Brief Outline: Requirements

Depending on context and purpose requirements can be understood as customary documents, contents, or work in progress.

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Partial & Biased Agendas
  1. Given that requirements are the entry point of engineering processes, nothing should be assumed regarding their form (natural, formal, or specific language), or content (business, functional, non-functional, or technical).
  2. Depending on the language used, requirements can be directly analyzed and engineered, or may have to be first formatted (aka captured).
  3. Since requirements reflect biased and partial agendas, taxonomy, in particular the distinction between functional and non functional concerns, is in the eyes of the stakeholders.
  4. Depending on content and context, requirements can be engineered as a whole (agile development models), or set apart as to tally with external dependencies (phased development models).

Further Reading

Focus: Users’ Stories & Use Cases

Preamble

Agile and phased development solutions are meant to solve different problems and therefore differ with artifacts and activities; that can be illustrated by requirements, understood as words in progress for the former, etched statements for the latter.

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Running Stories (Kara Walker)

Ignoring that distinction is to make stories stutter from hiccupped iterations, or phases sputter along ripped milestones.

Agile & Phased Tell Different Stories Differently

As illustrated by ill-famed waterfall, assuming that requirements can be fully set upfront often put projects at the hazards of premature commitments; conversely, giving free rein to expectations could put requirements on collision courses.

That apparent dilemma can generally be worked out by setting apart business outlines from users’ stories, the latter to be scripted and coded on the fly (agile), the former analysed and documented as a basis for further developments (phased). To that end project managers must avoid a double slip:

  • Mission creep: happens when users’ stories are mixed with business models.
  • Jump to conclusions: happens when enterprise business cases prevail over the specifics of users’ concerns.

Interestingly, the distinction between purposes (users concerns vs business functions) can be set along one between language semantics (natural vs modeling).

Semantics: Capture vs Analysis

Beyond methodological contexts (agile or phased), a clear distinction should be made between requirements capture (c) and modeling (m): contrary to the former which translates sequential specifications from natural to programming (p) languages without breaking syntactic and semantic continuity, the latter carries out a double translation for dimension (sequence vs layout) and language (natural vs modeling.)

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Semantic continuity (c>p) and discontinuity (c>m>p)

The continuity between natural and programming languages is at the root of the agile development model, enabling users’ stories to be iteratively captured and developed into code without intermediate translations.

That’s not the case with modeling languages, because abstractions introduce a discontinuity. As a corollary, requirements analysis is to require some intermediate models in order to document translations.

The importance of discontinuity can be neatly demonstrated by the use of specialization and generalization in models: the former taking into account new features to characterize occurrences (semantic continuity), the latter consolidating the meaning of features already defined (semantic discontinuity).

Confusion may arise when users’ stories are understood as a documented basis for further developments; and that confusion between outcomes (coding vs modeling) is often compounded by one between intents (users concerns vs business cases).

Concerns: Users’ Stories vs Business Cases

As noted above, users’ stories can be continuously developed into code because a semantic continuity can be built between natural and programming languages statements. That necessary condition is not a sufficient one because users’ stories have also to stand as complete and exclusive basis for applications.

Such a complete and exclusive mapping to application is de-facto guaranteed by continuous and incremental development, independently of the business value of stories. Not so with intermediate models which, given the semantic discontinuity, may create back-doors for broader concerns, e.g when some features are redefined through generalization. Hence the benefits of a clarity of purpose:

  • Users’ stories stand for specific requirements meant to be captured and coded by increments. Documentation should be limited to application maintenance and not confused with analysis models.
  • Use cases should be introduced when stories are to be consolidated or broader concerns factored out , e.g the consolidation of features or business cases.

Sorting out the specifics of users concerns while keeping them in line with business models is at the core of business analysts job description. Since that distinction is seldom directly given in requirements, it could be made easier if aligned on modeling options: stories and specialization for users concerns, models and generalization for business features.

From Stories to Cases

The generalization of digital environments entails structural and operational adjustments within enterprise architectures.

At enterprise level the integration of homogeneous digital flows and heterogeneous symbolic representations can be achieved through enterprise architectures and profiled ontologies. But that undertaking is contingent on the way requirements are first dealt with, namely how the specifics of users’ needs are intertwined with business designs.

As suggested above, modeling schemes could help to distinguish as well as consolidate users narratives and business outlooks, capturing the former with users’ stories and the latter with use cases models.

