Multithreading
The ability of an operating system to execute different parts of a program, called threads, simultaneously. The programmer must carefully design the program in such a way that all the threads can run at the same time without interfering with each other
Hyper-Threading
A technology developed by Intel that enables multithreaded software applications to execute threads in parallel on a single multi-core processor instead of processing threads in a linear fashion. Older systems took advantage of dual-processing threading in software by splitting instructions into multiple streams so that more than one processor could act upon them at once
thread
(1) In online discussions, a series of messages that have been posted as replies to each other. A single forum or conference typically contains many threads covering different subjects. By reading each message in a thread, one after the other, you can see how the discussion has evolved. You can start a new thread by posting a message that is not a reply to an earlier message.
(2) In programming, a part of a program that can execute independently of other parts. Operating systems that support multithreading enable programmers to design programs whose threaded parts can execute concurrently.
operating system
The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.
For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.
Operating systems can be classified as follows:
multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
multiprocessing : Supports running a program on more than one CPU.
multitasking : Allows more than one program to run concurrently.
multithreading : Allows different parts of a single program to run concurrently.
real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.
Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.
As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.
multiprocessing
(1) Refers to a computer system's ability to support more than one process (program) at the same time. Multiprocessing operating systems enable several programs to run concurrently. UNIX is one of the most widely used multiprocessing systems, but there are many others, including OS/2 for high-end PCs. Multiprocessing systems are much more complicated than single-process systems because the operating system must allocate resources to competing processes in a reasonable manner.
(2) Refers to the utilization of multiple CPUs in a single computer system. This is also called parallel processing.
parallel processing
The simultaneous use of more than one CPU to execute a program. Ideally, parallel processing makes a program run faster because there are more engines (CPUs) running it. In practice, it is often difficult to divide a program in such a way that separate CPUs can execute different portions without interfering with each other.
Most computers have just one CPU, but some models have several. There are even computers with thousands of CPUs. With single-CPU computers, it is possible to perform parallel processing by connecting the computers in a network. However, this type of parallel processing requires very sophisticated software called distributed processing software.
Note that parallel processing differs from multitasking, in which a single CPU executes several programs at once.
Parallel processing is also called parallel computing.
distributed processing
Refers to any of a variety of computer systems that use more than one computer, or processor, to run an application. This includes parallel processing, in which a single computer uses more than one CPU to execute programs. More often, however, distributed processing refers to local-area networks (LANs) designed so that a single program can run simultaneously at various sites. Most distributed processing systems contain sophisticated software that detects idle CPUs on the network and parcels out programs to utilize them.
Another form of distributed processing involves distributed databases, databases in which the data is stored across two or more computer systems. The database system keeps track of where the data is so that the distributed nature of the database is not apparent to users.
multitasking
The ability to execute more than one task at the same time, a task being a program. The terms multitasking and multiprocessing are often used interchangeably, although multiprocessing implies that more than one CPU is involved.
In multitasking, only one CPU is involved, but it switches from one program to another so quickly that it gives the appearance of executing all of the programs at the same time.
There are two basic types of multitasking: preemptive and cooperative. In preemptive multitasking, the operating system parcels out CPU time slices to each program. In cooperative multitasking, each program can control the CPU for as long as it needs it. If a program is not using the CPU, however, it can allow another program to use it temporarily. OS/2, Windows 95, Windows NT, the Amiga operating system and UNIX use preemptive multitasking, whereas Microsoft Windows 3.x and the MultiFinder (for Macintosh computers) use cooperative multitasking.
cooperative multitasking
A type of multitasking in which the process currently controlling the CPU must offer control to other processes. It is called cooperative because all programs must cooperate for it to work. If one program does not cooperate, it can hog the CPU. In contrast, preemptive multitasking forces applications to share the CPU whether they want to or not. Versions 8.0-9.2.2 of Macintosh OS and Windows 3.x operating systems are based on cooperative multitasking, whereas UNIX, Windows 95, Windows NT, OS/2, and later versions of Mac OS are based on preemptive multitasking
semaphore
A hardware or software flag. In multitasking systems, a semaphore is a variable with a value that indicates the status of a common resource. It's used to lock the resource that is being used. A process needing the resource checks the semaphore to determine the resource's status and then decides how to proceed.
deadlock
A condition that occurs when two processes are each waiting for the other to complete before proceeding. The result is that both processes hang. Deadlocks occur most commonly in multitasking and client/server environments. Ideally, the programs that are deadlocked, or the operating system, should resolve the deadlock, but this doesn't always happen.
A deadlock is also called a deadly embrace.
The 7 Layers of the OSI Model
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Application
(Layer 7) This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Presentation
(Layer 6) This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session
(Layer 5) This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport
(Layer 4) This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network
(Layer 3) This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Data Link
(Layer 2) At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical
(Layer 1) This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.
applet
A program designed to be executed from within another application. Unlike an application, applets cannot be executed directly from the operating system. With the growing popularity of OLE (object linking and embedding), applets are becoming more prevalent. A well-designed applet can be invoked from many different applications.
Web browsers, which are often equipped with Java virtual machines, can interpret applets from Web servers. Because applets are small in files size, cross-platform compatible, and highly secure (can't be used to access users' hard drives), they are ideal for small Internet applications accessible from a browser.
J2EE
Short for Java 2 Platform Enterprise Edition. J2EE is a platform-independent, Java-centric environment from Sun for developing, building and deploying Web-based enterprise applications online. The J2EE platform consists of a set of services, APIs, and protocols that provide the functionality for developing multitiered, Web-based applications.
