[edit] Database management systems

[edit] Real-time databases

In cryptography, encryption is the process of transforming information (referred to as plaintext) using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext). In many contexts, the word encryption also implicitly refers to the reverse process, decryption (e.g. “software for encryption” can typically also perform decryption), to make the encrypted information readable again (i.e. to make it unencrypted).

Encryption has long been used by militaries and governments to facilitate secret communication. Encryption is now commonly used in protecting information within many kinds of civilian systems. For example, in 2007 the U.S. government reported that 71% of companies surveyed utilized encryption for some of their data in transit.^[1] Encryption can be used to protect data "at rest", such as files on computers and storage devices (e.g. USB flash drives). In recent years there have been numerous reports of confidential data such as customers' personal records being exposed through loss or theft of laptops or backup drives. Encrypting such files at rest helps protect them should physical security measures fail. Digital rights management systems which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering (see also copy protection) are another somewhat different example of using encryption on data at rest.

Encryption is also used to protect data in transit, for example data being transferred via networks (e.g. the Internet, e-commerce), mobile telephones, wireless microphones, wireless intercom systems, Bluetooth devices and bank automatic teller machines. There have been numerous reports of data in transit being intercepted in recent years.^[2] Encrypting data in transit also helps to secure it as it is often difficult to physically secure all access to networks.

Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code (MAC) or a digital signature. Standards and cryptographic software and hardware to perform encryption are widely available, but successfully using encryption to ensure security may be a challenging problem. A single slip-up in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See, e.g., traffic analysis, TEMPEST, or Trojan horse.

One of the earliest public key encryption applications was called Pretty Good Privacy (PGP), according to Paul Rubens. It was written in 1991 by Phil Zimmermann and was purchased by Network Associates (now PGP Corporation) in 1997.

There are a number of reasons why an encryption product may not be suitable in all cases. First, e-mail must be digitally signed at the point it was created to provide non-repudiation for some legal purposes, otherwise the sender could argue that it was tampered with after it left their computer but before it was encrypted at a gateway according to Paul. An encryption product may also not be practical when mobile users need to send e-mail from outside the corporate network.^[3]
Data Flow Diagram

A data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).

On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.

A DFD provides no information about the timing or ordering of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).

Physical data independence deals with hiding the details of the storage structure from user applications. The application should not be involved with these issues, since there is no difference in the operation carried out against the data.

The data independence and operation independence together gives the feature of data abstraction. There are two levels of data independence. The logical structure of the data is known as the schema definition. In general, if a user application operates on a subset of the attributes of a relation, it should not be affected later when new attributes are added to the same relation. Logical data independence indicates that the conceptual schema can be changed without affecting the existing schemas. The physical structure of the data is referred to as "physical data description". Physical data independence deals with hiding the details of the storage structure from user applications. The application should not be involved with these issues since, conceptually, there is no difference in the operations carried out against the data. There are two types of data independence:

Data Mart: A data mart is a subset of an organizational data store, usually oriented to a specific purpose or major data subject, that may be distributed to support business needs.^[1] Data marts are analytical data stores designed to focus on specific business functions for a specific community within an organization. Data marts are often derived from subsets of data in a data warehouse, though in the bottom-up data warehouse design methodology the data warehouse is created from the union of organizational data marts.
Data Mining: Data mining is the process of extracting patterns from data. As more data are gathered, with the amount of data doubling every three years,^[1] data mining is becoming an increasingly important tool to transform these data into information. It is commonly used in a wide range of profiling practices, such as marketing, surveillance, fraud detection and scientific discovery.

While data mining can be used to uncover patterns in data samples, it is important to be aware that the use of non-representative samples of data may produce results that are not indicative of the domain. Similarly, data mining will not find patterns that may be present in the domain, if those patterns are not present in the sample being "mined". There is a tendency for insufficiently knowledgeable "consumers" of the results to attribute "magical abilities" to data mining, treating the technique as a sort of all-seeing crystal ball. Like any other tool, it only functions in conjunction with the appropriate raw material: in this case, indicative and representative data that the user must first collect. Further, the discovery of a particular pattern in a particular set of data does not necessarily mean that pattern is representative of the whole population from which that data was drawn. Hence, an important part of the process is the verification and validation of patterns on other samples of data.

Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks. For example, B-trees are particularly well-suited for implementation of databases, while compiler implementations usually use hash tables to look up identifiers.

