Meta-Model and UML Profile for Requirements Management of Software and Embedded Systems

Abstract

Software and embedded system companies today encounter problems related to requirements management tool integration, incorrect tool usage, and lack of traceability. This is due to utilized tools with no clear meta-model and semantics to communicate requirements between different stakeholders. This paper presents a comprehensive meta-model for requirements management. The focus is on software and embedded system domains. The goal is to define generic requirements management domain concepts and abstract interfaces between requirements management and system development. This leads to a portable requirements management meta-model which can be adapted with various system modeling languages. The created meta-model is prototyped by translating it into a UML profile. The profile is imported into a UML tool which is used for rapid evaluation of meta-model concepts in practice. The developed profile is associated with a proof of concept report generator tool that automatically produces up-to-date documentation from the models in form of web pages. The profile is adopted to create an example model of embedded system requirement specification which is built with the profile.

1. Introduction

Requirements Management is a crucial part of systems development. Several solutions and tools for requirements management process have been provided for software development [1]. However, surveys have indicated problems in requirements management processes used by practitioners related to tool integration, incorrect tool usage, and lack of traceability of requirements to development artifacts [2]. The most common tools currently used for requirements management in industry are word-processing packages, spreadsheet applications, and internal web pages. Using such tools with no clearly defined templates can lead to errors in communicating requirements between different stakeholders. As these errors are related to requirements capture, they happen early in the development process, and thus, they are likely to incur high in cost in later development phases.

Unified Modeling Language version 2 (UML2) [3] is a standard language for modeling systems. UML2 offers a rich set of diagrams for software architecture modeling but also expansion and tailoring methods for other domains. One of such expansion methods is to use UML profiles that add new domain-specific model elements to the UML language with stereotypes, tag definitions, and constraints [4].

Meta-model (i.e., domain model) is a structure of concepts in a certain domain. Meta-model captures abstractions and relationships of the target domain concepts. Meta-Object Facility (MOF) [5] is a standard language for creating meta-models. It is a subset of the UML language.

This paper presents a comprehensive meta-model for requirements management. The focus is on software and embedded system domains. Our goal is to define generic requirements management domain concepts and to establish abstract interfaces between requirements management and systems development. This leads to a portable requirements management meta-model which can be used with various system modeling languages. The meta-model also focuses on temporal aspects of requirements management during development process, such as states of requirements and changing requirements. Hence, the goal is to capture the whole requirements management process, not just the concepts needed for requirements specification. The MOF language is used for meta-model definition.

The created meta-model is prototyped by translating it into a UML profile. The profile is imported into a UML tool which is used for rapid evaluation of meta-model concepts in practice. The UML tool is further customized to remove unnecessary features from the tool to create a designer-friendly interface. The primary purpose of the profile is to prototype the meta-model, but it is also suitable to be used in real requirements management as such. For this purpose, the profile is associated with a proof of concept report generator tool. Its task is to form web pages documenting the current state of requirements. We illustrate how the profile is adopted in practice with a case study of an embedded system design. The work has been carried out in a joint project with several embedded system companies that are defining a common requirements management process and tool.

The paper is structured as follows. Section 2 presents the approach for prototyping and evaluating requirements management concepts. Related work on requirement management methods, meta-models, and UML profiles are discussed in Section 3. The general requirements process concepts are discussed in Section 4. Our meta-model for requirements management is presented in Section 5. The UML profile definition and tool customization is covered in Section 6. The example model is presented in Section 7. The report generator tool is presented in Section 8. Section 9 concludes the paper.

2. Our Approach for Prototyping Requirements Management Meta-Model

Figure 1 presents the general phases for creating, deploying, and using a UML profile. It also shows the outcome of this work regarding each phase. Figure 2 presents the concrete flow used in this work.

Figure 1

General phases for creating, deploying, and using a UML profile included with outcome of this work.

Figure 2

Flow for defining, deploying, and using UML profile for requirements management.

There is a widely used de facto standard approach for creating UML profiles proposed by Selic [6]. We follow this approach of designing a UML profile built on conceptual meta-model (domain model). The first phases are to discover the key abstractions and relationships of the target domain and to form a meta-model of the problem. The idea is to identify the concepts that simplify the modeled reality from aspects relevant to the particular domain. It should be emphasized that the meta-model does not imply the model notation when using the resulting UML profile. Instead, it forms the abstract syntax of the profile.

