUnited States, Australia and mainland Europe.[3]
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[edit] Advantages

Exchange large volumes of data using Electronic Data Interchange (EDI) Share product catalogs exclusively with trade partners Collaborate with other companies on joint development efforts

Jointly develop and use training programs with other companies Provide or access services provided by one company to a group of other

companies, such as an online banking application managed by one companyon behalf of affiliated banks

[edit] Disadvantages

Extranets can be expensive to implement and maintain within anorganization (e.g., hardware, software, employee training costs), if hostedinternally rather than by an application service provider.

Security of extranets can be a concern when hosting valuable or proprietaryinformation.

Companies can use an extranet to:

Exchange large volumes of data using Electronic Data Interchange (EDI) Share product catalogs exclusively with wholesalers or those "in the trade" Collaborate with other companies on joint development efforts Jointly develop and use training programs with other companies Provide or access services provided by one company to a group of other companies, such

as an online banking application managed by one company on behalf of affiliated banks Share news of common interest exclusively with partner companies

An intranet is a private computer networkthat uses Internet Protocol technology to securelyshare any part of an organization's information ornetwork operating system within thatorganization. The term is used in contrast to internet, a network between organizations, andinstead refers to a network within an organization. Sometimes the term refers only to theorganization's internalwebsite, but may be a more extensive part of the organization's

information technology infrastructure. It may host multiple private websites and constitute animportant component and focal point of internal communication and collaboration.
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Uses

Increasingly, intranets are being used to deliver tools and applications, e.g., collaboration (tofacilitate working in groups and teleconferencing) or sophisticated corporate directories, salesandcustomer relationship managementtools,project managementetc., to advance productivity.

Intranets are also being used as corporate culture-change platforms. For example, large numbersof employees discussing key issues in an intranet forum application could lead to new ideas inmanagement, productivity, quality, and other corporate issues.

In large intranets, website traffic is often similar to public website traffic and can be betterunderstood by using web metrics software to track overall activity. User surveys also improveintranet website effectiveness. Larger businesses allow users within their intranet to accesspublic internet through firewall servers. They have the ability to screen messages coming andgoing keeping security intact.

When part of an intranet is made accessible to customers and others outside the business, thatpart becomes part of an extranet. Businesses can send private messages through the publicnetwork, using special encryption/decryption and other security safeguards to connect one part oftheir intranet to another.

Intranet user-experience, editorial, and technology teams work together to produce in-housesites. Most commonly, intranets are managed by the communications,HRorCIO departments oflarge organizations, or some combination of these.

Because of the scope and variety of content and the number of system interfaces, intranets ofmany organizations are much more complex than their respective public websites. Intranets and

their use are growing rapidly. According to the Intranet design annual 2007 fromNielsenNorman Group, the number of pages on participants' intranets averaged 200,000 over the years2001 to 2003 and has grown to an average of 6 million pages over 20052007.[2]

[edit] Benefits

Workforce productivity: Intranets can help users to locate and viewinformation faster and use applications relevant to their roles andresponsibilities. With the help of a web browser interface, users can accessdata held in any database the organization wants to make available, anytimeand - subject to security provisions - from anywhere within the company

workstations, increasing employees' ability to perform their jobs faster, moreaccurately, and with confidence that they have the right information. It alsohelps to improve the services provided to the users.

Time: Intranets allow organizations to distribute information to employees onan as-neededbasis; Employees may link to relevant information at theirconvenience, rather than being distracted indiscriminately by electronic mail.

Communication: Intranets can serve as powerful tools for communicationwithin an organization, vertically and horizontally. From a communicationsstandpoint, intranets are useful to communicate strategic initiatives that
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have a global reach throughout the organization. The type of information thatcan easily be conveyed is the purpose of the initiative and what the initiativeis aiming to achieve, who is driving the initiative, results achieved to date,and who to speak to for more information. By providing this information onthe intranet, staff have the opportunity to keep up-to-date with the strategicfocus of the organization. Some examples of communication would be chat,

email, and or blogs. A great real world example of where an intranet helped acompany communicate is when Nestle had a number of food processingplants in Scandinavia. Their central support system had to deal with anumber of queries every day.[3] When Nestle decided to invest in an intranet,they quickly realized the savings. McGovern says the savings from thereduction in query calls was substantially greater than the investment in theintranet.