Data structures are used in almost every program or software system. Specific data structures are essential ingredients of many efficient algorithms, and make possible the management of huge amounts of data, such as large databases and internet indexing services. Some formal design methods and programming languages emphasize data structures, rather than algorithms, as the key organizing factor in software design.


Data Warehouse: is a repository of an organization's electronically stored data. Data warehouses are designed to facilitate reporting and analysis^[1].

A Data Warehouse houses a standardized, consistent, clean and integrated form of data sourced from various operational systems in use in the organization, structured in a way to specifically address the reporting and analytic requirements.

This definition of the data warehouse focuses on data storage. However, the means to retrieve and analyze data, to extract, transform and load data, and to manage the data dictionary are also considered essential components of a data warehousing system. Many references to data warehousing use this broader context. Thus, an expanded definition for data warehousing includes business intelligence tools, tools to extract, transform, and load data into the repository, and tools to manage and retrieve metadata.
Database Administrator (DBA): A database administrator (DBA) is a person responsible for the design, implementation, maintenance and repair of an organization's database. They are also known by the titles Database Coordinator or Database Programmer, and is closely related to the Database Analyst, Database Modeler, Programmer Analyst, and Systems Manager. The role includes the development and design of database strategies, monitoring and improving database performance and capacity, and planning for future expansion requirements. They may also plan, co-ordinate and implement security measures to safeguard the database.^[1] Employing organizations may require that a database administrator have a certification or degree for database systems (for example, the Microsoft Certified Database Administrator).

Database Management System (DBMS): A Database Management System (DBMS) is a set of computer programs that controls the creation, maintenance, and the use of the database of an organization and its end users. It allows organizations to place control of organization-wide database development in the hands of database administrators (DBAs) and other specialists. DBMSes may use any of a variety of database models, such as the network model or relational model. In large systems, a DBMS allows users and other software to store and retrieve data in a structured way. It helps to specify the logical organization for a database and access and use the information within a database. It provides facilities for controlling data access, enforcing data integrity, managing concurrency controlled, restoring database.

Database Structure:
Database Tuning: describes a group of activities used to optimize and homogenize the performance of a database. It usually overlaps with query tuning, but refers to design of the database files, selection of the database management system (DBMS), operating system and CPU the DBMS runs on.

The goal is to maximize use of system resources to perform work as efficiently and rapidly as possible. Most systems are designed to manage work efficiently, but it is possible to greatly improve performance by customizing settings and the configuration for the database and the DBMS being tuned.

Typical information that a decision support application might gather and present are:

• 
  an inventory of all of your current information assets (including legacy and relational data sources, cubes, data warehouses, and data marts),
  
• 
  comparative sales figures between one week and the next,
  
• 
  projected revenue figures based on new product sales assumptions;
  

One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

Denial-of-service attacks are considered violations of the IAB's Internet proper use policy, and also violate the acceptable use policies of virtually all Internet Service Providers. They also commonly constitute violations of the laws of individual nations.^[

Digital Certificate: In cryptography, a public key certificate (also known as a digital certificate or identity certificate) is an electronic document which uses a digital signature to bind together a public key with an identity — information such as the name of a person or an organization, their address, and so forth. The certificate can be used to verify that a public key belongs to an individual.

In a typical public key infrastructure (PKI) scheme, the signature will be of a certificate authority (CA). In a web of trust scheme, the signature is of either the user (a self-signed certificate) or other users ("endorsements"). In either case, the signatures on a certificate are attestations by the certificate signer that the identity information and the public key belong together.

For provable security this reliance on something external to the system has the consequence that any public key certification scheme has to rely on some special setup assumption, such as the existence of a certificate authority.^[1]

Certificates can be created for Unix-based servers with tools such as OpenSSL's ssl-ca.^[2] or SuSE's gensslcert. Similarly, Microsoft Windows 2003 contains Certificate Authority for the creation of digital certificates. In Windows Server 2008 the capability is in Active Directory Certification Authority.

While there is no single definition of a distributed system,^[5] the following defining properties are commonly used:

• 
  There are several autonomous computational entities, each of which has its own local memory.^[6]
  
• 
  The entities communicate with each other by message passing.^[7]
  

A distributed system may have a common goal, such as solving a large computational problem.^[8] Alternatively, each computer may have its own user with individual needs, and the purpose of the distributed system is to coordinate the use of shared resources or provide communication services to the users.^[9]

Other typical properties of distributed systems include the following:

• 
  The system has to tolerate failures in individual computers.^[10]
  
• 
  The structure of the system (network topology, network latency, number of computers) is not known in advance, the system may consist of different kinds of computers and network links, and the system may change during the execution of a distributed program.^[11]
  
• 
  Each computer has only a limited, incomplete view of the system. Each computer may know only one part of the input.^[12
  

Louis Gerstner, the former CEO of IBM, in his book, Who Says Elephants Can't Dance? attributes the term "e-Business" to IBM's marketing and Internet teams in 1996.