Next, the concepts of the meta-model are translated into a UML profile. This is done by creating stereotypes from domain modeling concepts and then mapping the stereotypes to UML meta-classes. The usage of stereotyped model elements is refined with tag definitions (stereotype attributes) and constraints. For example, a UML classifier can be given stereotype Requirement which means that the resulting model element behaves in diagrams as a classifier but also has the distinct properties of a requirement. Tag definitions of the stereotype refine the properties of the new model element. Constraints Constraining can be performed, for example, with Object Constraint Language (OCL) [7].

It should be noted that not all concepts of the meta-model necessarily become stereotypes. Only those concepts should be covered that are necessary in practical modeling situations. Moreover, as stereotypes are extending the UML meta-model, new stereotype should not be created if the corresponding concept is already covered by the UML language.

The benefit of this kind of profile design approach is that the features of the UML language do not bias the definition of the domain-modeling problem. This is because the semantics of the model become separated from the notation of the model.

After creating a UML profile, additional tool customization can be carried out to remove unnecessary features from the UML tool that are not needed for requirements management purposes. We use MagicDraw UML 16.5 tool from No Magic [8] as the UML tool. It allows creating custom diagrams and to customize model elements using a proprietary DSL engine. In practice, this means that only the model elements of the profile are visible for the modeler while other UML-related elements are hidden from the tool. This makes usage of the profile easier for designers.

After these phases, the deployed UML profile is ready to be used in modeling. After creating the requirements specification model, an automated report generation can be carried out using a dedicated model processing tool. It produces up-to-date documentation of the requirements and takes care of managing different versions of requirements releases. Only phases 5 and 6 are used in everyday requirements engineering work.

2.1. On UML Profiling for Prototyping a Meta-Model

It must be emphasized that there are several other ways of prototyping a meta-model as a tool. For example, Eclipse Graphical Modeling Framework (GMF) [9] provides a model-driven approach to generating graphical editors in Eclipse from a meta-model. The meta-model can be also implemented directly as a dedicated software program using a software programming language, such as Java. Our requirements management meta-model can be, and has been, implemented as a spreadsheet template in Microsoft Excel application.

The benefit of UML profiling is the fast and easy transformation of MOF meta-model into a deployable UML profile. This is because UML is defined with MOF [6]. Therefore, same UML tool can be used for defining them both. Furthermore, UML is a widely-adopted standard modeling language with extensive tool support. Yet, although UML profiles allow the reuse of tooling infrastructure the meta-model implementation may suffer from the limitations of the UML language itself. Nevertheless, UML profiling enables a cost-effective solution for prototyping a meta-model.

3. Related Work

Related work has various recommendations, meta-models, and UML profiles for requirements management. In the following, some of the closest to our work are examined.

3.1. Recommendations for Requirements Management Process

The IEEE computer society's Guide to the Software Engineering Body of Knowledge gives recommendations on requirements management for software systems [10]. It defines the basic concepts and presents general guidelines for requirements management. It gives a detailed description of the phases of iterative requirements management process. Some of its core concepts are reused in this work. Other references on general requirement management of software systems include for example, [11, 12]. There are several proposals for proprietary requirements management templates and processes, such as [13, 14].

3.2. Meta-Models and UML Profiles for Requirements Management

Researchers have developed various meta-models and taxonomies for requirements management, each focusing on different aspects. Kabanda et al. [15] define a requirements meta-model for software systems that incorporates social features: users, policies, and culture. Firesmith [16] presents a detailed taxonomy for security-related requirements, and Glinz [17] focuses on nonfunctional requirements. Ramesh and Jarke [18] present reference models for requirements traceability based on focus groups and interviews conducted in 26 software development organizations. The synthesized models were validated with several case studies and incorporated in a number of commercial traceability tools.

SysML [19] is a standard UML profile that, among other things, defines model elements and a specialized diagram for documenting requirements. Models created with SysML can be also attached to other UML models that make it generic in nature. Berenbach and Gall [20] present UML stereotype extensions for integrating the modeling of functional and nonfunctional requirements as well as hazards in use case diagrams. Zhu and Gordon [21] propose a UML profile for modeling design decisions and an associated UML profile for modeling nonfunctional requirements. Pardillo et al. [22] present a meta-model and UML profile for measurable requirements that connect goals, requirements, and measures.

3.3. Summary of Related Work

The related work proposals have been, in most cases, targeted for software systems. Although requirements management process for software systems can be equally well used in embedded system development due to similar nature of development processes, the related proposals lack following characteristics important for both domains in practical requirements management:

• (i) documenting and tracking the states of requirements during the development process,

• (ii) allocating and reusing requirements among product families, products, product variants, and system components (e.g., processors, busses, SW modules, etc.),

• (iii) establishing relationships to system modeling, analysis, and verification.

The main contribution of this paper is the definition of a meta-model and UML profile for requirements management to be utilized in practical software and embedded system development which, among other things, incorporates the above-mentioned aspects.

4. Requirements Management Process

The core of our meta-model definition is based on the general requirements management process [10]. In the following, an introduction to the process phases is given.

The requirements management process is composed of five main phases shown in Figure 3. Requirements elicitation is the first phase of the process. It involves investigating possible stakeholders and listing their main requirements for the product. The discovery of stakeholders is also called stakeholder analysis. As the result of the requirements elicitation, all stakeholders and their main requirements should be listed.

Figure 3

Iterative requirements management process.

The second step is requirements analysis. The first task of the analysis is to make sure that no requirements are in conflict with each other. Conflicting requirements usually originate from different objectives and motivations of different stakeholders, but can be also due to errors in the elicitation phase. Conflict resolution is always a compromise in which one or several requirements must change. The second task of the analysis is to refine the requirements and form hierarchies and other relationships between requirements. For example, this can be translating end user requirements to derived system requirements. The third task is to allocate requirements to design components, system models, and tests that will verify them.

The third step is requirements specification. This step is creating or changing requirements documentation based on the elicitation and analysis. The documentation can be in form of electronic document or internal web pages of the company, for example. The specification acts as the first and foremost vehicle in communicating requirements to system developers who use it as the basis for design and verification. For this reason, it is important that there is a common understanding among all stakeholders about the semantics of the specification. The best approach to foster this understanding and reduce misinterpretations is to force specification writers to utilize disciplined and well-defined templates, meta-models when writing the requirements document.

The fourth step is requirements validation. The first task of the validation is to make sure that the formed requirements define the right system. That is to make sure that requirements truly correspond to stakeholders' intentions before resources are committed to development. The second task of the validation is to make an assessment whether the requirements are feasible. Traditional techniques for validation include reviews of requirements documents and building early system models and prototypes.

In embedded system domain, the requirement validation is closely related to design-space exploration (DSE) [23]. DSE is optimizing the platform and mapping based on measuring relevant system parameters (e.g., power, execution time, and area) of several single-design points by static analysis or simulation. From requirements management aspect, the performance results of design-space exploration iterations should be compared against the set requirements for early requirement validation.

The final step comes after the design and implementation phase and it is requirements verification. Although it comes after the development phase, it is an important part of the requirement management process. The purpose is to verify that the end product or development process meet the given requirements. For this purpose, the requirements specification can include additional verification plans and models that refine how the requirement is supposed to be verified.

5. Meta-Model for Requirements Management

This section presents our meta-model for requirements management. The meta-model is depicted by the class diagrams presented in Figures 4–7. In the following, each element of the meta-model is discussed separately.

Figure 4

Main view of the requirements management meta-model.

Figure 5

Requirement management meta-model relationships, system parameters, and tests.

Figure 6

Requirement management meta-model interface to project management.

Figure 7

Products, product families, variants, and configurations.

5.1. Requirements and Their Properties

Figure 4 presents the main abstractions related to requirements. Requirement is a property that must be exhibited by a product, some of its part (e.g., subsystem, module), or its development process. Requirement has description, identifier, version, type, state, owner, and stakeholders as its attributes.

Description is a verbal expression of the requirement. The description should be expressed unambiguously and, if possible, quantitatively [10]. This concerns especially to nonfunctional requirements.

Identifier is a unique fingerprint of a requirement. It is used to separate a requirement from other requirements with a unique character string. On the other hand, a requirement may have several Versions. This is to track small changes of requirements after their first definition. Type classifies requirements to belong into certain category based on their nature. The possible categories have been defined in Figure 4 as an enumeration attribute. If the type of the requirement is Restriction, then its nature can be further refined with Restriction type.

State of a requirement is composed of four attributes that characterize its present status in the requirements management process. The first component is Conflict state. It defines whether the requirement has been analysed and if it is in conflict. Validation state defines whether the requirement has been validated. Authorization state determines whether the requirement is authorized for development. This attribute represents the final approval for committing resources to development in the context of a single requirement. The requirement can also be rejected. This means that it will not be considered in the development at all. Verification state defines whether the requirement has been verified after the development phase.

A requirement has also an owner and one or several stakeholders. Owner is an actor (person or company) who is responsible for the life span of the requirement. The owner takes care that the requirement is carried along the project and that the actions taken in the project comply with fulfilling the requirement. Stakeholder is an actor for whom the requirement is somehow meaningful. A stakeholder always gains or loses something based on the result of fulfilling the requirement.

Contracts are elements that bundle requirements together. A new contract typically brings new requirements to the product or development process.

5.2. Requirements Relationships

The network of requirements relationships is typically very complex in system design and the nature of the relationships may be ambiguous. However, it is important that the dependencies between requirements are identified and documented. This helps in later stages of development if requirements need to be changed. Changing a requirement may require that several other related requirements need to be reanalysed. Identification of the relationships helps to narrow down the number of requirements that need to be considered in requirement analysis due to a change in a requirement. We define three basic relationships between requirements which are composite, derive, and conflict. They are presented in Figure 5. All the basic relationships can be further refined with a free-form description to give additional semantics for them.

The composite relationship is used to decompose a complex requirement into several subrequirements. This allows to form requirement hierarchies. For example, composition can be used to divide responsibility of fulfilling a requirement between several design teams. The parent requirement is fulfilled after all its child requirements are fulfilled. The owner of requirement may change between hierarchy levels. This is to allow division of responsibility. The stakeholders are inherited from the upper levels of hierarchy to the lower ones while allowing to add new stakeholders to lower levels. The derive relationship can be used to express derivation dependencies between requirements. Derived requirements need to be reanalysed when the source requirement changes. Good example is a channel throughput requirement that is analysed to derive requirements for, data bus width in bits, data compression ratio, and max bit error rate (BER). The conflict relationship indicates that two or more requirements are in conflict which needs to be resolved before committing resources to development.

There is usually other information related to a requirement in addition to its textual description. We define three types of additional information. There are system models that refine the requirement describing how the requirement should be considered in the system. For example, a UML sequence diagram can be used to describe the protocol that refines the requirement "User must authenticate during login prior to usage of the service". In embedded system domain, the system models can be built using for example the standard profile for Modeling and Analysis of Real-Time Embedded system (MARTE) [24], profile for Schedulability, Performance and Time (SPT) [25], or some proprietary profile such as the TUT-Profile [26].

Verification models also refine requirements. They are blueprints of test benches which are used to verify that the particular requirement is met in the final implementation. Documentation is all other external documentation that is desired to be attached along with the requirement definition. For example, a processor data sheet can be attached to a requirement that restricts the underlying platform to utilize the particular processor core as its foundation. These abstract concepts and relationships allow attaching requirements and system models without binding to any specific modeling language.

5.3. Requirements and System Parameters

System parameter is a concept that models some feature or quantity of a product (e.g., power consumption, area, performance, etc.). A requirement then determines the possible values (or value boundaries) for such quantity.

Parameters help requirements engineers to piece together the relationships between requirements. In requirement elicitation, identifying system parameters and investigating their relationships is an efficient way of defining new relevant requirements.

System parameters are analysed to make early design decisions and validate requirements. As a result of the analysis, requirements are modified if it is observed that current requirements are not feasible to implement as such. In the final system, there will be some realized values for the defined parameters that are verified by measurements or some other observations.

A useful property for a requirement management tool is the capability to define such system parameters, their associations to requirements, and carry out at least simple calculations with them. More demanding analysis must be naturally separated to external analysis tools.

The system parameters have the following attributes in the meta-model.

• (i) Freeform description of the parameter.

• (ii) Unit of measurement (e.g., watt, meter, kilogram, etc.) in case the parameter is quantifiable.

• (iii) Equations describing parameter's relations to other system parameters.

• (iv) Estimated value is a value assigned for analysis. It can be based on engineer's best guess, previous knowledge, datasheets, or measurements from prototype.

• (v) Realized value is the actual value of the parameter in the implementation. This value is compared to requirements in the verification phase.

• (vi) Visibility, that is, whether the property is a user parameter or system parameter. User parameter is a feature that is visible to product user (external property) and system parameter is an internal property shown only for the developer. Typically user properties (and requirements) are used to derive system properties (and requirements).

• (vii) Relation defines association to other system parameter. It is a directed relationship that indicates how increasing the value of a parameter affects the other parameter. It can either increase or decrease it. The intensity can be additive, linear, polynomial, or exponential.

5.4. Allocating Requirements to Design Hierarchy

In a typical software and embedded system development, the requirements are refined and they become closer to actual design components when information in the project is increased. Various design decisions during the project lead into decomposition of the system into smaller and smaller components and modules that have their own specific requirements. This creates an evident need to allocate requirements to certain components in a design hierarchy.

For this purpose, the meta-model defines an abstraction called design part. Design parts can be hierarchical and requirements can be allocated to them. A requirement can have one of the three visibilities from the point of view of a design part: internal, external, and interface. Internal means that requirement is implementation-dependent and comes from the inside of the design part development. This is also called a white box requirement. External requirement comes from the outside. This is a requirement that the environment of the design part requires. This is also called a black box requirement. Interface requirement concerns the interface of the design part. This involves how the design part communicates with its surrounding environment and other design parts. It should be noted that a single requirement can be external for one design part and at the same time internal for another.

5.5. Requirements and Verification

Verification is closely related to requirements management. Tests are the main vehicle for verifying requirements. We prefer that tests are tracked together with requirements, but they should be separated to different logical trees. These trees are then linked together with relationships. There may be several tests for one requirement and one test can be a part of verifying several requirements. Tests have the following attributes.

• (i) Description: a free-form description of the test.

• (ii) Owner: actor responsible of the test in the test tree. This role is not necessarily responsible of conducting the actual test event but definition and change of it during product development (similar to requirement owner).

• (iii) Test objectives: clearly stated objectives for the test. Which requirements and which aspects of them the test is verifying.

• (iv) Test methods: description of the methods that are used in testing. This includes describing the test process phases, used measurement techniques and tools.

• (v) Technical details: defines notes and possible restrictions of the used testing methods.

One test can have several test events. These are instances of test and they must be planned and recorded during the product development. The attributes of a test event are as follows.

• (i) Description: short description of the test event.

• (ii) Tester: actor responsible for conducting the test.

• (iii) Deadline date: date when the test must be (or should have been) taken place.

• (iv) Testing date: actual testing date or planned testing date.

• (v) Test report: external documentation that reports the testing event process and results of the test event.

• (vi) Test result: according to test event, the test can be passed, failed or pending.

5.6. Change Management and Change Sets

Change set is a temporal concept in requirements management process which contains a set of requirements needed to be changed for some common reason. The purpose of the change sets is to allow controlled change in requirements and allow tracking the changes later on by associating them to some specific goal.

Change set description defines the purpose of the changes to be made for the selected set of requirements. Change set state defines whether the change set is active, completed, or in planning state. In active state, the changes are currently being made for the defined set of requirements. The change sets in completed states should be archived for traceability of changes later on. Sets in planning states are yet to be made and thus still inactive.

There may be several change sets active at the same time, but careful consideration has to be carried out when two or more active change sets contain overlapping requirements. It is preferred that these kinds of overlapping change sets should be prohibited completely to avoid uncontrolled corruption of the requirements tree due to simultaneous change of same requirements for different goals in mind.

5.7. Development Process Related Abstractions

There are three fundamental abstractions that are related to requirements management but which are somewhat always dependent on the development process and project management. Their metrics may differ according to policies used by the organization. In the following, we provide examples of these possible metrics. They are presented in Figure 6.

Risk describes the possibility of the requirement not becoming fulfilled and estimated losses if the requirement fails. The losses can be economical, time, and failure-propagation to other requirements. Failure propagation can be presented by composite and derive relationships of requirements. If one of the child requirements fails, then the parent fails as well. If a derived requirement fails, it directly implies a failure in at least one of its source requirements.

Priority is a property of a requirement that characterizes its importance in the development. The priorities of requirements can be used as basis for setting priorities of project tasks. On the other hand, priorities can make the requirement analysis more complex as different stakeholders demand different priorities. The most simple prioritization is dividing requirements into mandatory and nice-to-have type of requirements. Other possibility is to use an integer value that represents the importance of the requirement in relation to other requirement.

Cost is a value that it takes individually to fulfill a requirement. Good quantities for characterizing cost are additional money and time. It should be noted that some requirements may have divided costs since two requirements may always share the same investments and project tasks. These are always development process-dependent and need to be considered in requirement-by-requirement basis.

5.8. Product Families, Products, Variants, and Configurations

The meta-model also considers the hierarchy of products and their variants in product families. From requirements management perspective, the idea is to identify and reuse the requirements between products in product families as well as between different product variants. The meta-model related to this classification is presented in Figure 7. Product is the basic item which is a result of the development effort that satisfies customers' needs. Product family is a set of different products sharing certain common features. The definition of product families is highly organization and domain specific. For example, it can depend on similar design and production techniques, common features, or common implementation platform. Product variant is a parallel development path or customization of a product. For instance, a color camera and black-and-white camera can be tailored from the same basic product components and requirements, but ultimately lead to different variants. Product configuration is a combination of a variant and its version.

6. UML Profile for Requirements Management

The evaluation of requirements management concepts continues from specifying the meta-model to creating a UML profile. Thereafter, profile importing and additional tool customization is carried out. These phases are presented separately in the following subsections.

6.1. Profile Definition

Figure 8 illustrates how the profile is constructed with stereotype extensions. Only three stereotypes are shown in the figure for simplicity. The stereotypes and their attributes correspond to abstractions of the meta-model presented in the previous section. Other stereotypes are similarly derived.

Figure 8

Example stereotype extensions for requirements management.

In the figure, it is shown how Requirement stereotype extends UML meta-class Classifier. Other model elements, except relationships, are also extensions of a classifier. This means that the stereotype can be applied to any UML classifier element (class, use case, actor, etc.). The resulting model element can be adopted in diagrams where the concerned classifier is allowed. This increases the flexibility of adopting the profile.

The requirement stereotype contains string type attributes for description, version, ID, authorization date, and cost. Thus, they can be typed by the modeler in free-form textual notation. Authorizators and owners are also stereotype attributes. They become selectable from the list of all defined actors (companies and persons). The enumeration attributes are attached to the stereotype the same way as in the meta-model. In the figure, only analysation state is shown for simplicity, other state-related attributes are defined in a similar manner. For a designer, these attributes are defined from a pull-down menu where the value can be selected from a set of allowed values. This forces the modeler to use only legal values and thus reduces errors in model construction if compared to free-form textual input. The icon shown in top right corner of the stereotype box is the unique symbol used for requirements in diagrams. Similarly, other stereotypes have their own defined symbols.

Figure 8 also shows stereotype extensions for Derive and Stakeholder relationships. Both extend the meta-class Dependency. The stakeholders are exceptionally defined with special relationship that is used to bind actors to requirements instead of being directly defined as an attribute of a requirement (as in the case of owners). This allows better emphasis in diagrams on how stakeholders request requirements. This is reasonable, because stakeholders are inherited to lower level requirements in the hierarchy, whereas owners and authorizators can change arbitrarily between requirement and its subrequirements. The stakeholder relationship has also an attribute description, that is used to explain the intentions of the stakeholder for the particular requirement. This is another reason for using a special relationship. Other relationship stereotypes are defined so that unidirectional relationships extend UML dependency (stakeholder, derive, refine), whereas bidirectional relationships extend UML association (composite and conflict).

6.2. Tool Customization

The customization for requirement management is done with in-build DSL engine of the MagicDraw tool. We use the following features of the engine to tailor the modeling language and tool for requirements management.

• (i) Hiding standard UML properties from the model elements. This enables the designer to focus on modeling the domain concepts and hiding irrelevant UML-related properties which are not needed.

• (ii) Defining allowed relationships between model elements. This reduces errors in model building as the tool prevents forming illegal relationships.

Figure 9 illustrates the customization of the stereotypes shown in Figure 8. A similar kind of customization is performed for the rest of the stereotypes. The customization is performed by defining tagged values of the stereotype Customization. This stereotype is tool specific and it is interpreted by the MagicDraw DSL engine. The customization target attribute is used to define the elements the particular customization is applied to. The requirement element is customized so that only certain relationships for it are allowed.

Figure 9

Example customizations for requirements management.

Further, the relationship elements are tailored by defining which types of elements they can connect. This is defined with types of source and types of target attributes. For example, the stakeholder relationship can only connect a company or person to a requirement, whereas derive relationship can be only established between two requirements. The DSL engine takes into account these rules during model building by preventing forming illegal relationships.

Other approach for this type of constraining would be to use a separate constraining language such as OCL. Such approach would be more portable. However, for our prototyping purposes the tool-specific constraining is sufficient.

7. Example Model

This section illustrates how the modeling is carried out in practice with the profile. Our example is requirements specification of image manipulation application on HW platform synthesized onto FPGA. The purpose of the application is to test the interoperability and data transmission capabilities between HW components of the underlying FPGA technology. The development of the HW architecture has been divided between different companies, each responsible of implementing one or several HW components for the platform. The three main requirements for the prototype are as follows.

• (i) Correct functionality of the application, data transmission sequence between HW entities.

• (ii) End-to-end data transmission throughput.

• (iii) Maximum resource consumption of the design on FPGA (logic elements and memory bits).

Figure 10 presents the main requirements model for the system. It shows the three top-level requirements, their child and derived requirements, and stakeholders requesting them. All stakeholder companies form a stakeholder group named project consortium using composition. The stakeholder group is a convenient concept to bind companies and persons together as stakeholders. It makes it easier to handle large groups of stakeholders. The stakeholder group is attached to all top-level requirements with the stakeholder relationship.

Figure 10

Requirements hierarchy and stakeholders of image manipulation application on FPGA.

The total area requirement of the design is driven by the maximum capacity of the target FPGA. It is divided into maximum number of logic elements, on-chip memory bits, and DSP blocks (on-chip multipliers). The total area requirement is divided into five subrequirements according to decomposition of the design to IP components. Thus, each IP component has its own requirement for area. These requirements are balanced so that the total area requirement is met if all IP area requirements are met. This follows the definition of a hierarchical requirement. Each subrequirement has its own owner, which in this case is the same as the actor responsible designing the particular IP.

The minimum throughput requirement is informed in Mbits/second. It has the underlying on-chip network throughput as its subrequirement. The minimum throughput is a derived requirement from the picture size and minimum frame rate requirements.

The overall functionality requirement defines the protocol between IP blocks in the system and their combined functionality. This requirement is refined by a UML sequence diagram that shows the control and data transfers between IP blocks in the active mode of the system. In addition, the requirement is further refined by the functional specification of the system represented in the model as external documentation.

Figure 11 shows the customized menu for specifying the properties of a requirement. In this case, it is shown for the total area requirement. Normally, this specification menu has UML-related properties of model element but due to customization of the tool, only domain-related concepts are visible for the modeler. The properties are the same as defined in the meta-model and profile for requirements management.

Figure 11

Requirement property dialog (customized MagicDraw menu).

8. Report Generator Tool

The developed profile has been associated with a report generator tool implemented as a MagicDraw plug-in in Java programming language. Together the profile and the report generator tool form the meta-model prototyping environment. The report generator plug-in is used to process the created model developed with the profile and automatically create a documentation for the requirements. When the states of the requirements change, new requirements appear, or existing ones change, the report generator is executed to form a new version of the documentation while maintaining the version history for a single product. By this way, the requirements model in MagicDraw can be freely modified and the new versions of the reports are produced as releases.

Figure 12 shows the main page of a generated report. It lists the requirements of our example application, their version, priority, and states. The state information indicates that none of the requirements have been verified so far except the communication bus area-related requirements. They are fixed and already conformed to meet the given requirements in the target technology. In addition to the previous ones, total area, transfer protocol, and minimum frame rate have been authorized to development. Several requirements, except most of component area-related requirements, have been analysed, and no conflicts have been detected between them. All requirements have been validated.

Figure 12

Main view of generated requirements report.

The requirements report can be browsed using hyperlinks in the sidebar. Product Info page shows general information on the product. Version history page allows the user to examine the requirements version history and enter any of the versions in the past. Requirement information can be examined in different tables according to their category as well as in from of graphs. Stakeholders and their information can be also accessed individually. The stakeholder graph shows how stakeholders are dependent on different requirements.

9. Conclusions and Future Work

In this paper, we have presented a meta-model and UML profile for requirements management of software and embedded systems. We have shown well-defined reasons behind the meta-model concepts and imported the UML profile in general-purpose UML tool. The meta-model covers several important aspects of requirements necessary in practical systems development. These include temporal aspects of requirements as well as interfaces to system modeling, analysis, and verification. The future work consists of utilizing the meta-model and profile in larger embedded system development projects. Currently, we are adopting the meta-model concepts to capture requirements of an electric car motor controller unit [27].