Web publishing allows cumbersome corporate knowledge to be maintainedand easily accessed throughout the company using hypermedia and Webtechnologies. Examples include: employee manuals, benefits documents,company policies, business standards, newsfeeds, and even training, can beaccessed using common Internet standards (Acrobat files, Flash files, CGI

applications). Because each business unit can update the online copy of adocument, the most recent version is usually available to employees usingthe intranet.

Business operations and management: Intranets are also being used asa platform for developing and deploying applications to support businessoperations and decisions across the internetworked enterprise.

Cost-effective: Users can view information and data via web-browser ratherthan maintaining physical documents such as procedure manuals, internalphone list and requisition forms. This can potentially save the businessmoney on printing, duplicating documents, and the environment as well asdocument maintenance overhead. For example, Peoplesoft "derivedsignificant cost savings by shifting HR processes to the intranet".[3] McGoverngoes on to say the manual cost of enrolling in benefits was found to beUSD109.48 per enrollment. "Shifting this process to the intranet reduced thecost per enrollment to $21.79; a saving of 80 percent". Another company thatsaved money on expense reports was Cisco. "In 1996, Cisco processed54,000 reports and the amount of dollars processed was USD19 million".[3]

Enhance collaboration: Information is easily accessible by all authorisedusers, which enables teamwork.

Cross-platform capability: Standards-compliant web browsers areavailable for Windows, Mac, and UNIX.

Built for one audience: Many companies dictate computer specificationswhich, in turn, may allow Intranet developers to write applications that onlyhave to work on one browser (no cross-browser compatibility issues). Beingable to specifically address your "viewer" is a great advantage. SinceIntranets are user-specific (requiring database/network authentication priorto access), you know exactly who you are interfacing with and canpersonalize your Intranet based on role (job title, department) or individual("Congratulations Jane, on your 3rd year with our company!").

Promote common corporate culture: Every user has the ability to viewthe same information within the Intranet.
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Immediate updates: When dealing with the public in any capacity, laws,specifications, and parameters can change. Intranets make it possible toprovide your audience with "live" changes so they are kept up-to-date, whichcan limit a company's liability.

Supports a distributed computing architecture: The intranet can also belinked to a companys management information system, for example a time

keeping system.

Ethernet is a family offrame-based computer networkingtechnologies forlocal area networks(LAN). The name came from the physical concept of theether. It defines a number of wiring andsignaling standards for the Physical Layerof the OSI networking model as well as a commonaddressing format and Media Access Control at the Data Link Layer.

Ethernet is standardized as IEEE 802.3. The combination of thetwisted pair versions of Ethernetfor connecting end systems to the network, along with the fiber optic versions for site backbones,

is the most widespread wired LAN technology. It has been used from around 1980[1] to thepresent, largely replacing competing LAN standards such as token ring,FDDI, andARCNET.

An Ethernet LAN typically uses coaxial cable or special grades oftwisted pairwires. Ethernet isalso used in wireless LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable andcompete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD )protocol.

Repeaters and hubs

Main article: Ethernet hub

For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size.Somewhat larger networks could be built by using an Ethernet repeater. Initial repeaters had only2 ports, but they gave way to 4, 6, 8, and more ports. People recognized the advantages ofcabling in a star topology, primarily that a fault in one of the legs affects operation of only thestations attached to that leg.[citation needed]
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A twisted pair Cat-3 or Cat-5 cable is used to connect 10BASE-T Ethernet

Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with10BASE-T, was designed for point-to-point links only, and all termination was built into the

device. This changed repeaters from a specialist device used at the center of large networks to adevice that every twisted pair-based network with more than two machines had to use. The treestructure that resulted from this made Ethernet networks more reliable by preventing faults withone peer or its associated cable from affecting other devices on the network.[note 8]

Despite the physical star topology, repeater based Ethernet networks still use half-duplex andCSMA/CD, with only minimal activity by the repeater, primarily the Collision Enforcementsignal, in dealing with packet collisions. Every packet is sent to every port on the repeater, sobandwidth and security problems are not addressed. The total throughput of the repeater islimited to that of a single link, and all links must operate at the same speed.