Electronic business methods enable companies to link their internal and external data processing systems more efficiently and flexibly, to work more closely with suppliers and partners, and to better satisfy the needs and expectations of their customers.

In practice, e-business is more than just e-commerce. While e-business refers to more strategic focus with an emphasis on the functions that occur using electronic capabilities, e-commerce is a subset of an overall e-business strategy. E-commerce seeks to add revenue streams using the World Wide Web or the Internet to build and enhance relationships with clients and partners and to improve efficiency using the Empty Vessel strategy. Often, e-commerce involves the application of knowledge management systems.

E-business involves business processes spanning the entire value chain: electronic purchasing and supply chain management, processing orders electronically, handling customer service, and cooperating with business partners. Special technical standards for e-business facilitate the exchange of data between companies. E-business software solutions allow the integration of intra and inter firm business processes. E-business can be conducted using the Web, the Internet, intranets, extranets, or some combination of these.

E-Commerce: commonly known as (electronic marketing) e-commerce or eCommerce, consists of the buying and selling of products or services over electronic systems such as the Internet and other computer networks. The amount of trade conducted electronically has grown extraordinarily with widespread Internet usage. The use of commerce is conducted in this way, spurring and drawing on innovations in electronic funds transfer, supply chain management, Internet marketing, online transaction processing, electronic data interchange (EDI), inventory management systems, and automated data collection systems. Modern electronic commerce typically uses the World Wide Web at least at some point in the transaction's lifecycle, although it can encompass a wider range of technologies such as e-mail as well.

A large percentage of electronic commerce is conducted entirely electronically for virtual items such as access to premium content on a website, but most electronic commerce involves the transportation of physical items in some way. Online retailers are sometimes known as e-tailers and online retail is sometimes known as e-tail. Almost all big retailers have electronic commerce presence on the World Wide Web.

Electronic commerce that is conducted between businesses is referred to as business-to-business or B2B. B2B can be open to all interested parties (e.g. commodity exchange) or limited to specific, pre-qualified participants (private electronic market). Electronic commerce that is conducted between businesses and consumers, on the other hand, is referred to as business-to-consumer or B2C. This is the type of electronic commerce conducted by companies such as Amazon.com.

Electronic commerce is generally considered to be the sales aspect of e-business. It also consists of the exchange of data to facilitate the financing and payment aspects of the business transactions.

EDI can be formally defined as 'The transfer of structured data, by agreed message standards, from one computer system to another without human intervention'. Most other definitions used are variations on this theme. Even in this era of technologies such as XML web services, the Internet and the World Wide Web, EDI is still the data format used by the vast majority of electronic commerce transactions in the world.

The term is used for a number of different concepts:

• 
  Cardholder-initiated transactions, where a cardholder makes use of a payment card
  
• 
  Direct deposit payroll payments for a business to its employees, possibly via a payroll services company
  
• 
  Direct debit payments from customer to business, where the transaction is initiated by the business with customer permission
  
• 
  Electronic bill payment in online banking, which may be delivered by EFT or paper check
  
• 
  Transactions involving stored value of electronic money, possibly in a private currency
  
• 
  Wire transfer via an international banking network (generally carries a higher fee)
  
• 
  Electronic Benefit Transfer Electronic Benefit Transfer (EBT) is an electronic system in the United States that allows state governments to provide financial and material benefits to authorized recipients via a plastic debit card. Common benefits provided via EBT are typically sorted into two general categories: Food Stamp and Cash benefits. Food stamp benefits are federally authorized benefits that can be used only to purchase food and non-alcoholic beverages. Cash benefits include State General Assistance, TANF (Temporary Aid for Needy Families) benefits and refugee benefits.
  

An ERP system typically has modular hardware and software units and "services" that communicate on local area networks, wide area networks, internet and intranet. The modular design allows a business to add or reconfigure modules (perhaps from different vendors) while preserving data integrity in one shared database that may be centralized or distributed

Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The methods used show some similarities to radio systems, although there are fundamental differences, such as the fact that it is much easier to detect collisions in a cable broadcast system than a radio broadcast. The common cable providing the communication channel was likened to the ether and it was from this reference that the name "Ethernet" was derived.

