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Objectives
At the end of this Topic you should be able to:
- Explain the purpose of data-flow diagrams.
- Describe the meaning of the symbols used in data-flow diagrams.
- Describe the generic framework activities at which data flow diagrams can be used and the corresponding roles of data-flow diagrams in these stages.
- Construct simple data-flow diagrams from a textual description.
- Construct a levelled set of data-flow diagrams.
- Understand how to check the consistency of related data-flow diagrams.
Introduction to data-flow diagrams
What are data-flow diagrams?
Data-flow diagrams (DFDs) model a perspective of the system that is most readily understood by users – the flow of information through the system and the activities that process this information. Data-flow diagrams provide a graphical representation of the system that aims to be accessible to computer specialist and non-specialist users alike. The models enable software engineers, customers and users to work together effectively during the analysis and specification of requirements. Although this means that our customers are required to understand the modeling techniques and constructs, in data-flow modeling only a limited set of constructs are used, and the rules applied are designed to be simple and easy to follow. These same rules and constructs apply to all data-flow diagrams (i.e., for each of the different software process activities in which DFDs can be used).
An example data-flow diagram
An example of part of a data-flow diagram is given below. Do not worry about which parts of what system this diagram is describing – look at the diagram to get a feel for the symbols and notation of a data-flow diagram.
Figure 1. An example data-flow diagram
As can be seen, the DFD notation consists of only four main symbols:
- Processes — the activities carried out by the system which use and transform information. Processes are notated as rectangles with three parts, such as “Order Supplies” and “Make Payments” in the above example.
- Data-flows — the data inputs to and outputs from to these activities. Data-flows are notated as named arrows, such as “Delivery” and “Supply Order” in the example above.
- External entities — the sources from which information flows into the system and the recipients of information leaving the system. External entities are notated as ovals, such as “Supplier” in the example above.
- Data stores — where information is stored within the system. Data stores are notated as rectangles with two parts, such as “Supplier Details” and “Orders” in the example above. The diagrams are supplemented by supporting documentation including a data dictionary , describing the contents of data-flows and data stores; and process definitions , which provide detailed descriptions of the processes identified in the data-flow diagram. The benefits of data-flow diagrams Data-flow diagrams provide a very important tool for software engineering, for a number of reasons:
- The system scope and boundaries are clearly indicated on the diagrams (more will be described about the boundaries of systems and each DFD later in this chapter).
- The technique of decomposition of high level data-flow diagrams to a set of more detailed diagrams, provides an overall view of the complete system, as well as a more detailed breakdown and description of individual activities, where this is appropriate, for clarification and understanding. Note Use-case diagrams also provide a partition of a software-system into those things which are inside the system and those things which are outside of the system.
Elements of data-flow diagrams
Four basic elements are used to construct data-flow diagrams:
- processes
- data-flows
- data stores
- external entities The rest of this section describes each of the four elements of DFDs, in terms of their purpose, how the element is notated and the rules associated with how the element relates to others in a diagram.
Notation and software
A number of different notations exist for depicting these elements, although it is only the shape of the symbols which vary in each case, not the underlying logic. This unit uses the Select SSADM notation in the description and construction of data-flow diagrams. As data-flow diagrams are not a part of the UML specification, ArgoUML and Umbrello do not support their creation. However, Dia is free software available for both Windows and Ubuntu which does support data-flow diagrams.
Processes
Purpose
Processes are the essential activities, carried out within the system boundary, that use information. A process is represented in the model only where the information which provides the input into the activity is manipulated or transformed in some way, so that the data-flowing out of the process is changed compared to that which flowed in. The activity may involve capturing information about something that the organisation is interested in, such as a customer or a customer's maintenance call. It may be concerned with recording changes to this information, a change in a customer's address for example. It may require calculations to be carried out, such as the quantity left in stock following the allocation of stock items to a customer's job; or it may involve validating information, such as checking that faulty equipment is covered by a maintenance contract.
Notation
Processes are depicted with a box, divided into three parts.
Figure 2. The notation for a process
The top left-hand box contains the process number. This is simply for identification and reference purposes, and does not in any way imply priority and sequence. The main part of the box is used to describe the process itself, giving the processing performed on the data it receives. The smaller rectangular box at the bottom of the process is used in the Current Physical Data-Flow Diagram to indicate the location where the processing takes place. This may be the physical location
— the Customer Services Department or the Stock Room , for example. However, it is more often used to denote the staff role responsible for performing the process. For example, Customer Services , Purchasing , Sales Support , and so on. Rules The rules for processes are:
- Process names should be an imperative verb specific to the activity in question, followed by a pithy and meaningful description of the object of the activity. Create Contract , or Schedule Jobs , as opposed to using very general or non-specific verbs, such as Update Customer Details or Process Customer Call.