[edit] Bridging and switching

Main articles: Ethernet switch andBridging (networking)

While repeaters could isolate some aspects ofEthernet segments, such as cable breakages, theystill forwarded all traffic to all Ethernet devices. This created practical limits on how manymachines could communicate on an Ethernet network. The entire network was one collisiondomain, and all hosts had to be able to detect collisions anywhere on the network. This limitedthe number of repeaters between the farthest nodes. Segments joined by repeaters had to alloperate at the same speed, making phased-in upgrades impossible.

To alleviate these problems, bridging was created to communicate at the data link layer whileisolating the physical layer. With bridging, only well-formed Ethernet packets are forwardedfrom one Ethernet segment to another; collisions and packet errors are isolated. Prior todiscovery of network devices on the different segments, Ethernet bridges (and switches) worksomewhat like Ethernet repeaters, passing all traffic between segments. However, as the bridgediscovers the addresses associated with each port, it forwards network traffic only to thenecessary segments, improving overall performance. Broadcasttraffic is still forwarded to allnetwork segments. Bridges also overcame the limits on total segments between two hosts andallowed the mixing of speeds, both of which became very important with the introduction ofFastEthernet.

Early bridges examined each packet one by one using software on a CPU, and some of themwere significantly slower than repeaters at forwarding traffic, especially when handling manyports at the same time.[citation needed] This was in part because the entire Ethernet packet would beread into a buffer, the destination address compared with an internal table of known MACaddresses, and a decision made as to whether to drop the packet or forward it to another or allsegments.
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In 1989, the networking company Kalpanaintroduced their EtherSwitch, the first Ethernetswitch.[note 9]This worked somewhat differently from an Ethernet bridge, in that only the headerof the incoming packet would be examined before it was either dropped or forwarded to anothersegment. This greatly reduced the forwarding latency and the processing load on the networkdevice. One drawback of this cut-throughswitching method was that packets that had been

corrupted would still be propagated through the network, so a jabbering station could continue todisrupt the entire network. The eventual remedy for this was a return to the original store andforward approach of bridging, where the packet would be read into a buffer on the switch in itsentirety, verified against its checksum and then forwarded, but using more powerful application-specific integrated circuits. Hence, the bridging is then done in hardware, allowing packets to beforwarded at full wire speed.

When a twisted pair or fiber link segment is used and neither end is connected to a repeater,full-duplex Ethernet becomes possible over that segment. In full-duplex mode, both devices cantransmit and receive to and from each other at the same time, and there is no collision domain.This doubles the aggregate bandwidth of the link and is sometimes advertised as double the link

speed (e.g., 200 Mbit/s).

[note 10]

The elimination of the collision domain for these connections alsomeans that all the link's bandwidth can be used by the two devices on that segment and thatsegment length is not limited by the need for correct collision detection.

Since packets are typically delivered only to the port they are intended for, traffic on a switchedEthernet is less public than on shared-medium Ethernet. Despite this, switched Ethernet shouldstill be regarded as an insecure network technology, because it is easy to subvert switchedEthernet systems by means such as ARP spoofing and MAC flooding.

The bandwidth advantages, the slightly better isolation of devices from each other, the ability toeasily mix different speeds of devices and the elimination of the chaining limits inherent in non-

switched Ethernet have made switched Ethernet the dominant network technology.