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Use cases describe the part played by systems taking into account all supported stories.

That would neatly align means (part played by supporting systems) with ends (users’ stories vs business cases):

  • Users’ stories describe specific objectives independently of the part played by supporting systems.
  • Use cases describe the part played by systems taking into account all supported stories.

It must be stressed that this correspondence is not a coincidence: the consolidation of users’ stories into broader business objectives becomes a necessity when supporting systems are taken into account, which is best done with use cases.

Aligning Stories with Cases

Stories and models are orthogonal descriptions, the former being sequenced, the latter laid out; it ensues that correspondences can only be carried out for individuals uniformly identified (#) at enterprise and systems level, specifically: roles (aka actors), events, business objects, and execution units.

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Crossing cases with stories: events, roles, business objects, and execution units must be uniformly and consistently identified (#) .

It must be noted that this principle is supposed to apply independently of the architectures or methodologies considered.

With continuity and consistency of identities achieved at architecture level, the semantic discontinuity between users’ stories and models (classes or use cases) can be managed providing a clear distinction is maintained between:

  • Modeling abstractions, introduced by requirements analysis and applied to artifacts identified at architecture level.
  • The semantics of attributes and operations, defined by users’ stories and directly mapped to classes or use cases features.
Scratch_USC3
From Capture to Analysis: Abstractions introduce a semantic discontinuity

Finally, stories and cases need to be anchored to epics and enterprise architecture.

Business Cases & Enterprise Stories

Likening epics to enterprise stories would neatly frame the panoply of solutions:

  • At process level users’ stories and use cases would be focused respectively on specific business concerns and supporting applications.
  • At architecture level epics and business cases would deal respectively with business models and objectives,  and supporting systems capabilities.
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A Squared frame for enterprise architectures governance

That would provide a simple yet principled basis for enterprise architectures governance.

Further Reading

External Links

Squaring Software To Value Chains

Preamble

Digital environments and the ubiquity of software in business processes introduces a new perspective on value chains and the assessment of supporting applications.

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Business & Supporting Processes (Michel Blazy)

At the same time, as software designs cannot be detached of architectures capabilities, the central question remains of allocating costs and benefits between primary and support activities .

Value Chains & Activities

The concept of value chain introduced by Porter in 1985 is meant to encompass the set of activities contributing to the delivery of a valuable product or service for the market.

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Porter’s generic model

Taking from Porter’s generic model, various value chains have been refined according to business specific categories for primary and support activities.

Whatever their merits, these approaches are essentially static and fall short when the objective is to trace changes induced by business developments; and that flaw may become critical with the generalization of digital business environments:

  • Given the role and ubiquity of software components (not to mention smart ones), predefined categories are of little use for impact analysis.
  • When changes in value chains are considered, the shift of corporate governance towards enterprise architecture puts the focus on assets contribution, cutting down the relevance of activities.

Hence the need of taking into account changes, software development, and enterprise architectures capabilities.

Value Added & Software Development

While the growing interest for value chains in software engineering is bound to agile approaches and business driven developments, the issue can be put in the broader perspective of project planning.

With regard to assessment, stakeholders, start with business opportunities and look at supporting systems from a black box perspective; in return, software providers are to analyze requirements from a white box perspective, and estimate corresponding development effort and time delivery.

Assuming transparency and good faith, both parties are meant to eventually align expectations and commitments with regard to features, prices, and delivery.

With regard to policies, stakeholders put the focus on returns on investment (ROI), obtained from total cost of ownership, quality of service, and timely delivery. Providers for their part try to minimize development costs while taking into account effective use of resources and costs of opportunities. As it happens, those objectives may be carried on as non-zero sum games:

  • Business stakeholders foretell the actualized returns (a) to be expected from the functionalities under consideration (b).
  • Providers consider the solutions (b’) and estimate actualized costs (a’).
  • Stakeholders and providers agree on functionalities, prices and deliveries (c).
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Developing the value chain

Assuming that business and engineering environments are set within different time-frames, there should be room for non-zero-sum games winding up to win-win adjustments on features, delivery, and prices.

Continuous vs Phased Alignments

Notwithstanding the constraints of strategic planning, business processes are by nature opportunistic, and their ability to be adjusted to circumstances is becoming all the more critical with the generalization of digital business environments.

