Lecture 1. Databases and file systems

In the first lecture, we will look at the general meaning of the concepts of database and DBMS. To begin with, from the very beginning of the development of computing technology, two main directions of its use were formed. The first direction is the use of computer technology to perform numerical calculations that are too long or impossible at all to be done manually. The formation of this direction has contributed to the intensification of methods for the numerical solution of complex mathematical problems, the development of a class of programming languages ​​oriented towards the convenient writing of numerical algorithms, the development of feedback with the developers of new computer architectures.

The second direction, which is directly related to the topic of our course, is the use of computer technology in automatic or automated information systems. In the broadest sense, an information system is a software package whose functions are to support reliable storage of information in a computer’s memory, perform application-specific information transformations and / or calculations, and provide users with a convenient and easily mastered interface. Typically, the amount of information with which such systems have to deal is quite large, and the information itself has a rather complex structure. The classic examples of information systems are banking systems, reservation systems for airline or railway tickets, hotel rooms, etc.

In fact, the second direction arose somewhat later than the first. This is due to the fact that at the dawn of computing computers had limited capabilities in terms of memory. It is clear that we can talk about reliable and long-term storage of information only if there are storage devices that store information after turning off the power supply. RAM usually does not possess this property. At the beginning, two kinds of external memory devices were used: magnetic tapes and drums. At the same time, the capacity of magnetic tapes was large enough, but by their physical nature they provided consistent access to data. Magnetic drums (they are most similar to modern magnetic disks with fixed heads) made it possible to randomly access data, but were of limited size.

It is easy to see that these restrictions are not very significant for purely numerical calculations. Even if a program needs to process (or produce) a large amount of information, during programming you can think through the arrangement of this information in external memory so that the program works as quickly as possible.

On the other hand, for information systems in which the need for current data is determined by the user, the presence of only magnetic tapes and drums is unsatisfactory. Imagine the buyer of a ticket who, standing at the ticket office, has to wait for the complete rewind of the magnetic tape. One of the natural requirements for such systems is the average speed of operations.

As it seems, it is precisely the requirements for computing from the side of unnumbered applications that caused the emergence of removable magnetic disks with moving heads, which was a revolution in the history of computing technology. These external memory devices had a significantly larger capacity than magnetic drums, provided a satisfactory speed of access to data in a random sampling mode, and the ability to change the disk packet on the device made it possible to have a virtually unlimited data archive.

With the advent of magnetic disks, the history of data management systems in external memory began. Prior to this, each application program that needed to store data in external memory itself determined the location of each piece of data on a magnetic tape or drum and performed exchanges between operational and external memory using low-level software and hardware (machine commands or calls to the corresponding operating system programs ). This mode of operation does not allow or makes it very difficult to maintain several archives of long-term stored information on one external medium. In addition, each application program had to solve the problems of naming data parts and structuring data in external memory.