IX Международная студенческая научная конференция Студенческий научный форум - 2017

Data flow diagrams (Data Flow Diagrams- DFD) is a hierarchy of functional processes related data flows. The purpose of this presentation -relise how each process converts its input to the output, and to identify the relationship between these process. For constructing DFD traditionally used two different notations, appropriate methods of Yourdon and DeMarco Gein Sersona. These notations are slightly different from each other by a graphical representation of characters (the notation used in the examples Gein Sersona).

In accordance with this method a model system is defined as a hierarchy of data flow diagrams describing asynchronous transformation process information from its input to output to the system user. Information sources (external entities) generate information flows (data streams) that carry information to the subsystems or processes. Those, in turn, converts the information and generate new streams that carry information to other processes or sub-systems, data storage devices or external entities - consumers of information. Diagrams of the upper levels of the hierarchy (context diagram) define the basic processes or sub-systems with external inputs and outputs. They are detailed in using lower-level diagrams. Such decomposition continues, creating a layered hierarchy charts until until it reaches the level of decomposition processes where detail hereinafter is not meaningful.

The main components of data flow diagrams are:

• external entities;

• systems and subsystems;

• processes;

• Data storage;

• Data streams.

Foreign entity is a material object or a natural person who is the source or receiver of information, such as customers, employees, suppliers, customers, warehouse. Determination of an object or system as an external entity indicates that it is outside the boundaries of the studied system. During the study, some external entity can be transferred into the investigated system diagram, if necessary, or, conversely, some processes can be brought outside the diagram and is represented as an external entity.

The external entity is designated square in Figure 4, located above the graph and gives her a shadow in order to be able to identify this symbol among other symbols.

When constructing a model of a complex system, it can be represented in general form as called for in a context diagram of a system as a whole, or may be decomposed into a number of subsystems.

subsystem number is used to identify it. In the name of the subsystem name is entered in the form of a proposal from the subject and the relevant definitions and additions. Process is the conversion of input data streams to the output in accordance with a certain algorithm. Physically, the process can be implemented in various ways: it can be an organizational unit (department), the processing of the input document and the release of the reports, a program implemented in hardware logic device, etc.

process number is used to identify it. The name field is entered the name of the process in the form of a proposal from the active unambiguous verb in the infinitive form (to calculate, calculate, verify, identify, create, receive), followed by nouns in the accusative, for example: "Enter information about taxpayers," " Print information on the operating costs "," Check the receipt of money. " The information in the physical implementation of the field shows which organizational unit, a program or hardware device performs the process. Drive data - this abstract data storage device that can be placed at any point in the drive, and after a while to extract, the extraction methods and facilities can be arbitrary. The drive data can be physically realized in the form of microfiche, box in the files, tables in memory, the file on magnetic media, etc.

The drive data is identified by the letter «D» and an arbitrary number. the drive name is selected from considerations of the highest information content for the designer. The drive data, in general, is the prototype of the future database and a description of the data stored in it must comply with the data model. The data flow identifies информацию, передаваемую через некоторое соединение от источника к приемнику. Реальный поток данных может быть информацией, передаваемой по кабелю между двумя устройствами, пересылаемыми по почте письмами, магнитными лентами или дискетами, переносимыми с одного компьютера на другой и т.д.

The main purpose of constructing a hierarchy of DFD is to make the description of the system clear and understandable at every level of detail, and break it into pieces with well-defined relationship between them. To achieve this, it is advisable to use the following guidelines:

• Placing on each chart from 3 to processes 6-7 (similarly SADT). The upper limit corresponds to the human ability to simultaneously read and understand the structure of a complex system with many internal links, the lower limit is selected for reasons of common sense: there is no need to detail the process diagram containing only one or two processes.

• Do not obstruct the charts irrelevant at this level of detail.

• Decomposition of data streams to carry out in parallel with the decomposition process. These two works must be carried out simultaneously rather than one after the other.

• Choose a clear, reflecting the essence of the matter the names of processes and threads, thus try not to use abbreviations.

The first step in constructing the hierarchy of DFD is to build context diagrams. Usually in the design relatively simple system built only context diagram with a star topology, the center of which is the so-called main process, coupled with the receivers and sources of information, through which users interact with the system and other external systems. Before constructing DFD context is necessary to examine the external events (external entities) that have an impact on the functioning of the system.

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