[17]

[edit] Advanced networking

Simple switched Ethernet networks, while a great improvement over repeater-based Ethernet,suffer from single points of failure, attacks that trick switches or hosts into sending data to amachine even if it is not intended for it, scalability and security issues with regard tobroadcastradiation and multicast traffic, and bandwidth choke points where a lot of traffic is forced down asingle link.[citation needed]

Advanced networking features in switches and routers combat these issues through a number of

means including spanning-tree protocolto maintain the active links of the network as a treewhile allowing physical loops for redundancy, port security and protection features such as MAClock down and broadcast radiation filtering, virtual LANs to keep different classes of usersseparate while using the same physical infrastructure, multilayer switching to route betweendifferent classes and link aggregationto add bandwidth to overloaded links and to provide somemeasure of redundancy.
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[edit] Varieties of EthernetMain article: Ethernet physical layer

The Ethernet physical layer evolved over a considerable time span and encompasses quite a fewphysical media interfaces and several magnitudes of speed. The most common forms used are

10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize Category 5 cablesand 8P8Cmodular connectors. They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively.Fiber opticvariants of Ethernet offer high performance, electrical isolation and distance (up to tens ofkilometers with some versions). In general, networkprotocol stacksoftware will work similarlyon all varieties.

An object database (also object-oriented database) is adatabase model in which information

is represented in the form ofobjects as used in object-oriented programming.

Example of an object-oriented model.[1]

Object databases are a niche field within the broaderDBMS market dominated by relationaldatabase management systems (RDBMS). Object databases have been considered since the early1980s and 1990s but they have made little impact on mainstream commercial data processing,though there is some usage in specialized areas.

Advantages and disadvantages

This section may contain original research. Please improve it by verifying

the claims made and adding references. Statements consisting only of

original research may be removed. More details may be available on the talk
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page. (October 2008)

The main benefit of creating a database with objects as data is speed. OODBMS are faster thanrelational DBMS because data isnt stored in relational rows and columns but as objects[8].

Objects have a many to many relationship and are accessed by the use of pointers. Pointers arelinked to objects to establish relationships. Another benefit of OODBMS is that it can beprogrammed with small procedural differences without affecting the entire system[9]. This is mosthelpful for those organizations that have data relationships that arent entirely clear or need tochange these relations to satisfy the new business requirements.

Benchmarks between OODBMSs and RDBMSs have shown that an OODBMS can be clearlysuperior for certain kinds of tasks. The main reason for this is that many operations areperformed using navigationalrather than declarative interfaces, and navigational access to data isusually implemented very efficiently by following pointers.

Compared to relational databases another major advantage of OODBMSs is that they do not needanyobject relational mapping layerand object marshaling to map the application object model tothe database object model. In RDBMS, this mapping is also source of the impedance mismatch,which does not occur when using OODBMS. Avoiding this layer also improves performance andsaves effort for implementation and maintenance.

Critics ofnavigational database-based technologies like ODBMS suggest that pointer-basedtechniques are optimized for very specific "search routes" or viewpoints; for general-purposequeries on the same information, pointer-based techniques will tend to be slower and moredifficult to formulate than relational. Thus, navigation appears to simplify specific known uses atthe expense of general, unforeseen, and varied future uses.[citation needed] However, with suitable

language support, direct object references may be maintained in addition to normalised, indexedaggregations, allowing both kinds of access; furthermore, a persistent language may indexaggregations on whatever its content elements return from a call to some arbitrary object accessmethod, rather than only on attribute value, which allows a query to 'drill down' into complexdata structures.

Other things that work against ODBMS seem to be the lack ofinteroperability with a greatnumber of tools/features that are taken for granted in the SQL world, including but not limited toindustry standard connectivity, reporting tools, OLAP tools, andbackupand recovery standards.[citation needed] Additionally, object databases lack a formal mathematical foundation, unlike therelational model, and this in turn leads to weaknesses in their query support. However, this

objection is offset by the fact that some ODBMSs fully support SQL in addition to navigationalaccess, e.g. Objectivity/SQL++, Matisse, and InterSystems CACH. Effective use may requirecompromises to keep both paradigms in sync.