Broadly speaking, the squaring of supporting applications to business value can be done continuously or by phases:

  • Phased alignments start with some written agreements with regard to features, delivery, and prices before proceeding with development phases.
  • Continuous alignment relies on direct collaboration and iterative development to shape applications according to business needs.
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Phased vs Continuous Adjustments

Beyond sectarian controversies, each approach has its use:

  • Continuous schemes are clearly better at harnessing value chains, providing that project teams be allowed full project ownership, with decision-making freed of external dependencies or delivery constraints.
  • Phased schemes are necessary when value chains cannot be uniquely sourced as they take roots in different organizational units, or if deliveries are contingent on technical constraints.

In any case, it’s not a black-and-white alternative as work units and projects’ granularity can be aligned with differentiated expectations and commitments.

Work Units & Architecture Capabilities

While continuous and phased approaches are often opposed under the guises of Agile vs Waterfall, that understanding is misguided as it extends the former to a motley of self-appointed agile schemes and reduces the latter to an ill-famed archetype.

Instead, a reasoned selection of a development models should be contingent on the problems at hand, and that can be best achieved by defining work-units bottom-up with regard to the capabilities targeted by requirements:

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Work units should be defined with regard to the capabilities targeted by requirements

Development patterns could then be defined with regard to architecture layers (organization and business, systems functionalities, platforms implementations) and capabilities footprint:

  • Phased: work units are aligned with architecture capabilities, e.g : business objects (a), business logic (b), business processes (c), users interfaces (d).
  • Iterative: work units are set across capabilities and defined dynamically according to development problems.
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A common development framework for phased and iterative projects

That would provide a development framework supporting the assessment of iterative as well as phased projects, paving the way for comprehensive and integrated impact analysis.

Value Chains & Architecture Capabilities

As far as software engineering is concerned, the issue is less the value chain itself than its change, namely how value is to be added along the chain.

Given the generalization of digitized and networked business environments, that can be best achieved by harnessing value chains to enterprise architecture capabilities, as can be demonstrated using the Caminao layout of the Zachman framework.

To summarize, the transition to this layout is carried out in two steps:

  1. Conceptually, Zachman’s original “Why” column is translated into a line running across column capabilities.
  2. Graphically, the five remaining columns are replaced by embedded pentagons, one for each architecture layer, with the new “Why” line set as an outer layer linking business value to architectures capabilities:
Mapping value chains to enterprise architectures capabilities

That apparently humdrum transformation entails a significant shift in focus and practicality:

  • The focus is put on organizational and business objectives, masking the ones associated to systems and platforms layers.
  • It makes room for differentiated granularity in the analysis of value, some items being anchored to specific capabilities, others involving cross dependencies.

Value chains can then be charted from business processes to supporting architectures, with software applications in between.

Impact Analysis

As pointed above, the crumbling of traditional fences and the integration of enterprise architectures into digital environments undermine the traditional distinction between primary and support activities.

To be sure, business drive is more than ever the defining factor for primary activities; and computing more than ever the archetype of supporting ones. But in between the once clear-cut distinctions are being blurred by a maze of digital exchanges.

In order to avoid a spaghetti heap of undistinguished connections, value chains are to be “colored” according to the nature of links:

  • Between architectures capabilities: business and organization (enterprise), systems functionalities, or platforms and technologies.
  • Between architecture layers: engineering processes.
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Tracing value chains across architectures layers

When set within that framework, value chains could be navigated in both directions:

  • For the assessment of applications developed iteratively: business value could be compared to development costs and architecture assets’ depreciation.
  • For the assessment of features (functional or non functional) to be shared across business applications: value chains will provide a principled basis for standard accounting schemes.

Combined with model based system engineering that could significantly enhance the integration of enterprise architecture into corporate governance.

Model Driven Architecture & Value Chains

Model driven architecture (MDA) can be seen as the main (only ?) documented example of model based systems engineering. Its taxonomy organizes models within three layers:

  • Computation independent models (CIMs) describe organization and business processes independently of the role played by supporting systems.
  • Platform independent models (PIMs) describe the functionalities supported by systems independently of their implementation.
  • Platform specific models (PSMs) describe systems components depending on platforms and technologies.

Engineering processes can then be phased along architecture layers (a), or carried out iteratively for each application (b).

When set across activities value chains could be engraved in CIMs and refined with PIMs and PSMs(a). Otherwise, i.e with business value neatly rooted in single business units, value chains could remain implicit along software development (b).