From this early and comparatively simple concept, Ethernet evolved into the complex networking technology that today underlies most LANs. The coaxial cable was replaced with point-to-point links connected by Ethernet hubs and/or switches to reduce installation costs, increase reliability, and enable point-to-point management and troubleshooting. StarLAN was the first step in the evolution of Ethernet from a coaxial cable bus to a hub-managed, twisted-pair network. The advent of twisted-pair wiring dramatically lowered installation costs relative to competing technologies, including the older Ethernet technologies.

Above the physical layer, Ethernet stations communicate by sending each other data packets, blocks of data that are individually sent and delivered. As with other IEEE 802 LANs, each Ethernet station is given a single 48-bit MAC address, which is used to specify both the destination and the source of each data packet. Network interface cards (NICs) or chips normally do not accept packets addressed to other Ethernet stations. Adapters generally come programmed with a globally unique address, but this can be overridden, either to avoid an address change when an adapter is replaced, or to use locally administered addresses.

Despite the significant changes in Ethernet from a thick coaxial cable bus running at 10 Mbit/s to point-to-point links running at 1 Gbit/s and beyond, all generations of Ethernet (excluding early experimental versions) share the same frame formats (and hence the same interface for higher layers), and can be readily interconnected.

Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it, and the reduced panel space needed by twisted pair Ethernet, most manufacturers now build the functionality of an Ethernet card directly into PC motherboards, eliminating the need for installation of a separate network card.

An Executive Information System (EIS) is a type of management information system intended to facilitate and support the information and decision-making needs of senior executives by providing easy access to both internal and external information relevant to meeting the strategic goals of the organization. It is commonly considered as a specialized form of a Decision Support System (DSS) ^[1]

The emphasis of EIS is on graphical displays and easy-to-use user interfaces. They offer strong reporting and drill-down capabilities. In general, EIS are enterprise-wide DSS that help top-level executives analyze, compare, and highlight trends in important variables so that they can monitor performance and identify opportunities and problems. EIS and data warehousing technologies are converging in the marketplace.

In recent years, the term EIS has lost popularity in favour of Business Intelligence (with the sub areas of reporting, analytics, and digital dashboards).

XML’s design goals emphasize simplicity, generality, and usability over the Internet.^[2] It is a textual data format, with strong support via Unicode for the languages of the world. Although XML’s design focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.

There are a variety of programming interfaces which software developers may use to access XML data, and several schema systems designed to aid in the definition of XML-based languages.

As of 2009^[update], hundreds of XML-based languages have been developed,^[3] including RSS, Atom, SOAP, and XHTML. XML-based formats have become the default for most office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork^[4]

XML documents may begin by declaring some information about themselves, as in the following example.



Extensible Business Reporting Language (XBRL)

XBRL was developed for business and accounting applications. It is an XML-based application used to create, exchange and analyze financial reporting information that was developed for worldwide use. The AICPA led consortium that developed XBRL has promoted the application as a freely licensed product. In typical usage, XBRL consists of an instance document, containing primarily the business facts being reported, and a collection of taxonomies (called a Discoverable Taxonomy Set (DTS)), which define metadata about these facts, such as what the facts mean and how they relate to one another. XBRL uses XML Schema, XLink, and XPointer standards.

The instance document begins with the root element. There may be more than one XBRL instance embedded in a larger XML document. The XBRL instance document itself holds the following information:

• 
  Business Facts – facts can be divided into two categories
  
  • 
    Items are facts holding a single value. They are represented by a single XML element with the value as its content.
    
  • 
    Tuples are facts holding multiple values. They are represented by a single XML element containing nested Items or Tuples.
    
  • 
    
    
• 
  
  
• 
  -
  
• 
  -
  
• 
    SG2003
  
• 
    Saucony
  
• 
    Grid 2003
  
• 
    female
  
• 
    7.5
  
• 
   
  54.99
  
  
• 
    USA Sports Distributor
  
• 
   
  
• 
  -
  
• 
    SJ2002
  
• 
    Saucony
  
• 
    Jazz 2002
  
• 
    female
  
• 
    8.0
  
• 
   
  77.50
  
  
• 
    USA Sports Distributor
  
• 
   
  
• 
  -
  
• 
    SO2001
  
• 
    Saucony
  
• 
    Omni 2001
  
• 
    female
  
• 
    7.5
  
• 
   
  98.99
  
  
• 
    USA Sports Distributor
  
• 
   
  
• 
  -
  
• 
    NA2003
  
• 
    Nike
  
• 
    Air Max 2003
  
• 
    female
  
• 
    7.5
  
• 
   
  99.50
  
  
• 
    USA Sports Distributor
  
• 
   
  
• 
   
  

• 
  Contexts define the entity (e.g. company or individual) to which the fact applies and the period of time the fact is relevant. Date and time information appearing in the period element must conform to ISO 8601. Scenarios provide further contextual information about the facts, such as whether the business values reported are actual, projected, or budgeted.
  
