Monday, April 28, 2008

Public transport

Public transport, public transportation, public transit or mass transit comprise all transport systems that transport members of the general public, usually charging set fares. While the above terms are generally taken to include rail and bus services, wider definitions might include scheduled airline services, ferries, taxicab services etc. A further restriction that is sometimes applied is that transit should occur in continuously shared vehicles, which would exclude taxis that are not shared-ride taxis.

The term public transport is preferred in the British Isles and most Commonwealth countries, whereas public transportation, public transit and mass transit are used most often in North America. term transit is less likely to include long-distance forms of public transportation, such as long-distance or commuter railroads, inter-city buses, or intercity railways.

Monday, April 21, 2008

The Bering Land Bridge

The Bering Land Bridge, the Jesuit scholar José de Acosta suggested that the peoples of the Americas arrived via a now-submerged land bridge from Asia as primitive hunters, later settling into sedentary communities and cities. In Notes on the State of Virginia (1781), Thomas Jefferson theorized that the ancestors of Native Americans crossed the Bering Strait from Asia, a viewpoint that came to prevail in the 20th century, as carbon dating and molecular genetics began to shed light on the origins of native populations.

Between the 1950s and the 1980s, the Bering Land Bridge theory came to be viewed as proven beyond any doubt. Most archaeologists came to believe that the native cultures of the Americas had been isolated from the Old World after the closing of the Bering land route, when they were still in the hunter-gatherer stage and developed without any outside influences for the next 9,000 years until the time of Columbus. It was also believed at the time that trans-oceanic travel only became possible in the 15th century, after key advances in Old World shipbuilding and navigation. This belief is supported by the lack of substantial evidence of Old World influences on American civilizations.

Monday, April 14, 2008

Computer-aided software engineering

Computer-aided software engineering (CASE) is the use of software tools to assist in the development and maintenance of software. Tools used to assist in this way are known as CASE Tools.

Some typical CASE tools are:

* Code generation tools

* Data modeling tools

* UML

* Refactoring tools

* QVT or Model transformation Tools

* Configuration management tools including revision control

All aspects of the software development lifecycle can be supported by software tools, and so the use of tools from across the spectrum can, arguably, be described as CASE; from project management software through tools for business and functional analysis, system design, code storage, compilers, translation tools, test software, and so on.

Monday, April 07, 2008

Durability (database systems)

In database systems, durability is the ACID property that guarantees that transactions that are successfully committed will survive permanently and will not be undone by system failure.

For example, if a flight booking database system reports that a seat has successfully been booked, then the seat will remain booked even if the system crashes. Durability also guarantees that the system will not rollback the transaction because the seat was double-booked.

The only way a transaction can be undone after it has been committed is by a compensatory transaction.

Friday, April 04, 2008

Database normalization

Database normalization, sometimes referred to as canonical synthesis, is a technique for designing relational database tables to minimize duplication of information and, in so doing, to safeguard the database against certain types of logical or structural problems, namely data anomalies. For example, when multiple instances of a given piece of information occur in a table, the possibility exists that these instances will not be kept consistent when the data within the table is updated, leading to a loss of data integrity. A table that is sufficiently normalized is less vulnerable to problems of this kind, because its structure reflects the basic assumptions for when multiple instances of the same information should be represented by a single instance only.