Computer World. Матросова Т.А. - 54 стр.

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53
are using the data. However, the DBMS goes far beyond the file system m
providing flexibility, such as data structures that support efficient access
to very large amounts of data.
2. Programming interface. A DBMS allows the user or an application program to
access and modify data through a powerful query language
Again, the advantage of a DBMS over a file system is the flexibility to
manipulate stored data in much more complex ways than the reading and
writing of files.
3. Transaction management. A DBMS supports concurrent access to data,
i.e., simultaneous access by many distinct processes (called «transactions») at
once. To avoid some of the undesirable consequences of simultaneous access, the
DBMS supports isolation, the appearance that transactions execute one at-a-time,
and atomicity, the requirement that transactions execute either completely or not
at all. A DBMS also supports durability, the ability to recover from failures or
errors of many types.
The Evolution of Database Systems
What is a database? In essence a database is nothing more than a collection of
information that exists over a long period of time, often many years. In common
parlance, the term database refers to a collection of data that is managed by a DBMS.
The DBMS is expected to:
1. Allow users to create new databases and spec ify their schema (logical
structure of the data), using a specialized language called a data-definition
language.
2. Give users the ability to query the data (a «query» is database lingo for
a question about the data) and modify the data, using an appropriate
language, often called a query language or data-manipulation language.
3. Support the storage of very large amounts of data – many gigabytes or
more – over a long period of time, keeping it secure from accident or
unauthorized use and allowing efficient access to the data for queries and
database modifications.
4. Control access to data from many users at once, without allow ing the
actions of one user to affect other users and without allowing simultaneous
accesses to corrupt the data accidentally.
Early Database Management Systems
The first commercial database management systems appeared in the late 1960's.
These systems evolved from file systems, which provide some of item (3) above, file
systems store data over a long period of time, and they allow the storage of large
amounts of data. However, file systems do not generally guarantee that data cannot
be lost if it is not backed up, and they don't support efficient access to data items
whose location in a particular file is not known.
Further, file systems do not directly support item (2), a query language for the
data in files Their support for (1) a schema for the data is limited to the creation

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