• 
  Units define the units used by numeric or fractional facts within the document, such as USD, shares. XBRL allows more complex units to be defined if necessary. Facts of a monetary nature must use a unit from the ISO 4217 namespace.
  

• 
  Schemas define Item and Tuple "concepts" using elements. Concepts provide names for the fact and indicate whether or not it's a tuple or an item, the data type (such as monetary, numeric, fractional, or textual), and potentially more metadata. Items and Tuples can be regarded as "implementations" of concepts, or specific instances of a concept. A good analogy for those familiar with object oriented programming would be that Concepts are the classes and Items and Tuples are Object instances of those classes. This is the source of the use of the "instance document" terminology. In addition to defining concepts, Schemas reference linkbase documents. Tuples instances are 1..n relationships with their parents; their metadata is simply the collection of their attributes.
  

• 
  Linkbases are a collection of Links, which themselves are a collection of locators, arcs, and potentially resources. Locators are elements that essentially reference a concept and provide an arbitrary label for it. In turn, arcs are elements indicating that a concept links to another concept by referencing the labels defined by the locators. Some arcs link concepts to other concepts. Other arcs link concepts to resources, the most common of which are human-readable labels for the concepts. The XBRL 2.1 specification defines five different kinds of linkbases.
  
  • 
    Label Linkbase – This linkbase provides human readable strings for concepts. Using the label linkbase, multiple languages can be supported, as well as multiple strings within each language.
    
  • 
    Reference Linkbase – This linkbase associates concepts with citations of some body of authoritative literature.
    
  • 
    Calculation Linkbase – This linkbase associates concepts with other concepts so that values appearing in an instance document may be checked for consistency.
    
  • 
    Definition Linkbase – This linkbase associates concepts with other concepts using a variety of arc roles to express relations such as is-a, whole-part, etc.
    
  • 
    Presentation Linkbase – This linkbase associates concepts with other concepts so that the resulting relations can guide the creation of a user interface, rendering, or visualisation.
    

An extranet is a private network that uses Internet protocols, network connectivity, and possibly the public telecommunication system to securely share part of an organization's information or operations with suppliers, vendors, partners, customers or other businesses. An extranet can be viewed as part of a company's intranet that is extended to users outside the company, usually via the Internet. It has also been described as a "state of mind" in which the Internet is perceived as a way to do business with a selected set of other companies (business-to-business, B2B), in isolation from all other Internet users. In contrast, business-to-consumer (B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users.

A fat client or rich client is a computer (client) in client-server architecture networks which typically provides rich functionality independently of the central server. Originally known as just a 'client' or 'thick client', the name is contrasted to thin client, which describes a computer heavily dependent on a server's applications.

A fat client still requires at least periodic connection to a network or central server, but is often characterised by the ability to perform many functions without that connection. In contrast, a thin client generally does as little processing as possible and relies on accessing the server each time input data needs to be processed or validated.
Field

In computer science, data that has several parts can be divided into fields. For example, a computer may represent today's date as three distinct fields: the day, the month and the year.

Relational databases arrange data as sets of database records, also called rows. Each record consists of several fields; the fields of all records form the columns.

Field Check

File

At the lowest level, many modern operating systems consider files simply as a one-dimensional sequence of bytes. At a higher level, where the content of the file is being considered, these binary digits may represent integer values, text characters, image pixels, audio or anything else. It is up to the program using the file to understand the meaning and internal layout of information in the file and present it to a user as more meaningful information (such as text, images, sounds, or executable application programs).

At any instant in time, a file might have a size, normally expressed as number of bytes, that indicates how much storage is associated with the file. In most modern operating systems the size can be any non-negative whole number of bytes up to a system limit. However, the general definition of a file does not require that its instant size has any real meaning, unless the data within the file happens to correspond to data within a pool of persistent storage.