Further Reading

 External Links

 

Focus: Requirements Reuse

Preamble

Requirements is what to feed engineering processes. As such they are to be presented under a wide range of forms, and nothing should be assumed upfront about forms or semantics.

 

What is to be reused: Sketches or Models  ? (John Devlin)

Answering the question of reuse therefore depends on what is to be reused, and for what purpose.

Documentation vs Reuse

Until some analysis can be carried out, requirements are best seen as documents;  whether such documents are to be ephemeral or managed would be decided depending on method (agile or phased), contents (business, supporting systems, implementation, or quality of services), or purpose (e.g governance, regulations, etc).

What is to be reused.

Setting apart external conditions, requirements documentation could be justified by:

  • Traceability of decision-making linking initial requests with actual implementation.
  • Acceptance.
  • Maintenance of deliverables during their life-cycle.

Depending on development approaches, documentation could limited to archives (agile development models) or managed as intermediate products (phased development models). In the latter case reuse would entail some formatting of requirements.

The Cases for Requirements Reuse

Assuming that requirements have been properly formatted, e.g as analysis models (with technical ones managed internally at system level), reuse could be justified by changes in business, functional, or quality of services requirements:

  • Business processes are meant to change with opportunities. With requirements available as analysis models, changes would be more easily managed (a) if they could be fine-grained. Business rules are a clear example, but that could also be the case for new features added to business objects.
  • Functional requirements may change even without change of business ones, e.g if new channels and users are introduced addressing existing business functions. In that case reusable business requirements (b) would dispense with a repeat of business analysis.
  • Finally, quality of service could be affected by operational changes like localization, number of users, volumes, or frequency. Adjusting architecture capabilities would be much easier with functional (c) and business (d) requirements properly documented as analysis models.
Cases for Reuse

Along that perspective, requirements reuse appears to revolve around two pivots, documents and analysis models. Ontologies could be used to bind them.

Requirements & Ontologies

Reusing artifacts means using them in contexts or for purposes different of native ones. That may come by design, when specifications can anticipate on shared concerns, or as an afterthought, when initially unexpected similarities are identified later on. In any case, reuse policies have to overcome a twofold difficulty:

  • Visibility: business and functional analysts must be made aware of potential reuse without having to spend too much time on research.
  • Overheads: ensuring transparency, traceability, and consistency checks on requirements (documents or analysis models) cannot be achieved without costs.

Ontologies could help to achieve greater visibility with acceptable overheads by framing requirements with regard to nature (documents or models) and context:

With regard to nature, the critical distinction is between document management and model based engineering systems. When framed as ontologies, the former is to be implemented as thesaurus targeting terms and documents, the latter as ontologies targeting categories specific to organizations and business domains.

Documents, models, and capabilities should be managed separately

With regard to context the objective should be to manage reusable requirements depending on the kind of jurisdiction and stability of categories, e.g:

  • Institutional: Regulatory authority, steady, changes subject to established procedures.
  • Professional: Agreed upon between parties, steady, changes subject to accord.
  • Corporate: Defined by enterprises, changes subject to internal decision-making.
  • Social: Defined by usage, volatile, continuous and informal changes.
  • Personal: Customary, defined by named individuals (e.g research paper).
Combining contexts of reuse with architectures layers (enterprise, systems, platforms) and capabilities (Who,What,How, Where, When).

Combined with artificial intelligence, ontology archetypes could crucially extend the benefits of requirements reuse, notably through the impact of deep learning for visibility.

On a broader perspective requirements should be seen as a source of knowledge, and their reuse managed accordingly.

Further Reading

Views, Models, & Architectures

Preamble

Views can take different meanings, from windows opening on specific data contexts (e.g DB relational theory), to assortments of diagrams dedicated to particular concerns (e.g UML).

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Deconstructing the Universe along Contexts and Concerns (Depero Fortunato)

Models for their part have also been understood as views, on DB contents as well as systems’ architecture and components, the difference being on the focus put on engineering. Due to their association with phased processes, models has been relegated to a back-burner by agile approaches; yet it may resurface in terms of granularity with model-based engineering frameworks.

Yet, whatever the terminology (layers vs levels), what is at stake is the alignment of two basic scales:

  • Architectures: enterprise (concepts), systems (functionalities), and platforms (technologies).
  • Models: conceptual (business context and organization), analysis (symbolic representations), design (physical implementation).

Views & Architectures

As far as systems engineering is concerned, understandings of views usually refer to Philippe Kruchten’s “4+1” View Model of Software Architecture” :

  • Logical view: design of software artifacts.
  • Process view: captures the concurrency and synchronization aspects.
  • Physical view: describes the mapping(s) of software artifacts onto hardware.
  • Development view: describes the static organization of software artifacts in development environments.

A fifth is added for use cases describing the interactions between systems and business environments.

Whereas these views have been originally defined with regard to UML diagrams, they may stand on their own meanings and merits, and be assessed or amended as such.

Apart from labeling differences, there isn’t much to argue about use cases (for requirements), process (for operations), and physical (for deployment) views; each can be directly associated to well identified parts of systems engineering that are to be carried out independently of organizations, architectures or methods.

Logical and development views raise more questions because they imply a distinction between design and implementation. That implicit assumption induces two kinds of limitations:

  • They introduce a strong bias toward phased approaches, in contrast to agile development models that combine requirements, development and acceptance into iterations.
  • They classify development processes with regard to predefined activities, overlooking a more critical taxonomy based on objectives, architectures and life-cycles: user driven and short-term (applications ) vs data-based and long-term (business functions).

These flaws can be corrected if logical and development views are redefined respectively as functional and application views, the former targeting business objects and functions, the latter business logic and users’ interfaces.

Architecture based views
Architecture based views

That make views congruent with architecture levels and consequently with engineering workshops. More importantly, since workshops make possible the alignment of products with work units, they are a much better fit to model-based engineering and a shift from procedural to declarative paradigm.

Model-based Systems Engineering & Granularity

At least in theory, model-based systems engineering (MBSE) should free developers from one-fits-all procedural schemes and support iterative as well as declarative approaches. In practice that would require matching tasks with outcomes, which could be done if responsibilities on the former can be aligned with models granularity of the latter.

With coarse-grained phased schemes like MDA’s CIM/PIM/PSM (a), dependencies between tasks would have to be managed with regard to a significantly finer artifacts’ granularity.

Managing changes at architecture (a) or application (b) level.
Managing changes at architecture (a) or application (b) level.

For agile schemes, assuming conditions on shared ownership and continuous deliveries are met, projects would put locks on “models” at both ends (users’ stories and deliveries) of development cycles (b), with backlogs items defining engineering granularity.

Backlogs mechanism can be used to manage customized granularity and hierarchical dependencies across model layers

From the enterprise perspective it would be possible to unify the management of changes in architectures across layers and responsibilities: business concepts and organization, functional architecture, and systems capabilities:

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Functional architecture as symbolic bridge between business needs and system capabilities.

From the engineering perspective it would be possible to unify the management of changes in artifacts at the appropriate level of granularity: static and explicit using milestones (phased), dynamic and implicit using backlogs (agile).

Fine grained model based frameworks could support phased as well as agile development solutions

Such a declarative repository would greatly enhance exchanges and integration across projects  and help to align heterogeneous processes independently of the methodologies used.

Further Reading

External Links

iStar and the Requirements Conundrum

Synopsis

Whenever software engineering problems are looked at, the blame is generally put on requirements, with each side of the business/system divide holding the other responsible.

Requirements inception

The iStar approach tries to tackle the problem with a conceptual language focused on interactions between business processes and supporting systems.

Dilemma

Conceptual approaches to requirements try to breach the dilemma between phased and agile development schemes: the former takes for granted that requirements can be fully and definitively set upfront; the latter takes a more pragmatic path and tries to reconcile business and system analysts through direct and continuous collaboration.

Setting apart frictions between specific methods, the benefits of agile principles and practices are now well-recognized, contingent on the limits of agile scope. Summarily, agile development is at its best when requirements capture and analysis can be weaved with development and tests. The question remains of what happens when requirements are to be dealt with separately.

The iStar’s answer shares with agile a focus on collaboration and doesn’t take side for business (e.g users’ stories) or systems (e.g use cases). Instead, iStar modeling language is meant to support a conceptual description of interactions between business processes and supporting systems in terms of actors’ goals and commitments, and the associated dependencies.

Actors & Goals

The defining aspect of the iStar modeling approach is to replace one-sided perspectives (business or system) by a systemic one focused on the interactions between agents. The interactive part of a requirement will therefore comprise three basic items:

  • A primary actor trigger an interaction in order to meet some goal; e.g a car owner want his car repaired.
  • Secondary actors may be involved during the ensuing exchanges: e.g body shop, appraiser, insurance company.
  • Functions to be performed: actual task; e.g appraise damages; qualification (soft goal), e.g fair appraisal; and resources, e.g premium payment.
Actors & dependencies
Actors & Dependencies

Dependencies Semantics

The factual description of interactions is both detailed and enriched by elements set within a broader scope:

  • Goal (strong) dependency: assertions about actual state of affairs: object, activity, or expectations.
  • Soft-goal dependency: assertions about expected outcomes.
  • Task dependency: organizational, functional, or technical constraints pertaining to the execution of activities.
  • Resource dependency: constraints or conditions on the availability of inputs, actual or symbolic.

It would be tempting to generalize the strong/soft distinction to dependencies as to make use of modal logic, strong dependencies associated with deontic rules, soft dependencies with alethic ones. That would .

iStar & Caminao

Since iStar modeling categories are directly aligned with UML Use Cases, they can easily mapped to core Caminao stereotypes for actors, objects, events, and activities.

Actors & dependencies
iStar with Caminao Stereotypes

Interestingly, the iStar strong/soft distinction could translate to the actual/symbolic one which constitute the conceptual backbone of the Caminao paradigm.

Assessment

From the business perspective, iStar must be credited with two critical tenets:

  • The focus on interactions between agents is essential for business and system analysts to collaborate. Such benefits appear clearly for the definition of primary and secondary roles (aka actors), intents (business) and capabilities (supporting environments).
  • The distinction between strong and soft goals, even if the logical basis remains unexploited.

Yet, the system perspective lacks a functional dimension, e.g:

  • Architecture levels (enterprise and organization, systems and functionalities, platforms and technologies) are not taken into consideration, nor the nature of capabilities, e.g strategic and operational.
  • The strong/soft dependencies distinction is not explicitly associated with systems capabilities.

On the whole these pros and cons reflect iStar’s declared intent on conceptual modeling; as a corollary these flaws mark also the limits of conceptual modeling when it is detached from the symbolic description of supporting systems functionalities.

Nonetheless, as illustrated by the research quoted below, iStar remains a sound basis for the specification of interactions between users and systems, either as use cases or users’ stories.

Further Reading

External Links

Focus: Business Processes & Abstraction

Preamble

Abstractions, and corollary inheritance, are primarily understood with objects. Yet, since business processes are meant to focus on activities, semantics may have to be refined when abstraction and inheritance are directly used for behaviors.

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How to apply abstraction to processes ?  (E. Gimenez Velilla)

Considering that the primary purpose of abstractions is to tackle business variants with regard to supporting systems, their representation with use cases provides a good starting point.

Business Variants: Use case’s <extend> & <include>

Taking use cases as a modeling nexus between business and systems realms, <extend> and <include> appear as the default candidates for the initial description of behaviors’ specialization and generalization.

  • <include>: to be compared to composition semantics, with the included behaviors performed  by instances identified (#) by the owner UC (a).
  • <extend>: to be compared to aggregation semantics, with the extending behaviors performed  by separate instances with reference to the owner ones (b).
Included UCs are meant to be triggered by owners (a); that cannot be clearly established for abstract use cases and generalization (c).
Included UCs are meant to be triggered by owners (a); that cannot be clearly established for abstract use cases and generalization (c).

Abstract use cases and generalization have also been mentioned by UML before being curiously overlooked in following versions. Since none has been explicitly discarded, some confusion remains about hypothetical semantics. Notionally, abstract UCs would represent behaviors never to be performed on their own (c). Compared to inclusion, used for variants of operations along execution paths, abstract use cases would describe the generic mechanisms to be applied to triggering events at UC inception independently of actual business operations carried out along execution paths.

Nonetheless, and more importantly, the mix-up surrounding the generalization of use cases points to a critical fault-line running under UML concepts: since both use cases and classes are defined as qualifiers, they are supposed to be similarly subject to generalization and specialization. That is misguided because use cases describe the business behaviors to be supported by systems, not to be confused with the software components that will do the job. The mapping between the former and the latter is to be set by design, and there is no reason to assume a full and direct correspondence between functional requirements and functional architecture.