- Processes may not act as data sources or sinks. Data flowing into a process must have some corresponding output, which is directly related to it. Similarly, data-flowing out of a process must have some corresponding input to which it is directly related.
- Normally only processes that transform system data are shown on data-flow diagrams. Only where an enquiry is central to the system is it included.
- Where a process is changing data from a data store, only the changed information flow to the data store (and not the initial retrieval from the data store) is shown on the diagram.
- Where a process is passing information from a data store to an external entity or another process, only the flow from the data store to the process is shown on the diagram. Data-flows Purpose A data-flow represents a package of information flowing between two objects in the data-flow diagram. Data-flows are used to model the flow of information into the system, out of the system, and between elements within the system. Occasionally, a data-flow is used to illustrate information flows between two external entities, which is, strictly speaking, outside of the system boundaries. However, knowledge of the transfer of information between external entities can sometimes aid understanding of the system under investigation, in which case it should be depicted on the diagram. Notation A data-flow is depicted on the diagram as a directed line drawn between the source and recipient of the data-flow, with the arrow depicting the direction of flow.
Figure 3. Notation for a data-flow
The directed line is labelled with the data-flow name, which briefly describes the information contained in the flow. This could be a Maintenance Contract , Service Call Details , Purchase Order , and so on. Data-flows between external entities are depicted by dashed, rather than unbroken, lines. Rules The rules for drawing data-flows are:
- Because of their function in the storage and retrieval of data, data stores often provide input dataflows to receive output flows from a number of processes. For the sake of clarity and to avoid crisscrossing of data-flows in the data-flow diagram, a single data store may be included in the diagram at more than one point. Where the depiction of a data store is repeated in this way, this is signified by drawing a second vertical line along the left-hand edge of the rectangle for each occurrence of the data store. External entities Purpose External entities are entities outside of the system boundary which interact with the system, in that they send information into the system or receive information from it. External entities may be external to the whole organisation — as in Customer and Supplier in our running example; or just external to the application area where users' activities are not directly supported by the system under investigation. Accounts and Engineering are shown as external entities as they are recipients of information from the system. Sales also provide input to the system. External entities are often referred to as sources and sinks. All information represented within the system is sourced initially from an external entity. Data can leave the system only via an external entity. Notation External entities are represented on the diagram as ovals drawn outside of the system boundary, containing the entity name and an identifier.
Figure 5. Notation for external entities
Names consist of a singular noun describing the role of the entity. Above the label, a lower case letter is used as the identifier for reference purposes. Rules The rules associated with external entities are:
- Each external entity must communicate with the system in some way, thus there is always a dataflow between an external entity and a process within the system.
- External entities may provide and receive data from a number of processes. It may be appropriate, for the sake of clarity and to avoid crisscrossing of data flows, to depict the same external entity at a number of points on the diagram. Where this is the case, a line is drawn across the left corner of the ellipse, for each occurrence of the external entity on the diagram. Customer is duplicated in this way in our example. Multiple copies of entities and data stores on the same diagram At times a diagram can be made much clearer by placing more than one copy of an external entity or data store in different places — this can avoid a tangle of crossing data-flows.
Where more than one copy of an external entity appears on a diagram it has a cut off corner in the top left, such as below:
Figure 6. How to notate duplicated external entities
When more than one copy of a data store appears on a diagram it has a cut off left-side, such as below:
Figure 7. How to notate duplicate data stores
Context diagrams
What is a context diagram?
The context diagram is used to establish the context and boundaries of the system to be modelled: which things are inside and outside of the system being modelled, and what is the relationship of the system with these external entities. A context diagram, sometimes called a level 0 data-flow diagram , is drawn in order to define and clarify the boundaries of the software system. It identifies the flows of information between the system and external entities. The entire software system is shown as a single process. A possible context diagram for the Video-Rental LTD case study is shown below.
Figure 8. A context diagram for Video-Rental LTD
The process of establishing the analysis framework by drawing and reviewing the context diagram inevitably involves some initial discussions with users regarding problems with the existing system and the specific requirements for the new system. These are formally documented along with any specific system requirements identified in previous studies. Having agreed on the framework, the detailed investigation of the current system must be planned. This involves identifying how each of the areas included within the scope will be investigated. This could be by interviewing users, providing questionnaires to users or clients, studying existing system documentation and procedures, observation and so on. Key users are identified and their specific roles in the investigation are agreed upon.