Information in a computer file can consist of smaller packets of information (often called "records" or "lines") that are individually different but share some trait in common. For example, a payroll file might contain information concerning all the employees in a company and their payroll details; each record in the payroll file concerns just one employee, and all the records have the common trait of being related to payroll—this is very similar to placing all payroll information into a specific filing cabinet in an office that does not have a computer. A text file may contain lines of text, corresponding to printed lines on a piece of paper. Alternatively, a file may contain an arbitrary binary image (a BLOB) or it may contain an executable.

The way information is grouped into a file is entirely up to the person designing the file. This has led to a plethora of more or less standardized file structures for all imaginable purposes, from the simplest to the most complex. Most computer files are used by computer programs. These programs create, modify and delete files for their own use on an as-needed basis. The programmers who create the programs decide what files are needed, how they are to be used and (often) their names.

In some cases, computer programs manipulate files that are made visible to the computer user. For example, in a word-processing program, the user manipulates document files that the user personally names. The content of the document file is arranged in a way that the word-processing program understands, but the user chooses the name and location of the file and provides the bulk of the information (such as words and text) that will be stored in the file.

A file attribute is metadata that describes or is associated with a computer file. For example, an operating system often keeps track of the date a file was created and last modified, as well as the file's size and extension (and what application to open it with). File permissions are also kept track of. The user may attach other attributes themselves, such as comments or color labels, as in Apple Computer's Mac OS X (version 10.3 or later).

In MS-DOS, OS/2 and Microsoft Windows the attrib command can be used to change and display file attributes.

Financial Risk

Firewall

A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices configured to permit, deny, encrypt, decrypt, or proxy all (in and out) computer traffic between different security domains based upon a set of rules and other criteria.

Firewalls can be implemented in either hardware or software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

Flowchart

Flowcharts are used in analyzing, designing, documenting or managing a process or program in various fields

A typical flowchart from Computer Science textbooks may have the following kinds of symbols:

Start and end symbols

Represented as circles, ovals or rounded rectangles, usually containing the word "Start" or "End", or another phrase signaling the start or end of a process, such as "submit enquiry" or "receive product".

Arrows

Processing steps

Represented as rectangles. Examples: "Add 1 to X"; "replace identified part"; "save changes" or similar.

Input/Output

Represented as a parallelogram. Examples: Get X from the user; display X.

Conditional or decision

Represented as a diamond (rhombus). These typically contain a Yes/No question or True/False test. This symbol is unique in that it has two arrows coming out of it, usually from the bottom point and right point, one corresponding to Yes or True, and one corresponding to No or False. The arrows should always be labeled. More than two arrows can be used, but this is normally a clear indicator that a complex decision is being taken, in which case it may need to be broken-down further, or replaced with the "pre-defined process" symbol.

A number of other symbols that have less universal currency, such as:

• 
  A Document represented as a rectangle with a wavy base;
  
• 
  A Manual input represented by parallelogram, with the top irregularly sloping up from left to right. An example would be to signify data-entry from a form;
  
• 
  A Manual operation represented by a trapezoid with the longest parallel side at the top, to represent an operation or adjustment to process that can only be made manually.
  
• 
  A Data File represented by a cylinder.
  

A fourth-generation programming language (1970s-1990) (abbreviated 4GL) is a programming language or programming environment designed with a specific purpose in mind, such as the development of commercial business software^[1]. In the evolution of computing, the 4GL followed the 3GL in an upward trend toward higher abstraction and statement power. The 4GL was followed by efforts to define and use a 5GL.

All 4GLs are designed to reduce programming effort, the time it takes to develop software, and the cost of software development. They are not always successful in this task, sometimes resulting in inelegant and unmaintainable code. However, given the right problem, the use of an appropriate 4GL can be spectacularly successful as was seen with MARK-IV and MAPPER (see History Section, Santa Fe real-time tracking of their freight cars – the productivity gains were estimated to be 8 times over COBOL). The usability improvements obtained by some 4GLs (and their environment) allowed better exploration for heuristic solutions than did the 3GL.

Gateway

Gateways work on all seven OSI layers . The main job of a gateway is to convert protocols among communications networks. A router by itself transfers, accepts and relays packets only across networks using similar protocols. A gateway on the other hand can accept a packet formatted for one protocol (e.g. AppleTalk) and convert it to a packet formatted for another protocol (e.g. TCP/IP) before forwarding it. A gateway can be implemented in hardware, software or both, but they are usually implemented by software installed within a router. A gateway must understand the protocols used by each network linked into the router. Gateways are slower than bridges, switches and (non-gateway) routers.

A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the networks in between are gateways. For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet