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Keep Clear Separation Between the Concerns that Each Architecture View Addresses

ISO/IEC 42010 advocates or, more likely, is predicated on the assumption that an architecture description of a complex system needs many architecture views.

Since the initial publication of IEEE Std 1471, there has been wide acceptance for the use of multiple viewpoints to express architectures of systems, and for system concerns as a primary motivation for the content of architectural descriptions. This revision builds upon these core concepts, and captures additional consensus in the areas of architecture frameworks and architecture description languages.

Each view describes the real world architecture and addresses a different set or concerns or questions posed. The typical concerns each view addresses are defined within an architecture viewpoint which is a specification for each view type. Note that this use of ‘viewpoint’ is not that same as that used in MODAF and DODAF where viewpoint is a collection of architecture views, not a specification.

Pretty obviously these need to overlap so that the reader can move around the architecture description and also understand the context for any part. For example there’s no point in showing a function without showing the thing that performs the function and this in turn might realise or implement part of a concept. Overlapping views are therefore a good thing. This in turn requires overlap between the defining viewpoints (ISO terminology).

Whilst some overlap is always good, too much overlap in invariably bad. If there is too much overlap in the things that can be shown in different view types then it becomes difficult if not impossible for the reader to understand the purpose of each view type and differentiate between them. It also leads to inconsistency in the way that an architect approaches the task since if the same content can be shown on 2 view types one architect might choose one view type and another might choose the second. From the reader’s point this is confusing since there is no clear expectation in terms of finding particular objects and relationships. In essence it is a human factors problem since with too much overlap the affordance suffers as there is no longer a clear distinction between view type and the likely view content. From the point of ISO/IEC 42010 the risk is that there is no longer any clear distinction in terms of the concerns addressed by each view type.

This is best illustrated by practical example. The following is taken from MODAF 1.2.004 and looks at the MODAF::SV-1 Resource Interaction Specification View vs the MODAF::SV-2 set (SV-2a System Port Specification View, SV-2b System Port Connectivity Description View and SV-2c System Connectivity Clusters View).

The MODAF System Viewpoint states:

The primary purpose of an SV-1 is to show resource structure; i.e. to identify the primary sub- systems, posts and roles and their interactions. SV-1 contributes to user understanding of the structural characteristics of the capability.

but goes on to add:

In its simplest form, an SV-1 can be used to depict systems and sub-systems, and identify the interfaces between them; however, this rarely adds more to that which can be shown in an SV-2, product.

and

If possible, an SV-1 will show resources and their interactions for the entire architecture on the same diagram

It’s immediately confusing since the stated intent is to show structure (despite the name of the view) but then adds interactions and itself identifies that there is hardly any difference between the SV-1 and SV-2 since both can legitimately be used to describe both structure and identify interfaces between the structural parts. In terms of the definition there is no distinction between the different concerns addressed and therefore you might choose the SV-1 to describe structure and interaction whilst I might choose the SV-2. [Note: The MODAF::SV-2 can only be used for System and Software not for Human Resource (Organisation, Job and Role) unlike the MODAF::SV-1]

At 1.2.004 there is even less differentiation since it now can be used not only to identify but to characterise interfaces:

If SV-1 is developed as a composite structure model (e.g. in SysML, UML), Resource Ports may be used to convey how interactions are dealt with internal to the resource when the resource has parts. Resource Ports may also specify the interfaces they require or provide.

(As an aside there are also inconsistencies between what appears in the textual description, the data objects and the fragment of simplified metamodel since the data objects section does not appear to list all the types of object that can appear on a SV-1.)

There is so much overlap between the MODAF:SV-1 and SV-2 that it is no longer clear what distinguishes the two (you can show structure and interactions on both) and therefore why you need both. Certainly the stated emphasis on structure isn’t reflected in the definition of the SV-1.

To be fair to MODAF this looks to be a problem inherited from DODAF:

Systems Viewpoint. SV-1: Systems Interface Description

A primary purpose of a SV-1 DoDAF-described Model is to show resource structure, i.e., identify the primary sub-systems, performer and activities (functions) and their interactions. SV-1 contributes to user understanding of the structural characteristics of the capability.

The NATO Architecture Framework, version 3.1 doesn’t state a structural intent quite as boldly although it does say for the NSV-1 System interface description:

In a sense NSV-1 and NSV-4 provide complementary representations (structure and function)

If structural were limited to composition and configuration relationships and exclude flows then the SV-1s would be structural. They don’t, however, and the fact that their name includes Interaction or Interface (not structure) suggests that the intended focus of each of these views are the flows. Very much a case of mixed messages which doesn’t help the architect.

This shows that it is vital to keep clear water between the focus and therefore the concerns addressed by each view type.

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External Links

• ISO/IEC FCD 42010 Systems and Software Engineering — Architecture Description - latest draft. 8th June 2010
• The MODAF System Viewpoint. 26th April 2010
• DODAF 2.02. SV-1:: Systems Interface Description
• ANNEX 1 TO AC/322(SC/1-WG/1)N(2009)0005-ADD2 NAF V3.1 CHAPTER 5 5.2.6.1 NSV-1 System interface description. 1st March 2010

Perspectives - Structure / Grouping of TRAK Viewpoints

—Edited to reflect changes in names of TRAK Enterprise and Concept Perspectives—

Architectural frameworks normally provide ways of grouping views that have a common aspect and these collections are known as perspectives.

TRAK provides the following perspectives:-

• Enterprise (was ‘Capability’) – concerned with the overall enterprise and capabilities needed.
• Concept (was ‘Operational’) – concerned with the logical ‘‘what’’
• Procurement – concerned with projects and delivery of solutions
• Solution – concerned with the behavioural and physical aspects of the solution
• Management – concerned with the management of architectural views

They provide a way of simplifying and organising the architectural description.

TRAK defines a set of architectural viewpoints and view contents. Elements shown on a view have to be part of the underlying metamodel and can only be connected using the allowed relationships. TRAK specifies what can be shown and how it is presented and organised. This is shown in the context of IEEE 1471 in Figure 4?1. It is the ‘architecture element conforms to metamodel’ relationship outside the IEEE 1471 space and the ‘framework defines architecture viewpoint’ and ‘framework defines perspective’ that provide conformity and consistency.

IEEE1471 Conceptual Model with TRAK Metamodel Elements Added

Each TRAK viewpoint (and therefore view) is designed to address specific concerns or questions.

Enterprise Perspective

This perspective covers the enduring capabilities that are needed as part of the bigger enterprise. These are high level needs that everything else contributes to and form part of the long term strategic objectives that need to be managed. It provides a mechanism to link into the higher level goals such as ‘Keep London moving’.

Concept Perspective

The concept perspective covers the logical view of what is needed. It covers the logical connection of concept nodes, for example a service control centre, to other nodes with no recognition of how this might be realised either by organisation or technology. It provides a means of stating the operational exchange needs and information required.

Procurement Perspective

The procurement perspective provides a top level view of the solution to the problem outlined in the capability perspective and developed in the operational perspective in that it provides a way of showing how projects deliver solutions to provide capability. It provides a way of showing time dependency between projects and is an essential prerequisite for investigating capability gaps in the capability perspective. It also provides a mechanism for showing how organisations and projects relate to the systems being delivered.

Solution Perspective

The solution perspective provides views of the solution or potential solution, recognising that there may be many potential solutions which might meet the logical needs expressed in the operational perspective. Functional views provide a means of describing the behaviour in terms of functions, activity, sequence, state and interactions. Physical views describe how the system is organised, how information is routed and where parts are or must be.

Management Perspective

The management perspective covers views that are concerned with the management and production of the architecture products. It enables the scope of any architecture task to be defined and the provides ways of recording what was done and capturing the intended understanding so that the architecture can be provided to others or re-used.

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External Links

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