Constructing a context diagram
In order to produce the context diagram and agree on system scope, the following must be identified:
- external entities
- data-flows
We can see that on this level 1 DFD there are a number of data stores, and data-flows between processes and the data stores. It is important to notice that the same data-flows to and from the external entities appear on this level 1 diagram and the level 0 context diagram. Each time a process is expanded to a lower level, the lower level diagram must show all the same data-flows into, and out of the higher level process it expands.
Constructing level 1 DFDs
If no context diagram exists, first create one before attempting to construct the level 1 DFD (or construct the context diagram and level 1 DFD simultaneously). The following steps are suggested to aid the construction of Level 1 DFD:
- Identify processes. Each data-flow into the system must be received by a process. For each dataflow into the system examine the documentation about the system and talk to the users to establish a plausible process of the system that receives the data-flow. Each process must have at least one output data-flow. Each output data-flow of the system must have been sent by a process; identify the processes that sends each system output.
- Draw the data-flows between the external entities and processes.
- Identify data stores by establishing where documents / data needs to be held within the system. Add the data stores to the diagram, labelling them with their local name or description.
- Add data-flows flowing between processes and data stores within the system. Each data store must have at least one input data-flow and one output data-flow (otherwise data may be stored, and never used, or a store of data must have come from nowhere). Ensure every data store has input and output data-flows to system processes. Most processes are normally associated with at least one data store.
- Check diagram. Each process should have an input and an output. Each data store should have an input and an output. Check the system details so see if any process appears to be happening for no reason (i.e., some “trigger” data-flow is missing, that would make the process happen).
Decomposing diagrams into level 2 and lower
hierarchical levels
What is a level 2 (or lower) DFD?
We have already seen how a level 0 context diagram can be decomposed (exploded) into a level 1 DFD. In DFD modeling terms we talk of the context diagram as the “parent” and the level 1 diagram as the “child”. This same process can be applied to each process appearing within a level 1 DFD. A DFD that represents a decomposed level 1 DFD process is called a level 2 DFD. There can be a level 2 DFD for each process that appears in the level 1 DFD. A possible level 2 DFD for process “2: Loan of video” of the level 1 DFD is as follows:
Figure 10. A level 2 data-flow diagram for Video-Rental LTD
Note, that every data-flow into and out of the parent process must appear as part of the child DFD. The numbering of processes in the child DFD is derived from the number of the parent process – so all processes in the child DFD of process 2, will be called 2.X (where X is the arbitrary number of the process on the level 2 DFD). Also there are no new data-flows into or out of this diagram – this kind of data-flow validation is called balancing. Look at the rectangular boundary for this level 2 DFD. Outside the boundary is the external entity “Customer”. Also outside the boundary are the two data stores – although these data stores are inside the system (see the level 1 DFD), they are outside the scope of this level 2 DFD. Constructing level 2 (and lower) DFDs — functional decomposition The level 1 data-flow diagram provides an overview of the system. As the software engineers' understanding of the system increases it becomes necessary to expand most of the level 1 processes to a second or even third level in order to depict the detail within it. Decomposition is applied to each process on the level 1 diagram for which there is enough detail hidden within the process. Each process on the level 2 diagrams also needs to be checked for possible decomposition, and so on. A process box that cannot be decomposed further is marked with an asterisk in the bottom right hand corner. A brief narrative description of each bottom-level process should be provided with the dataflow diagrams to complete the documentation of the data-flow model. These make up part of the process definitions which should be supplied with the DFD. Each process on the level 1 diagram is investigated in more detail, to give a greater understanding of the activities and data-flows. Normally processes are decomposed where:
- There are more than six data-flows around the process
- The process name is complex or very general which indicates that it incorporates a number of activities. The following steps are suggested to aid the decomposition of a process from one DFD to a lower level DFD. As you can see they are very similar to the steps for creating a level 1 DFD from a context diagram:
- Make the process box on the level 1 diagram the system boundary on the level 2 diagram that decomposes it. This level 2 diagram must balance with its “parent” process box — the dataflows to and from the process on the level 1 diagram will all become data-flows across the system boundary on the level 2 diagram. The sources and recipients of data-flows across the level 2 system boundary are drawn outside the boundary and labelled exactly as they are on the level 1 diagram. Note that these sources and recipients may be data stores as well external entities or other processes — this is because a data store in a level 1 diagram will be outside the boundary of a level 2 process that sends or receives data-flows to/from the data store.
Always bear in mind that levelling is meant to simplify and clarify the diagrams, and if this cannot be done then it may be best to leave the diagram as it is.