Use Cases Distilled

As far as use cases are considered, mapping business behaviors to supporting systems functionalities can be carried out at two levels:

  • Objects: UCs being identified by triggering agents, events, and goals, they are to be matched with corresponding users interfaces and controllers, the former for the description of I/O flows, the latter for the continuity and integrity of interactions.
  • Methods: As it’s safe to assume that use cases are underpinned by shared business functions and system features, a significant part of their operations are to be realized by methods of shared business entities or services.
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Setting apart UIs and controllers, no direct mapping should be assumed between use cases and functional qualifiers.

The business variants distilled into objects’ or services’ methods can be generalized and specialized according to OOD principles; and the same principles can be applied to specific users’ interfaces. But since purely behavioral aspects of UCs can neither be distilled into objects’ methods, nor directly translated into controller objects, their abstraction semantics have to be reconsidered.

Inheritance Semantics: Structural vs Functional

As far as software artifacts are concerned, abstraction semantics are set by programming languages, and while they may differ, the object-oriented (OO) paradigm provides some good enough consolidation. Along that perspective, inheritance emerges as a critical issue due to its direct impact on the validity of programs.

Generally speaking, inheritance describes how structural or behavioral traits are passed from ancestors to descendants, either at individual or type level. OO design is more specific and puts the focus on the intrinsic features (attributes and operations) supported by types or classes, which ensues that behaviors are not considered as such but through the objects’ methods that realize them:

  • Structural inheritance deals with attributes and operations set for the whole life-cycle of instances. As a consequence corresponding inheritance is bound to identities (#) and multiple ascendants (i.e identities) are ruled out.
  • Functional inheritance deal with objects behaviors which may or may not be frozen to whole life-cycles. Features can therefore be inherited from multiple ascendants.

That structural vs functional distinction matches the one between composition and aggregation used to characterize the links between objects and parts which, as noted above, can also be applied to uses cases.

Use Cases & Abstraction

Assuming that the structural/functional distinction defined for objects can also be applied to behaviors, use cases provide a modeling path from variants in business processes to OOD of controllers:

  • Behaviors included by UCs (a) are to be set along the execution paths triggered by UC primary events (#). Inheritance is structural, from UCs base controllers to corresponding (local) ones, and covers features (e.g views on business objects) and associated states (e.g authorizations) defined by use case triggering circumstances.
  • Behaviors extending UCs (b) are triggered by secondary events generated along execution paths. Inheritance is functional, from extending UCs (e.g text messaging) to UCs primary controllers.

Yet this dual scheme may not be fully satisfactory as it suffers from two limitations:

  • It only considers the relationships between UCs, not with the characteristics of the use cases themselves.
  • It ignores the critical difference between the variants of business logic and the variants of triggering conditions.

Both flaws can be patched up if abstract use cases are specifically introduced to factor out triggering circumstances (c):

Use cases provide a principled modeling path from variants in business processes to the OOD of corresponding controllers.
Use cases provide a principled modeling path from variants in business processes to the OOD of corresponding controllers.
  • Undefined triggering circumstances is the only way to characterize abstraction independently of what happens along execution paths.
  • Abstract use cases can then be used to specify inception mechanisms to be inherited by concrete use cases.

That understanding of abstract use cases comes with clear benefits with regard to security and confidentiality.

What is at Stake

Abstraction can significantly reinforce the bridging role of use cases between business and UML models.

On one side specialized use cases can be associated to operations and functions directly implemented, e.g  by factoring out authentication and authorization:

One standard solution is to define a common use case controlling accesses for all users providing they can be identified before being subsequently (i.e during UC execution) qualified and authorized. Apparently, that could be done with <<include>> (a) or <<extend>> (b) connectors.

PtrnUC_abst

But the second option would not be possible with the semantic distinction suggested above for UC patterns, which specifies that use cases can only be extended from existing sessions.

A more generic approach (possibly with patterns) could try to “abstract” Open Session UC, e.g to cover a broader range of actors and identification mechanisms.

Understanding UC abstraction in terms of a partial specification to be <<included>> and run by the current thread will be inconsistent because there would be no concrete actor for the identification mechanisms (c).

By contrast, since inheritance connectors apply to types and not to instances (i.e execution threads), abstracted identification mechanisms are meant to be part of Manage Session and can be applied to triggering actors (d).

Such a clear distinction between the specification of threads (using connectors) and activities (using inheritance) should provide the basis of architecture-based UC patterns.

Al in all, that will greatly help to align business cases, business opportunities, and functional architectures.

Further Reading