In fact there is an intrinsic tension between the notion ofencapsulation, which hides data andmakes it available only through a published set of interface methods, and the assumptionunderlying much database technology, which is that data should be accessible to queries basedon data content rather than predefined access paths. Database-centric thinking tends to view the
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world through a declarative and attribute-driven viewpoint, while OOP tends to view the worldthrough a behavioral viewpoint, maintaining entity-identity independently of changing attributes.This is one of the many impedance mismatch issues surrounding OOP and databases.

Although some commentators have written off object database technology as a failure, the

essential arguments in its favor remain valid, and attempts to integrate database functionalitymore closely into object programming languages continue in both the research and the industrialcommunities.[citation

A distributed database management system ('DDBMS') is a software systemthat permits themanagement of a distributed databaseand makes the distribution transparent to the users. Adistributed database is a collection of multiple, logically interrelated databases distributed over acomputer network. Sometimes "distributed database system" is used to refer jointly to thedistributed database and the distributed DBMS.

[edit] Overview

Distributed database management systems is a software for managing databases stored onmultiple computers in a network. A distributed database is a set of databases stored on multiplecomputers that typically appears to applications on a single database. Consequently, anapplication can simultaneously access and modify the data in several databases in a network.

DDBMS is specially developed for heterogeneous database platforms, focusing mainly onheterogeneous database management systems (HDBMS).

Centralized Systems: A modern, general-purpose computer system consists of one to a few

CPUs & a number of device controllers that are connected through a common bus that

provides access to shared memory. The CPUs have local cache memories that store local

copies of parts of the memory, to speed up access to data. Each device controller is in

charge of a specific type of device (for example, a disk drive, an audio device, or a video

display). The CPUs & the device controllers can execute concurrently, competing for

memory access. Cache memory reduces the contention for memory access, since it reducesthe number of times that the CPU needs to access shared memory.

In a distributed database system, the database is stored on several computers. The

computers in a distributed system communicate with one another through various

communication media, such as high-speed networks or telephone lines. They do not share

main memory or disks. The computers in a distributed system may vary in size & function,

ranging from workstations up to mainframe systems.
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The main difference between centralized & distributed databases is that the distributed

databases are typically geographically separated, are separately administered, & have

slower interconnection. Also in distributed databases we differentiate between local &

global transactions. A local transaction is one that accesses data only from sites where the

transaction originated. A global transaction, on the other hand, is one that either accesses

data in a site different from the one at which the transaction was initiated, or accessed datain several different sites.

Advantages of distributed databases

Management of distributed data with different levels of transparency. Increase reliability and availability. Easier expansion. Reflects organizational structure database fragments are located in the

departments they relate to. Local autonomy a department can control the data about them (as they

are the ones familiar with it.) Protection of valuable data if there were ever a catastrophic event such as

a fire, all of the data would not be in one place, but distributed in multiplelocations.

Improved performance data is located near the site of greatest demand,and the database systems themselves are parallelized, allowing load on thedatabases to be balanced among servers. (A high load on one module of thedatabase won't affect other modules of the database in a distributeddatabase.)

Economics it costs less to create a network of smaller computers with thepower of a single large computer.

Modularity systems can be modified, added and removed from the

distributed database without affecting other modules (systems). Reliable transactions - Due to replication of database. Hardware, Operating System, Network, Fragmentation, DBMS, Replication

and Location Independence. Continuous operation...

Distributed Query processing. Distributed Transaction management.

Single site failure does not affect performance of system. All transactions follow A.C.I.D.property: a-atomicity, the transaction takes place as whole or not at all; c-consistency, maps one

consistent DB state to another; i-isolation, each transaction sees a consistent DB; d-durability,the results of a transaction must survive system failures. The Merge Replication Method used toconsolidate the data between databases.

[edit] Disadvantages of distributed databases

Complexity extra work must be done by the DBAs to ensure that thedistributed nature of the system is transparent. Extra work must also be done
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departments, projects, and so on, and physically distributed into offices,

plants, factories, where each unit maintains its own operational data. The

sharing ability of the data and the efficiency of

data access should be improved by the development of a distributeddatabase system that reflects this organizational structure, makes the data

in all units accessible, and stores data proximate to the location where it is

most frequently used.

computer virtual memory

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