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| 4.7 Basic Method "Data Flow Modeling" (DFM) |
4.7 Elementarmethode "Datenfluß-Modellierung" (DFM)
1 Identification/Definition of the Method
/Hatley, 1987/Appendix A, A.2 and A.3
2 Brief Characteristic of the Method
Objective and PurposeIt is the objective of the Data Flow Modeling (DFM) to define the functional structure of a system by means of the combined consideration of functions and data. The data flows are the interfaces between the functions. The data flow modeling abstracts from physical facts of a projected system.
Means of Representation
The means of representation are the components of the data flow diagram (DFD) which are listed in the following. Each individual element of a diagram can be identified by giving it a name.
- Processes:
The basic processing items of a data flow diagram. They are used to transform incoming data flows into outgoing data flows. Processes that are not further decomposed have to be described by means of a textual specification. This text defines how the input data of the process are transformed into output data.
Note: A DFM process cannot be compared with a process of the V-Model (SW Architecture). A process in the sense of the DFM can rather be compared with an abstract item of the modeling which is realized by subsystems, segments, SWCI's, etc. depending on the level of design. - Terminators:
Data producers (data sources) or data consumers (data "sinks") outside of the system; - Dataflows:
Logical channels ("pipelines") in which data are transported, they are represented by arrows connecting the processes; - Data store:
Storage space from which data can be read with a time delay after writing them; without processing component.
Operational Sequence
In a top-down-oriented procedure more and more detailed levels of the future system are specified. This is based on a context diagram which only represents the data flows of the system from and to its environment. When refining the data flow model, the functions identified in the functional hierarchy (see method FCTD - Functional Decomposition) are refined by means of a DFD of the corresponding level (also see /Martin, 1987/ chap. 9).
The DFD of a certain hierarchical level can be made to represent the cooperation of processes that are connected via data flows. A refinement of the DFM on the one hand is always realized in balance with the corresponding refinement of the functional hierarchy (see FCTD - Functional Decomposition) on the other.
When modeling the data flows it is important to find a logical internal structure of the planned system that is stable and independent of design decisions and hardware environment.
3 Limits of the Methods Application
A DFD defines the possible structure of the system functions and the way they are linked via data flows. Many of the influencing factors of the later system remain unnoticed in this model (time behavior, marginal hardware conditions, database aspects). Further modeling means have to supplement the data flow model. Particularly method CFM - Control Flow Modeling has to be applied in case of realtime applications as a supplement to method DFM.
4 Specification of the Methods Allocation
| No. | Activity | Description |
|---|---|---|
| 4.1 | SD1.1 - Recording of Actual Status and Analysis |
A data flow model has to be generated in order to document the actual state. The hierarchical level must be limited since only the essential structure of the old system is to be represented.
The method covers the subproduct User Requirements.Actual Status and Current Analysis together with FCTD - Functional Decomposition on the function side. |
| 4.2 | SD1.2 - Description of Application System |
In order to represent all external components of the system (data sources and data sinks) a data flow diagram has to be generated on the highest hierarchical level (context diagram). This is used to illustrate the overall system function and the system-external interfaces.
The method covers subproduct User Requirements.Preliminary System Description, together with function FCTD - Functional Decomposition on the function side. |
| 4.3 | SD1.5 - User-Level System Structure |
To model the system, data flow diagrams have to be designed in top-down method down to the segment level. This is not meant to anticipate a design but to model the system in its essence, independent of any realization decisions.
The method covers subproduct User Requirements.Description of the Functionality only in part, i. e. from the point of view of a preliminary collaboration of functions; therefore it must be completed by further methods. |
| 4.4 | SD2.1 - Technical System Design |
The functional units (processes) of the data flow model with its data flows are adjusted to the identified elements of the System Architecture (segments, SW Units, HW Units) and with the external and internal system interfaces.
The method covers subproducts Interface Overview.System-External Interfaces and Interface Overview.System-Internal Interfaces. |
| 4.5 | SD3.2 - Specification of Requirements for External Interfaces of SW/HW Unit |
The external interfaces have to be adjusted to the external terminators of the DFM.
Subproduct Technical Requirements.Technical Requirements for the Interfaces is covered to the extent that logical information flows, describing in/output, are modeled. It may be necessary to apply method CFM - Control Flow Modeling as well. |
| 4.6 | SD3.3 - Definition of Requirements for the Functionality |
A data flow model must be generated and possibly improved for the SW Unit. If necessary, a restructuring is required in comparison with the DFD from SD2.2 - Realization of Efficiency Analysis, particularly in the case where individual functions have been distributed to different SW Units.
The method completely covers subproduct Technical Requirements.Overall Function of Element only in part, i. e. from the point of view of a preliminary function collaboration; it must be completed by other methods. |
5 Interfaces
| No. | Interface | Observation | Information in Annex 1 |
|---|---|---|---|
| 5.1 | DFM-ER |
The data flow diagram must be generated by means of reciprocal adjustments with the ER - E/R Modeling (allocation of E/R submodels to the elements of the data flow diagram).
Note: |
4.3 Interface DFM-ER |
| 5.2 | DFM-FCTD | The results in chapter Functional Structure are brought in the execution of the basic method Data Flow Modeling in order to define the functional level. | 4.4 Interface DFM-FCTD |
| 5.3 | DFM-CFM | Control flow models have to be generated on the basis of data flow models. The components of CFM - Control Flow Modeling are integrated into the corresponding data flow models. | 4.5 Interface DFM-CFM |
6 Literature
/DeMarco, 1978/ Structured Analysis and System Specification/Hatley, 1987/ Strategies for Real-Time System Specification
/Martin, 1987/ Diagramming Standards for Analysts & Programmers
/McMenamin, 1984/ Essential Systems Analysis
7 Functional Tool Requirements
SSD07 - Supporting Modeling of Information Flows
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