DBMS management systems (database)
DBMS management systems (database) is an important type of software system used today in the largest and the smallest computers. There are two main characteristics that differ from other types of DBMS software systems.
- Ability to manage persistent data;
- Opportunity for efficient access to large amounts of data.
The first characteristic is related to the fact that there is a permanent database and its contents are data that the DBMS access and manage. The second characteristic distinguishes DBMS of a filing system that also manages persistent data, but is not usually provides quick access to any part of the data. The capabilities of DBMS are necessary when the amount of data is very large, since a small amount of data apply simple access methods such as linear scan of the data. Along with these main features of DBMS there are many others, such as:
- Providing at least one model of data or mathematical abstraction through which the user can view the data;
- Providing high-level language, which allows the user to define data structures to access data and process them;
- Transaction management, the ability to ensure correct parallel database access multiple users simultaneously;
- Access management, ability to restrict access to data from unauthorized users the opportunity to check the validity of the data;
- Flexibility, the ability to recover from failures in the system without loss of data.
Each DBMS provides at least an abstract data model that allows the user to view information, not as a series of bits, and in a comprehensible manner. It is usually possible to see the data at several levels of abstraction. A relatively low level DBMS allows data to be seen as composed of files.
Example 4.1. Any communication administration usually keeps a file with information about their subscribers a recording for each subscriber can have five fields: first name, last name, customer number, home address, type of terminal and possibly other information. Suppose that in the record kept the name and address of the subscriber. The record has the structure as SQL defined at Iamaccepted.com:
The file itself is a sequence of records for each subscriber to the network. In one of the data models that will look at the file of records is an abstraction of what is called a relation and can be described by
SUBSCRIBERS here is the name of the relation corresponding to the file of example 4.1, NAME and ADDR are field names; fields are often called attributes when talking about relations.
The relation is an abstraction of the file where the types of data fields almost do not matter, and where the order of entries is not defined. Entries in relation called tyupli (tuple)  or tuples . Thus the file is regarded as a list of entries and relation, as many of tyupli.
Effective access to the file
The ability to store information is so important feature of DBMS, each file system associated with the operating system does this. It is important that access to the data in the file. For example, suppose that you need to find the address of the subscriber with the name John Doe. Ako communication administration has thousands of subscribers demand throughout the file is too expensive. DBMS enables the use of index files that will give access to records in a row, no matter how large the file. Similarly, the introduction of new entries or delete old can be made permanent a very short time, regardless of the length of the file.
Another feature of the DBMS allows browse through the files. Values from two or more files are combined to obtain the information that is being sought. The following example illustrates navigation.
Example 4.2. Suppose that records per subscriber is stored information about the subscriber number, not your home address. In another file called NUMBERS has records that connect subscriber numbers and home addresses. In this case, there is the relation:
Now, if you need to find emergency number and the subscriber named John Doe is necessary to move from SUBSCRIBERS to NUMBERS, using ADDR field in the two files. Thus we find the first entry in SUBSCRIBERS, which NAME = “John Doe” and this record take value ADDR, as “Slivnitsa, N73”. Then look in the file NUMBERS recording field ADDR = “Slivnitsa, N73”, and find that NUM = “345 678”. If you put the correct indexes, we get access to the data in a very short time constant that does not depend on the length of the files.
Languages operations data
To make access to files easier DBMS provides a query language or language of operations data for expressing operations in the file. Main operations data differ in the level of detail they require the user as usual, languages based on relational data models usually require fewer details of the languages based on other models.
Example 4.3. The reference, discussed in Example 4.2 can be written in the language SQL, which is based on relational data model as shown below:
(1) SELECT NUM
(2) FROM SUBSCRIBERS, NUMBERS
(3) WHERE SUBSCRIBERS.NAME=”Иван Петров”
(4) AND SUBSCRIBERS.ADDR=NUMBERS.ADDR
Red (1) tells the DBMS print subscriber number by response line (2) says to look at the relations SUBSCRIBERS and NUMBERS row (3) says that the user name is “John Doe,” the last line says that the number it is associated with the same address, which is also connected to the subscriber.
Below is the same query written in simple language version of DML operations data network model.
(1) SUBSCRIBERS.NAME:=”Иван Петров”
(2) FIND SUBSCRIBERS RECORD BY CALC=KEY
(3) FIND OWNER OF CURRENT NAME-ADDR SET
(4) FIND NUMBERS RECORD IN CURRENT ADDR-NUM SET
(5) print NUMBERS.NUM
Lines (1) and (2) tell the DBMS to find records for John Doe file SUBSCRIBERS. Red (3) uses a SET structure that connects a user name and his address. Red (4) uses the assumption that there is another SET structure, ADDR = NUM, linking the addresses of subscribers to their numbers. On line (5) we find and print the number of the subscriber. Note that the print is not part of the language of the operations data as part of its accompanying programming language.
It can easily see that the navigation between files is made far more difficult than in DML SQL, ie the developer of DML more efforts are needed. The difference is not in most lines, but mainly in how the second example allows you to move from one record to the next, while the first tells how the response is associated with the data. This declarative SQL and other languages based on the relational model is the reason systems based on this model are becoming more popular.
Another important feature of DBMS is the ability to simultaneously manage a large number of transactions that are procedures for database operations. Some databases are so large that they can be useful only if used simultaneously by many computers. Often these computers are distributed worldwide. A typical example is database management systems used in telecommunications.
Sometimes two access to data not interfere with each other. For example, multiple requests to read the number of a subscriber will not interfere. But if at the same time an amendment is made to these data, the result of the two transactions, if they are not coordinated is unpredictable. Thus, the transaction, which amends elements of data need to “lock” for other transactions that are trying to read or write at the same time. DBMS should provide some form of control parallelism to prevent uncoordinated access the same element data from more than one transaction.
Problems can be compounded if the database is spread across many computer systems, such as modern telecommunica-tion systems. These problems are associated with duplication of data so as to ensure the most rapid local access and protect data from destruction in case any of the computers damaged.
Secrecy of data
DBMS should not only provide data storage at fault, but it should protect them from unauthorized access. For example, some users may have limited access to information on the web – operators can modify elements of it, while the average user it is not allowed.DBMS must be able to connect to different users with their privileges, ie the ability to access files fields in files or other subsets of data in the database. DBMS must maintain a table containing information about each user with privileges to access any data element. For example, a user may be allowed to read the file but can not insert or delete data, other may not be allowed access to the file at all, while others can read and modify the entire file.
To ensure adequate rich variety of structures so that the user can see parts of files without being able to see everything DBMS often offers view (view). View allows the creation of imaginary objects defined in a precise way of real objects such as files or relations.
Example 4.4. Let’s SUBSCRIBERS a file with the following fields:
and requires most people to have access to all other fields except ACCOUNT (number of current account). In SQL language can be defined vision SAFE-SUBS by:
CREATE VIEW SAFE-SUBS BY
SELECT NAME, ADDR, NUM
SAFE-SUBS view consists of fields with your name, address and number of subscribers, but not right number of current account. SAFE-SUBS can be viewed as described by the relation
Seeing SAFE-SUBS there physically, as a file, however, it may make an inquiry, if it exists.For example, we can ask for the number of John Doe. Expressed SQL query would look like this:
WHERE NAME=”Иван Петров”
The normal user has access to view SAFE-SUBS, but not to SUBSCRIBERS. Consumers the right to know the numbers of the current account subscriber is given access SUBSCRIBERS relation, ie they may alter these numbers.
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