Balancing
The key to successfully levelling is to make the diagrams balance. For example, if a second level diagram expands a first level process then all the inputs to the process must be inputs to the second level diagram, and all the outputs from the process must be outputs on the second level diagram. Moreover, there must be no other inputs and outputs. To be particular, all the inputs and outputs of the system which appear on the context diagram must appear on the level one diagram and there should be no other inputs and outputs on the level one diagram. This does not mean there can be no changes to the higher levels of a set of diagrams. When producing a lower level diagram, the software engineer may realise that a new input is needed for the process to be able to carry out its task. In which case, the software engineer should add this data-flow as an input and then add the input as a data-flow to the original process. If needs be, this input may be added at several levels higher up. The software engineer may add new outputs in the same way. As long the diagrams always balance, inputs and outputs can be added and removed wherever necessary.
Numbering
Numbering in a levelled set of diagrams is important, as the numbers help you to find your way around the levels. It is easily described by example. Suppose Receive Order is the process numbered 3 on the level one diagram (remember, numbers do not indicate any order, they are simply labels) and this is expanded to a level two diagram. The process numbers on the level two diagram will be 3.1, 3.2, 3. and so on. Suppose now that process 3.4 on the level two diagram is Register New Customer and needs further expansion to a level three diagram. The process numbers on this diagram will be 3.4.1, 3.4.2, 3.4.3 and so on. The rule used here is this: if X is the number of the process you wish to expand, then the numbers on the next level are X.1, X.2, X.3... The same applies for data stores. Data stores that appear in a level two diagram expanding a process labelled 4 in the level one diagram will be numbered D4.1. D4.2, D4.3 and so on. Deeper levels will be D4.1.1, D4.1.2, the numbering scheme being just the same as for processes. Note though, it is not the data stores that are expanded. They may simply appear in the expansion of a process.
Process descriptions
A software engineer may define a process where no further expansion is appropriate because there are no separate sub-processes which may make up the original process. However, the software engineer may still wish to describe the process in more detail as it is a particularly difficult or tricky process. In this case, the software engineer writes down a process description for the process. This can take any form which the software engineer thinks appropriate. Traditional flowcharts could be used or plain English. More common is what is called structured English. This looks like English only it is written more like a computer language. It used to avoid the problem that different people reading the same piece of plain English may understand it in different ways.
Validation
It should be clear that producing data-flow diagrams can be complicated. A routine check using the following questions should make sure that you find any simple mistakes. The first set of questions refer to a single diagram, so if you have a set of levelled data-flow diagrams then these checks need to be made for each diagram.
- Is every data-flow attached to a process at either the beginning or the end of the arrow?
- Is every data-flow labelled with a sensible noun?
- Does every process have at least one input and at least one output?
- Is every process named sensibly (no uses of words such as “process” or “handle”) with an action and what is acted upon? (The template is “Do something to something”)
- Is every data store named with the type of thing it stores in the plural?
- Where data stores and external entities have been shown several times on one diagram, do all instances have a “diagonal” line?
- Are there any data-flows which cross? If so, try and add more duplicate external entities or data stores to avoid the crossing. This second set of questions is specifically about levelling and so should be asked about the set of diagrams as a whole.
- Do all diagrams balance? That is, where a diagram expands a process in a higher level, are the inputs and outputs to the process identical to the inputs and outputs on the expanded, lower level diagram?
- Are all external entities shown on both the context diagram and level one diagram?
- Are all of the processes and data stores numbered correctly? All data-flow diagrams are an aid to communication between software engineers and their customers. Although they may be correct and accurate, a messy or tangled data-flow model will reduce communication as surely as a long-winded text description. To avoid this, as the diagrams evolve, redraw them whenever they begin to get cluttered or have several corrections on them. A simple rearrangement of the components may be sufficient to greatly improve a diagram.
An example in constructing a data-flow
diagram
Just as for logical data structures, to make a data-flow diagram we must analyse the requirements and describe the system in terms of the components of data-flow diagrams. Several attempts will be needed before a final and complete model of the system can be produced. Unfortunately, there is no straightforward way to progress through developing a data-flow diagram. We are aiming therefore to build a skeleton model on paper which we can then work with and develop more fully. Each stage will be illustrated with examples from the following Estate Agent case study (repeated from ???).