Some of the key features and services of J2EE:
At the client tier, J2EE supports pure HTML, as well as Java applets or applications. It relies on Java Server Pages and servlet code to create HTML or other formatted data for the client.
Enterprise JavaBeans (EJBs) provide another layer where the platform's logic is stored. An EJB server provides functions such as threading, concurrency, security and memory management. These services are transparent to the author.
Java Database Connectivity (JDBC), which is the Java equivalent to ODBC, is the standard interface for Java databases.
The Java servlet API enhances consistency for developers without requiring a graphical user interface.
MAC Layer
The Media Access Control Layer is one of two sublayers that make up the Data Link Layer of the OSI model. The MAC layer is responsible for moving data packets to and from one Network Interface Card (NIC) to another across a shared channel.
See a breakdown of the seven OSI layers in the Quick Reference section of Webopedia.
The MAC sublayer uses MAC protocols to ensure that signals sent from different stations across the same channel don't collide.
Different protocols are used for different shared networks, such as Ethernets, Token Rings, Token Buses, and WANs.
flow control
(1) In communications, the process of adjusting the flow of data from one device to another to ensure that the receiving device can handle all of the incoming data. This is particularly important where the sending device is capable of sending data much faster than the receiving device can receive it.
There are many flow control mechanisms. One of the most common flow control protocols for asynchronous communication is called xon-xoff. In this case, the receiving device sends a an xoff message to the sending device when its buffer is full. The sending device then stops sending data. When the receiving device is ready to receive more data, it sends an xon signal.
Flow control can be implemented in hardware or software, or a combination of both.
(2) In programming, the statements and other constructs that control the order in which operations are executed. For example, common looping statements such as for…next and while are known as flow control statements. Branching statements, such as if…then are also part of a programming language's flow control mechanism.
100Base-T
A networking standard that supports data transfer rates up to 100 Mbps (100 megabits per second). 100BASE-T is based on the older Ethernet standard. Because it is 10 times faster than Ethernet, it is often referred to as Fast Ethernet. Officially, the 100BASE-T standard is IEEE 802.3u.
Like Ethernet, 100BASE-T is based on the CSMA/CD LAN access method. There are several different cabling schemes that can be used with 100BASE-T, including:
Telnet
(tel´net) (n.) A terminal emulation program for TCP/IP networks such as the Internet. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. This enables you to control the server and communicate with other servers on the network. To start a Telnet session, you must log in to a server by entering a valid username and password. Telnet is a common way to remotely control Web servers.
CSMA/CD
Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD. This standard enables devices to detect a collision. After detecting a collision, a device waits a random delay time and then attempts to re-transmit the message. If the device detects a collision again, it waits twice as long to try to re-transmit the message. This is known as exponential back off.
CSMA/CD is a type of contention protocol.
distributed processing
Refers to any of a variety of computer systems that use more than one computer, or processor, to run an application. This includes parallel processing, in which a single computer uses more than one CPU to execute programs. More often, however, distributed processing refers to local-area networks (LANs) designed so that a single program can run simultaneously at various sites. Most distributed processing systems contain sophisticated software that detects idle CPUs on the network and parcels out programs to utilize them.
Another form of distributed processing involves distributed databases, databases in which the data is stored across two or more computer systems. The database system keeps track of where the data is so that the distributed nature of the database is not apparent to users.
FTP
Short for File Transfer Protocol, the protocol for exchanging files over the Internet. FTP works in the same way as HTTP for transferring Web pages from a server to a user's browser and SMTP for transferring electronic mail across the Internet in that, like these technologies, FTP uses the Internet's TCP/IP protocols to enable data transfer.
FTP is most commonly used to download a file from a server using the Internet or to upload a file to a server (e.g., uploading a Web page file to a server).
TCP/IP
(pronounced as separate letters) Short for Transmission Control Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, also support TCP/IP.
switch
(1) In networks, a device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or, in the case of Ethernet networks, switched Ethernet LANs.
(2) A small lever or button. The switches on the back of printers and on expansion boards are called DIP switches. A switch that has just two positions is called a toggle switch.
(3) Another word for option or parameter -- a symbol that you add to a command to modify the command's behavior.
protocol
An agreed-upon format for transmitting data between two devices. The protocol determines the following:
the type of error checking to be used
data compression method, if any
how the sending device will indicate that it has finished sending a message
how the receiving device will indicate that it has received a message
There are a variety of standard protocols from which programmers can choose. Each has particular advantages and disadvantages; for example, some are simpler than others, some are more reliable, and some are faster.
From a user's point of view, the only interesting aspect about protocols is that your computer or device must support the right ones if you want to communicate with other computers. The protocol can be implemented either in hardware or in software.
HTTP
Short for HyperText Transfer Protocol, the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page.
The other main standard that controls how the World Wide Web works is HTML, which covers how Web pages are formatted and displayed.
HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it. This is the main reason that it is difficult to implement Web sites that react intelligently to user input. This shortcoming of HTTP is being addressed in a number of new technologies, including ActiveX, Java, JavaScript and cookies.
Also see The Difference Between FTP and HTTP in the Did You Know . . . ? section of Webopedia.
SMTP
(pronounced as separate letters) Short for Simple Mail Transfer Protocol, a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.
routing
(n.) In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path.
Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.
An application that runs on a personal computer or workstation and enables you to send, receive and organize e-mail. It's called a client because e-mail systems are based on a client-server architecture. Mail is sent from many clients to a central server, which re-routes the mail to its intended destination.
The Difference Between FTP and HTTP
File Transfer Protocol, or FTP, is a protocol used to upload files from a workstation to a FTP server or download files from a FTP server to a workstation. It is the way that files get transferred from one device to another in order for the files to be available on the Internet. When ftp appears in a URL it means that the user is connecting to a file server and not a Web server and that some form of file transfer is going to take place. Most FTP servers require the user to log on to the server in order to transfer files.
In contrast, Hyper Text Transfer Protocol, or HTTP, is a protocol used to transfer files from a Web server onto a browser in order to view a Web page that is on the Internet. Unlike FTP, where entire files are transferred from one device to another and copied into memory, HTTP only transfers the contents of a web page into a browser for viewing. FTP is a two-way system as files are transferred back and forth between server and workstation. HTTP is a one-way system as files are transported only from the server onto the workstation's browser. When http appears in a URL it means that the user is connecting to a Web server and not a file server. The files are transferred but not downloaded, therefore not copied into the memory of the receiving device.
Web server
A computer that delivers (serves up) Web pages. Every Web server has an IP address and possibly a domain name. For example, if you enter the URL http://www.pcwebopedia.com/index.html in your browser, this sends a request to the server whose domain name is pcwebopedia.com. The server then fetches the page named index.html and sends it to your browser.
Any computer can be turned into a Web server by installing server software and connecting the machine to the Internet. There are many Web server software applications, including public domain software from NCSA and Apache, and commercial packages from Microsoft, Netscape and others.
gateway
(n.) (1) A node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.
In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.
(2) A computer system located on earth that switches data signals and voice signals between satellites and terrestrial networks.
(3) An earlier term for router, though now obsolete in this sense as router is commonly used
ActiveX control
A control using ActiveX technologies. An ActiveX control can be automatically downloaded and executed by a Web browser. ActiveX is not a programming language, but rather a set of rules for how applications should share information. Programmers can develop ActiveX controls in a variety of languages, including C, C++, Visual Basic, and Java.
An ActiveX control is similar to a Java applet. Unlike Java applets, however, ActiveX controls have full access to the Windows operating system. This gives them much more power than Java applets, but with this power comes a certain risk that the applet may damage software or data on your machine. To control this risk, Microsoft developed a registration system so that browsers can identify and authenticate an ActiveX control before downloading it. Another difference between Java applets and ActiveX controls is that Java applets can be written to run on all platforms, whereas ActiveX controls are currently limited to Windows environments.
Related to ActiveX is a scripting language called VBScript that enables Web authors to embed interactive elements in HTML documents.
platform
The underlying hardware or software for a system. For example, the platform might be an Intel 80486 processor running DOS Version 6.0. The platform could also be UNIX machines on an Ethernet network.
The platform defines a standard around which a system can be developed. Once the platform has been defined, software developers can produce appropriate software and managers can purchase appropriate hardware and applications. The term is often used as a synonym of operating system.
The term cross-platform refers to applications, formats, or devices that work on different platforms. For example, a cross-platform programming environment enables a programmer to develop programs for many platforms at once.
script
Another term for macro or batch file, a script is a list of commands that can be executed without user interaction. A script language is a simple programming language with which you can write scripts.
Apple Computer uses the term script to refer to programs written in its HyperCard or AppleScript language.
Application Server -
A software program that handles all application operations between users and an organization's backend business applications or databases. Application servers are typically used for complex transaction-based applications. To support high-end needs, an application server has to have built-in redundancy, monitors for high-availability, high-performance distributed application services and support for complex database access. These are the leading Java App Servers:
Apache/Jakarta Tomcat (the open source, and official reference implementation of the Java Servlet and JSP technologies) BEA WebLogic IBM WebSphere iPlanet Application Server Oracle Application Server
Enterprise JavaBeans (EJB) - A Java API developed by Sun Microsystems that defines a component architecture for multi-tier client/server systems. EJB systems allow developers to focus on the actual business architecture of the model, rather than worry about endless amounts of programming and coding needed to connect all the working parts. This task is left to EJB server vendors. Developers just design (or purchase) the needed EJB components and arrange them on the server. Because EJB systems are written in Java, they are platform independent. Being object oriented, they can be implemented into existing systems with little or no recompiling and configuring. (from Webopedia)
Java - A high-level object-oriented programming language developed by Sun Microsystems. Java is similar to C++, but simplified to eliminate language features that cause common programming errors. Java source code files (files with a .java extension) are compiled into a format called bytecode (files with a .class extension), which can then be executed by a Java interpreter. Compiled Java code can run on most computers because Java interpreters and runtime environments, known as Java Virtual Machines (VMs), exist for most operating systems, including UNIX, the Macintosh OS, and Windows. Bytecode can also be converted directly into machine language instructions by a just-in-time compiler (JIT).
Java 2 Platform Enterprise Edition (J2EE) - J2EE is a platform-independent, Java-centric environment from Sun for developing, building and deploying Web-based enterprise applications online. The J2EE platform consists of a set of services, APIs, and protocols that provide the functionality for developing multitiered, Web-based applications.
Java Applet - A program designed to be executed from within another application. Unlike an application, applets cannot be executed directly from the operating system. Applets became more prevalent with the popularity growth of OLE (object linking and embedding). A well-designed applet can be invoked from many different applications. Web browsers, which are often equipped with Java virtual machines, can interpret applets from Web servers. Because applets are small in file size, cross-platform compatible, and highly secure (can't be used to access users' hard drives), they are ideal for small Internet applications accessible from a browser. (from Webopedia)
Java Beans - A specification developed by Sun Microsystems that defines how Java objects interact. An object that conforms to this specification is called a JavaBean, and is similar to an ActiveX control. It can be used by any application that understands the JavaBeans format. The principal difference between ActiveX controls and JavaBeans are that ActiveX controls can be developed in any programming language but executed only on a Windows platform, whereas JavaBeans can be developed only in Java, but can run on any platform. (from Webopedia)
Java Database Connectivity (JDBC) - A Java API that enables Java programs to execute SQL statements. This allows Java programs to interact with any SQL-compliant database. Since nearly all relational database management systems (DBMSs) support SQL, and because Java itself runs on most platforms, JDBC makes it possible to write a single database application that can run on different platforms and interact with different DBMSs. (from Webopedia)
Java Server Pages (JSP) - A server-side technology that serve as an extension to the Java servlet technology that was developed by Sun as an alternative to Microsoft's ASPs (Active Server Pages). JSPs have dynamic scripting capability that works in tandem with HTML code, separating the page logic from the static elements -- the actual design and display of the page. Embedded in the HTML page, the Java source code and its extensions help make the HTML more functional, being used in dynamic database queries, for example. JSPs are not restricted to any specific platform or server. (from Webopedia)
Java Servlet - An applet that runs on a server. The term usually refers to a Java applet that runs within a Web server environment. This is analogous to a Java applet that runs within a Web browser environment. Java servlets are becoming increasingly popular as an alternative to CGI programs. The biggest difference between the two is that a Java applet is persistent. This means that once it is started, it stays in memory and can fulfill multiple requests. In contrast, a CGI program disappears once it has fulfilled a request. The persistence of Java applets makes them faster because there's no wasted time in setting up and tearing down the process. (from Webopedia)
macro
(1) A symbol, name, or key that represents a list of commands, actions, or keystrokes. Many programs allow you to create macros so that you can enter a single character or word to perform a whole series of actions. Suppose, for example, that you are editing a file and want to indent every third line five spaces. If your word processor supports macros, you can create one that consists of the following keystrokes:
Move Cursor to Beginning of Line
Move Cursor Down 1 Line
Move Cursor Down 1 Line
Move Cursor Down 1 Line
Insert 5 Spaces
Now you can enter the name of the macro, and the word processor will perform all these commands at once.
You can also use macros to enter words or phrases that you use frequently. For example, you could define a macro to contain all the keystrokes necessary to begin a letter -- your name, address, and a code that inserts the current date. Then, whenever you write a letter, you just press the macro key to include the letter header.
In a way, macros are like simple programs or batch files. Some applications support sophisticated macros that even allow you to use variables and flow control structures such as loops.
(2) In dBASE programs, a macro is a variable that points to another variable where the data is actually stored. In most other applications, this would be called a link.
batch file Last modified: Monday, September 01, 1997
A file that contains a sequence, or batch, of commands. Batch files are useful for storing sets of commands that are always executed together because you can simply enter the name of the batch file instead of entering each command individually.
In DOS systems, batch files end with a.BAT extension. For example, the following DOS batch file prints the date and time and sets the prompt to GO>:
date
time
prompt [GO>]
Whenever you boot a DOS -based computer, the system automatically executes the batch file named AUTOEXEC.BAT, if it exists.Many operating systems use the terms command file or shell script in place of batch file.
cross-platform Last modified: Sunday, September 01, 1996
Refers to the capability of software or hardware to run identically on different platforms. Many applications for Windows and the Macintosh, for example, now produce binary-compatible files, which means that users can switch from one platform to the other without converting their data to a new format.
Cross-platform computing is becoming increasingly important as local-area networks become better at linking machines of different types.
ISP
Short for Internet Service Provider, a company that provides access to the Internet. For a monthly fee, the service provider gives you a software package, username, password and access phone number. Equipped with a modem, you can then log on to the Internet and browse the World Wide Web and USENET, and send and receive e-mail.
In addition to serving individuals, ISPs also serve large companies, providing a direct connection from the company's networks to the Internet. ISPs themselves are connected to one another through Network Access Points (NAPs).
ISPs are also called IAPs (Internet Access Providers).
proxy server
A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server.
Proxy servers have two main purposes:
Improve Performance: Proxy servers can dramatically improve performance for groups of users. This is because it saves the results of all requests for a certain amount of time. Consider the case where both user X and user Y access the World Wide Web through a proxy server. First user X requests a certain Web page, which we'll call Page 1. Sometime later, user Y requests the same page. Instead of forwarding the request to the Web server where Page 1 resides, which can be a time-consuming operation, the proxy server simply returns the Page 1 that it already fetched for user X. Since the proxy server is often on the same network as the user, this is a much faster operation. Real proxy servers support hundreds or thousands of users. The major online services such as Compuserve and America Online, for example, employ an array of proxy servers.
Filter Requests: Proxy servers can also be used to filter requests. For example, a company might use a proxy server to prevent its employees from accessing a specific set of Web sites.
See the Server Types page in the quick reference section of Webopedia for a comparison of server types.
World Wide Web
A system of Internet servers that support specially formatted documents. The documents are formatted in a markup language called HTML (HyperText Markup Language) that supports links to other documents, as well as graphics, audio, and video files. This means you can jump from one document to another simply by clicking on hot spots. Not all Internet servers are part of the World Wide Web. There are several applications called Web browsers that make it easy to access the World Wide Web; Two of the most popular being Netscape Navigator and Microsoft's Internet Explorer.
World Wide Web is not synonymous with the Internet.
Compatible
(n) Indicates that a product can work with or is equivalent to another, better-known product. The term is often used as a shorthand for IBM-compatible PC , a computer that is compatible with an IBM PC. Another term for a compatible is clone. (adj) The ability of one device or program to work with another device or program. The term compatible implies different degrees of partnership. For example, a printer and a computer are said to be compatible if they can be connected to each other. An IBM compatible PC, on the other hand, is a computer that can run the same software as an IBM PC. Compatibility of two devices, such as printers, usually means that they react to software commands in the same way. Some printers achieve compatibility by tricking the software into believing that the printer is a different machine. This is called emulation. Be aware, however, that hardware compatibility does not always extend to expansion slots. For example, two compatible printers may not accept the same font cartridges. Complete hardware compatibility is denoted by the term plug compatible.Software products are compatible if they use the same data formats. For example, many programs are compatible with dBASE. This means that the files they produce can easily be transformed into a dBASE database or that they can import dBASE files.
API
Abbreviation of application program interface, a set of routines, protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer puts the blocks together. Most operating environments, such as MS-Windows, provide an API so that programmers can write applications consistent with the operating environment. Although APIs are designed for programmers, they are ultimately good for users because they guarantee that all programs using a common API will have similar interfaces. This makes it easier for users to learn new programs.
ODBC
(pronounced as separate letters) Short for Open DataBase Connectivity, a standard database access method developed by the SQL Access group in 1992. The goal of ODBC is to make it possible to access any data from any application, regardless of which database management system (DBMS) is handling the data. ODBC manages this by inserting a middle layer, called a database driver , between an application and the DBMS. The purpose of this layer is to translate the application's data queries into commands that the DBMS understands. For this to work, both the application and the DBMS must be ODBC-compliant -- that is, the application must be capable of issuing ODBC commands and the DBMS must be capable of responding to them. Since version 2.0, the standard supports SAG SQL.
servlet
A small program that runs on a server. The term usually refers to a Java applet that runs within a Web server environment. This is analogous to a Java applet that runs within a Web browser environment.
Java servlets are becoming increasingly popular as an alternative to CGI programs. The biggest difference between the two is that a Java applet is persistent. This means that once it is started, it stays in memory and can fulfill multiple requests. In contrast, a CGI program disappears once it has fulfilled a request. The persistence of Java applets makes them faster because there's no wasted time in setting up and tearing down the process.
java
A high-level programming language developed by Sun Microsystems. Java was originally called OAK, and was designed for handheld devices and set-top boxes. Oak was unsuccessful so in 1995 Sun changed the name to Java and modified the language to take advantage of the burgeoning World Wide Web.
Java is an object-oriented language similar to C++, but simplified to eliminate language features that cause common programming errors. Java source code files (files with a .java extension) are compiled into a format called bytecode (files with a .class extension), which can then be executed by a Java interpreter. Compiled Java code can run on most computers because Java interpreters and runtime environments, known as Java Virtual Machines (VMs), exist for most operating systems, including UNIX, the Macintosh OS, and Windows. Bytecode can also be converted directly into machine language instructions by a just-in-time compiler (JIT).
Java is a general purpose programming language with a number of features that make the language well suited for use on the World Wide Web. Small Java applications are called Java applets and can be downloaded from a Web server and run on your computer by a Java-compatible Web browser, such as Netscape Navigator or Microsoft Internet Explorer.
database management system Last modified: Wednesday, December 03, 2003
A collection of programs that enables you to store, modify, and extract information from a database. There are many different types of DBMSs, ranging from small systems that run on personal computers to huge systems that run on mainframes. The following are examples of database applications:
computerized library systems
automated teller machines
flight reservation systems
computerized parts inventory systems
From a technical standpoint, DBMSs can differ widely. The terms relational, network, flat, and hierarchical all refer to the way a DBMS organizes information internally. The internal organization can affect how quickly and flexibly you can extract information.
Requests for information from a database are made in the form of a query, which is a stylized question. For example, the query SELECT ALL WHERE NAME = "SMITH" AND AGE > 35
requests all records in which the NAME field is SMITH and the AGE field is greater than 35. The set of rules for constructing queries is known as a query language. Different DBMSs support different query languages, although there is a semi-standardized query language called SQL (structured query language). Sophisticated languages for managing database systems are called fourth-generation languages, or 4GLs for short.
The information from a database can be presented in a variety of formats. Most DBMSs include a report writer program that enables you to output data in the form of a report. Many DBMSs also include a graphics component that enables you to output information in the form of graphs and charts.
SQL
Abbreviation of structured query language, and pronounced either see-kwell or as separate letters. SQL is a standardized query language for requesting information from a database. The original version called SEQUEL (structured English query language) was designed by an IBM research center in 1974 and 1975. SQL was first introduced as a commercial database system in 1979 by Oracle Corporation.
Historically, SQL has been the favorite query language for database management systems running on minicomputers and mainframes. Increasingly, however, SQL is being supported by PC database systems because it supports distributed databases (databases that are spread out over several computer systems). This enables several users on a local-area network to access the same database simultaneously.
Although there are different dialects of SQL, it is nevertheless the closest thing to a standard query language that currently exists. In 1986, ANSI approved a rudimentary version of SQL as the official standard, but most versions of SQL since then have included many extensions to the ANSI standard. In 1991, ANSI updated the standard. The new standard is known as SAG SQL.
database
(1) Often abbreviated DB. A collection of information organized in such a way that a computer program can quickly select desired pieces of data. You can think of a database as an electronic filing system.
Traditional databases are organized by fields, records, and files. A field is a single piece of information; a record is one complete set of fields; and a file is a collection of records. For example, a telephone book is analogous to a file. It contains a list of records, each of which consists of three fields: name, address, and telephone number.
An alternative concept in database design is known as Hypertext. In a Hypertext database, any object, whether it be a piece of text, a picture, or a film, can be linked to any other object. Hypertext databases are particularly useful for organizing large amounts of disparate information, but they are not designed for numerical analysis.
To access information from a database, you need a database management system (DBMS). This is a collection of programs that enables you to enter, organize, and select data in a database.
(2) Increasingly, the term database is used as shorthand for database management system.
CGI Last modified: Tuesday, March 02, 2004
Abbreviation of Common Gateway Interface, a specification for transferring information between a World Wide Web server and a CGI program. A CGI program is any program designed to accept and return data that conforms to the CGI specification. The program could be written in any programming language, including C, Perl, Java, or Visual Basic.
CGI programs are the most common way for Web servers to interact dynamically with users. Many HTML pages that contain forms, for example, use a CGI program to process the form's data once it's submitted. Another increasingly common way to provide dynamic feedback for Web users is to include scripts or programs that run on the user's machine rather than the Web server. These programs can be Java applets, Java scripts, or ActiveX controls. These technologies are known collectively as client-side solutions, while the use of CGI is a server-side solution because the processing occurs on the Web server.
One problem with CGI is that each time a CGI script is executed, a new process is started. For busy Web sites, this can slow down the server noticeably. A more efficient solution, but one that it is also more difficult to implement, is to use the server's API, such as ISAPI or NSAPI. Another increasingly popular solution is to use Java servlets.
mainframe
A very large and expensive computer capable of supporting hundreds, or even thousands, of users simultaneously. In the hierarchy that starts with a simple microprocessor (in watches, for example) at the bottom and moves to supercomputers at the top, mainframes are just below supercomputers. In some ways, mainframes are more powerful than supercomputers because they support more simultaneous programs. But supercomputers can execute a single program faster than a mainframe.
10.2 Pointers and Arrays; Pointer Arithmetic
[This section corresponds to K&R Sec. 5.3]
Pointers do not have to point to single variables. They can also point at the cells of an array. For example, we can write
int *ip;
int a[10];
ip = &a[3];
and we would end up with ip pointing at the fourth cell of the array a (remember, arrays are 0-based, so a[0] is the first cell). We could illustrate the situation like this:
We'd use this ip just like the one in the previous section: *ip gives us what ip points to, which in this case will be the value in a[3].
Once we have a pointer pointing into an array, we can start doing pointer arithmetic. Given that ip is a pointer to a[3], we can add 1 to ip:
ip + 1
What does it mean to add one to a pointer? In C, it gives a pointer to the cell one farther on, which in this case is a[4]. To make this clear, let's assign this new pointer to another pointer variable:
ip2 = ip + 1;
Now the picture looks like this:
If we now do
*ip2 = 4;
we've set a[4] to 4. But it's not necessary to assign a new pointer value to a pointer variable in order to use it; we could also compute a new pointer value and use it immediately:
*(ip + 1) = 5;
In this last example, we've changed a[4] again, setting it to 5. The parentheses are needed because the unary ``contents of'' operator * has higher precedence (i.e., binds more tightly than) the addition operator. If we wrote *ip + 1, without the parentheses, we'd be fetching the value pointed to by ip, and adding 1 to that value. The expression *(ip + 1), on the other hand, accesses the value one past the one pointed to by ip.
Given that we can add 1 to a pointer, it's not surprising that we can add and subtract other numbers as well. If ip still points to a[3], then
*(ip + 3) = 7;
sets a[6] to 7, and
*(ip - 2) = 4;
sets a[1] to 4.
Up above, we added 1 to ip and assigned the new pointer to ip2, but there's no reason we can't add one to a pointer, and change the same pointer:
ip = ip + 1;
Now ip points one past where it used to (to a[4], if we hadn't changed it in the meantime). The shortcuts we learned in a previous chapter all work for pointers, too: we could also increment a pointer using
ip += 1;
or
ip++;
Of course, pointers are not limited to ints. It's quite common to use pointers to other types, especially char. Here is the innards of the mystrcmp function we saw in a previous chapter, rewritten to use pointers. (mystrcmp, you may recall, compares two strings, character by character.)
char *p1 = &str1[0], *p2 = &str2[0];
while(1)
{
if(*p1 != *p2)
return *p1 - *p2;
if(*p1 == '\0' || *p2 == '\0')
return 0;
p1++;
p2++;
}
The autoincrement operator ++ (like its companion, --) makes it easy to do two things at once. We've seen idioms like a[i++] which accesses a[i] and simultaneously increments i, leaving it referencing the next cell of the array a. We can do the same thing with pointers: an expression like *ip++ lets us access what ip points to, while simultaneously incrementing ip so that it points to the next element. The preincrement form works, too: *++ip increments ip, then accesses what it points to. Similarly, we can use notations like *ip-- and *--ip.
As another example, here is the strcpy (string copy) loop from a previous chapter, rewritten to use pointers:
char *dp = &dest[0], *sp = &src[0];
while(*sp != '\0')
*dp++ = *sp++;
*dp = '\0';
(One question that comes up is whether the expression *p++ increments p or what it points to. The answer is that it increments p. To increment what p points to, you can use (*p)++.)
When you're doing pointer arithmetic, you have to remember how big the array the pointer points into is, so that you don't ever point outside it. If the array a has 10 elements, you can't access a[50] or a[-1] or even a[10] (remember, the valid subscripts for a 10-element array run from 0 to 9). Similarly, if a has 10 elements and ip points to a[3], you can't compute or access ip + 10 or ip - 5. (There is one special case: you can, in this case, compute, but not access, a pointer to the nonexistent element just beyond the end of the array, which in this case is &a[10]. This becomes useful when you're doing pointer comparisons, which we'll look at next.)
Using an array name as a pointer
An array name is really a pointer to the first element of the array. For example, the following is legal.
int b[100]; // b is an array of 100 ints.
int* p; // p is a pointer to an int.
p = b; // Assigns the address of first element of b to p.
p = &b[0]; // Exactly the same assignment as above.
Array name is a const pointer
When you declare an array, the name is a pointer, which cannot be altered. In the previous example, you could never make this assignment.
p = b; // Legal -- p is not a constant.
b = p; // ILLEGAL because b is a constant, altho the correct type.
Pointer arithmetic
"Meaningful" arithmetic operations are allowed on pointers.
· Add or subtract integers to/from a pointer. The result is a pointer. Subtract two pointers to the same type. The result is an int. Multiplying, adding two pointers, etc. don't make sense.
Pointer addition and element size
When you add an integer to a pointer, the integer is multiplied by the element size of the type that the pointer points to.
// Assume sizeof(int) is 4.
int b[100]; // b is an array of 100 ints.
int* p; // p is a a pointer to an int.
p = b; // Assigns address of first element of b. Ie, &b[0]
p = p + 1; // Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]
Fifth Normal Form (5NF)
The normal forms discussed so far required that the given relation R if not in the given normal form be decomposed in two relations to meet the requirements of the normal form. In some rare cases, a relation can have problems like redundant information and update anomalies because of it but cannot be decomposed in two relations to remove the problems. In such cases it may be possible to decompose the relation in three or more relations using the 5NF.
The fifth normal form deals with join-dependencies which is a generalisation of the MVD. The aim of fifth normal form is to have relations that cannot be decomposed further. A relation in 5NF cannot be constructed from several smaller relations.
A relation R satisfies join dependency (R1, R2, ..., Rn) if and only if R is equal to the join of
R1, R2, ..., Rn where Ri are subsets of the set of attributes of R.
A relation R is in 5NF (or project-join normal form, PJNF) if for all join dependencies at least one of the following holds.
(a) (R1, R2, ..., Rn) is a trivial join-dependency (that is, one of Ri is R)
(b) Every Ri is a candidate key for R.
An example of 5NF can be provided by the example below that deals with departments, subjects and students.
dept subject student
Comp. Sc.
Mathematics
Comp. Sc.
Comp. Sc.
Physics
Chemistry CP1000
MA1000
CP2000
CP3000
PH1000
CH2000 John Smith
John Smith
Arun Kumar
Reena Rani
Raymond Chew
Albert Garcia
The above relation says that Comp. Sc. offers subjects CP1000, CP2000 and CP3000 which are taken by a variety of students. No student takes all the subjects and no subject has all students enrolled in it and therefore all three fields are needed to represent the information.
The above relation does not show MVDs since the attributes subject and student are not independent; they are related to each other and the pairings have significant information in them. The relation can therefore not be decomposed in two relations
(dept, subject), and
(dept, student)
without loosing some important information. The relation can however be decomposed in the following three relations
(dept, subject), and
(dept, student)
(subject, student)
and now it can be shown that this decomposition is lossless.
Multivalued Normalisation -Fourth Normal Form
We have considered an example of Programmer(Emp name, qualification, languages) and discussed the problems that may arise if the relation is not normalised further. We also saw how the relation could be decomposed into P1(Emp name, qualifications) and P2(Emp name, languages) to overcome these problems. The decomposed relations are in fourth normal form (4NF) which we shall now define.
We are now ready to define 4NF. A relation R is in 4NF if, whenever a multivalued dependency
X -> Y holds then either
(a) the dependency is trivial, or
(b) X is a candidate key for R.
As noted earlier, the dependency X ->> ø or X ->> Y in a relation R (X, Y) is trivial since they must hold for all R (X, Y). Similarly (X, Y) -> Z must hold for all relations R (X, Y, Z) with only three attributes.
In fourth normal form, we have a relation that has information about only one entity. If a relation has more than one multivalue attribute, we should decompose it to remove difficulties with multivalued facts.
Intuitively R is in 4NF if all dependencies are a result of keys. When multivalued dependencies exist, a relation should not contain two or more independent multivalued attributes. The decomposition of a relation to achieve 4NF would normally result in not only reduction of redundancies but also avoidance of anomalies.
Single-Valued Dependencies
Initially Codd (1972) presented three normal forms (1NF, 2NF and 3NF) all based on functional dependencies among the attributes of a relation. Later Boyce and Codd proposed another normal form called the Boyce-Codd normal form (BCNF). The fourth and fifth normal forms are based on multivalue and join dependencies and were proposed later.
The primary objective of normalization is to avoid some of the anomalies that we discussed at the beginning of the chapter.
Example Normalisation
Un-Normalised Data
ORDER DETAILS 1
WID-008
Green Widget
Data is presented in a single column. In tis caste the following data is contained:
Quantity, Item ID and item Description. Since multiple unrelated values are stored in a column, this data is considered un-normalised.
First normal Form
ORDER DETAILS ItemCode Description Quantity
WID-008 Green Widget 1
Data is now in First Normal Form (1NF). Data items have been put in their own columns, and the columns furnished with a description. For data to be in 1NF each piece of information should be in its own column,and each row of data should contain the same columns.
Second Normal Form
ORDER DETAILS OrderID ItemCode Description Quantity
2345 WID-008 Green Widget 1
Data is now in Second Normal Form (2NF). 2NF is based on 1NF, with the addition that a unique identifier is added to each row - In this case the OrderID value. In most cases this unique value is a counter, rather than being related to the data. Each non-key attribute should be functionally dependent on the primary key for the table.
Third Normal Form
ORDER DETAILS OrderID ItemCode Quantity
2345 WID-008 1
ITEM DETAILS ItemCode Description
WID-008 Green Widget
Data is now in Third Normal Form (3NF). All attributes not dependent on the primary key should be moved to a separate table. Repetition of data is also eliminated. In this case, the tables are linked by the ItemCode attribute.
First Normal Form (1NF)
A table satisfying the properties of a relation is said to be in first normal form. As discussed in an earlier chapter, a relation cannot have multivalued or composite attributes. This is what the 1NF requires.
A relation is in 1NF if and only if all underlying domains contain atomic values only.
The first normal form deals only with the basic structure of the relation and does not resolve the problems of redundant information or the anomalies discussed earlier. All relations discussed in these notes are in 1NF.
For example consider the following example relation:
student (sno, sname, dob)
Add some other attributes so it has anomalies and is not in 2NF
The attribute dob is the date of birth and the primary key of the relation is sno with the functional dependencies sno -> sname and sno -> dob. The relation is in 1NF as long as dob is considered an atomic value and not consisting of three components (day, month, year). The above relation of course suffers from all the anomalies that we have discussed earlier and needs to be normalized. (add example with date of birth)
Second Normal Form (2NF)
The second normal form attempts to deal with the problems that are identified with the relation above that is in 1NF. The aim of second normal form is to ensure that all information in one relation is only about one thing.
A relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation.
To understand the above definition of 2NF we need to define the concept of key attributes. Each attribute of a relation that participates in at least one candidate key of is a key attribute of the relation. All other attributes are called non-key.
The concept of 2NF requires that all attributes that are not part of a candidate key be fully dependent on each candidate key. If we consider the relation
student (sno, sname, cno, cname)
and the functional dependencies
sno -> sname
cno -> cname
and assume that (sno, cno) is the only candidate key (and therefore the primary key), the relation is not in 2NF since sname and cname are not fully dependent on the key. The above relation suffers from the same anomalies and repetition of information as discussed above since sname and cname will be repeated. To resolve these difficulties we could remove those attributes from the relation that are not fully dependent on the candidate keys of the relations. Therefore we decompose the relation into the following projections of the original relation:
S1 (sno, sname)
S2 (cno, cname)
SC (sno, cno)
Use an example that leaves one relation in 2NF but not in 3NF.
We may recover the original relation by taking the natural join of the three relations.
If however we assume that sname and cname are unique and therefore we have the following candidate keys
(sno, cno)
(sno, cname)
(sname, cno)
(sname, cname)
The above relation is now in 2NF since the relation has no non-key attributes. The relation still has the same problems as before but it then does satisfy the requirements of 2NF. Higher level normalization is needed to resolve such problems with relations that are in 2NF and further normalization will result in decomposition of such relations.
Third Normal Form (3NF)
Although transforming a relation that is not in 2NF into a number of relations that are in 2NF removes many of the anomalies that appear in the relation that was not in 2NF, not all anomalies are removed and further normalization is sometime needed to ensure further removal of anomalies. These anomalies arise because a 2NF relation may have attributes that are not directly related to the thing that is being described by the candidate keys of the relation. Let us first define the 3NF.
A relation R is in third normal form if it is in 2NF and every non-key attribute of R is non-transitively dependent on each candidate key of R.
To understand the third normal form, we need to define transitive dependence which is based on one of Armstrong's axioms. Let A, B and C be three attributes of a relation R such that A -> B and B -> C. From these FDs, we may derive A -> C. As noted earlier, this dependence A -> C is transitive.
The 3NF differs from the 2NF in that all non-key attributes in 3NF are required to be directly dependent on each candidate key of the relation. The 3NF therefore insists, in the words of Kent (1983) that all facts in the relation are about the key (or the thing that the key identifies), the whole key and nothing but the key. If some attributes are dependent on the keys transitively then that is an indication that those attributes provide information not about the key but about a kno-key attribute. So the information is not directly about the key, although it obviously is related to the key.
Consider the following relation
subject (cno, cname, instructor, office)
Assume that cname is not unique and therefore cno is the only candidate key. The following functional dependencies exist
cno -> cname
cno -> instructor
instructor -> office
We can derive cno -> office from the above functional dependencies and therefore the above relation is in 2NF. The relation is however not in 3NF since office is not directly dependent on cno. This transitive dependence is an indication that the relation has information about more than one thing (viz. course and instructor) and should therefore be decomposed. The primary difficulty with the above relation is that an instructor might be responsible for several subjects and therefore his office address may need to be repeated many times. This leads to all the problems that we identified at the beginning of this chapter. To overcome these difficulties we need to decompose the above relation in the following two relations:
s (cno, cname, instructor)
ins (instructor, office)
s is now in 3NF and so is ins.
An alternate decomposition of the relation subject is possible:
s(cno, cname)
inst(instructor, office)
si(cno, instructor)
The decomposition into three relations is not necessary since the original relation is based on the assumption of one instructor for each course.
The 3NF is usually quite adequate for most relational database designs. There are however some situations, for example the relation student(sno, sname, cno, cname) discussed in 2NF above, where 3NF may not eliminate all the redundancies and inconsistencies. The problem with the relation student(sno, sname, cno, cname) is because of the redundant information in the candidate keys. These are resolved by further normalization using the BCNF.