and so should have a different name from the input data. Property Details are taken and recorded as a property on the file so the output could be just something like Recorded Property or more simply, Property. Now start again only using this output as a new input. It must either go to another process, to a data store or to an external entity. It should be clear from the case study what happens. If a new process is needed then do the same again. Find a sensible name for the process using the case study, determine and label the outputs and then follow the outputs. If the data is stored then add a data store to the diagram, name it sensibly from the case study and draw the output arrow going into the data store. This is what happens in Property and so we add the data store Properties. If the output is an output from the system then simply add the external entity which receives the output. When the data is finally output or comes to rest in a data store, go back and follow any of the other outputs which may have been defined on the way. When they are exhausted, choose a new input and follow that through in exactly the same way. Follow events Another way to approach building up a data-flow model is to consider what happens in the system. The case study will outline a number of events. There must be processes in the system which respond to these events or even make them happen. Identify these processes and then add the data inputs which are used by the process and determine the outputs. For example, in the estate agent example, there is the phrase, “When a sale is completed...”. This is an event: a sale is completed. From the case study we see that lots of things then happen: the buyer confirms exchange of contracts so this is an input to some process; the client details are removed from the file and invoice is sent out. This is the process. A sensible name might be Record Sale or possibly Receive Sale Confirmation. The data needed is the input from the Buyer and Client details which are on file. This must mean there is a data store somewhere on the diagram holding this information. If there is not one there already then add it. And the output must be an invoice to the Client. From here on the approach is the same as following inputs. For any new outputs, work out where those outputs must go and if it is to a process follow them as if they were inputs to the new process. Most processes can be found in the case study using either technique of following inputs or following events. However, some processes are related to temporal events and so can only be found by following events. As the name suggests, temporal events are events which occur at specific times. They are not prompted to happen by the arrival of new data, but rather because a certain time has been reached. These events often appear in case studies beginning with phrases such as, “Once a month...” or, “At the end of every day,...”. However, once these have been identified, producing the model by following this event is exactly the same as for any other event. In the estate agent system, there are two temporal events: there is a weekly matching of potential buyers with properties; invoices and reminders are sent out on a monthly basis. Though time is the trigger the processes carrying out temporal events, time is generally not shown on the data-flow diagram. This is because the time aspect is often just a practical implementation rather than rigid necessity. For example, the matching of buyers and properties at the estate agents need not be weekly. It is probably done weekly so that it always gets done, and also so that it does not interrupt the other daily business. With an automated system it may be possible to match buyers with properties as soon as any new details on either arrive. Where time is crucial to a process, say accounting done at the end of a financial year, then this can be reflected in the name of the process. For example, “Calculate end of year profits”.
Fill in gaps After building a model that handles each input or each event, it is worth going over the processes defined so far. For each process, ask the question, “Does this process have all the information it needs to perform its task?” For instance, if a process sends out invoices, does it have all the details of the invoice and the address of where the invoice should go? If the answer is No , then add a data-flow into the process which consists of the data needed by the process. If there are several, clearly distinct items of data needed, then you may need an arrow for each item. Now try to identify the source of the data. First, see if the data can be found already inside the system, either on a data store or as a result of a process. If not, it may be that the data can be obtained by processing some of the existing data in, which case add a new process that takes the existing data and makes the data you require. Or, the data may be available but from the case study it is clear that there is a time-lag between the process that produces the data and the process which uses it. Simply add a data store where the data can reside till it is needed. If there is still no source for the data then it could be from an external entity. In which case, this is a new input to the system. It may not be explicitly mentioned in the case study, but if it is necessary then it should be added. Having added the new input from the appropriate entity, go back and correct the context diagram. This is an important task. If there is not enough data to support a task then the system will not function properly. Of course, to be on the safe side, you could have all the data going to all the processes! But this is not really a solution because with a large system this would be impractical. Having examined all the processes, check that all the outputs have been generated. All of the inputs should have been covered already, but this does not mean that all the outputs have been produced. If there is still an output which does not appear on the diagram, see if there is a process where it could come from. If there is no sensible candidate, add a process and begin to work backwards. What inputs does the new process need? Where do these inputs come from? This task is almost the same as the one just described. Any left over outputs must have come from a process. Outputs cannot come from data stores or external entities. If there is no sensible way to fit the output into the diagram then it may be that it is not a sensible output for the system you are currently considering. Use the case study to confirm this. Finally, check the data stores. Data must enter a data store somehow and generally data on a data store is read. For each data store, identify when the store is either written or read by considering the processes which may use the data. Also, use the case study to see that you have not missed any arrows to or from a data store. Repeat By this stage, you will have considered all the inputs, all the outputs and produced a first draft of the data-flow model of the system. Review the case study, looking for functionality described which is not performed by the model. In particular, look for temporal events as these are sometimes hidden implicitly in the text. Where necessary, add new processes that perform the omitted functions and use the method of following events to work out their inputs and outputs. Fill in the gaps of the model in exactly the same way as was done to produce the first attempt.
