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Sat, 2008-Jun-07

4+1 View Scenarios, and the rest

This is the final chapter in a series I have been running on my evolving understanding of 4+1 view architectural descriptions. This time around I am covering scenarios, and other things that might end up in an architectural description. We have already established the set of components in our architecture, and the functional interfaces to external systems. We have drawn links between components, but not really elaborated on where responsibilities lie between a given pairing of linked components.


The scenarios view seems to be the most flexible of the views, and seems to capture "other". I think I am sticking fairly closely to the Architectural Blueprints definition when I use this view primarily to convey behaviours of components and internal interfaces. I have been using collaboration diagrams showing component interactions in arrangements that roughly correlate to the Process View presentation. As with other views I have generally tried to align things to the Logical View classification, and attempted to come up with one or more scenarios for each logical function.

Sales Scenario: Record Sale

Recording of sales is a fairly straightforward transfer of Sales Records from Web Browser to the Sales Manager Component. Financial Transactions pertaining to these records are submitted to the General Ledger for further analysis.

It is possible that we could talk more about the user interface than I have in this diagram. We could talk about how the browser navigates to the page that lets them enter the Sales Records. I have deliberately kept things fairly simple here, trying to focus on clarifying roles within the architecture rather than getting too tied up in the detail of various interfaces involved. Even my method invocations don't look right from a UML perspective. They are really more along the lines of data flows, and I don't expect a literal analogue to exist for any of these method invocations.

Another way to approach the Scenarios View would be to take things back up a notch. Use cases may better capture the end user's intent, whether they be UML-style use cases or more general Systems Engineering -style use cases. I think there is a lot of flexibility in this area, which makes things difficult to nail down. I share the experience of how I have been approaching my specific problem domain, and hope that it leads to some useful light-bulbs going off in someone else's head.

Inventory Scenario: Stocktake

The Stocktake scenario is instructive because it tells us that Inventory Manager is directly involved in the stocktake procedure. It is not an idle bystander that accepts a bulk update at the end of the stocktake. The Stocktake GUI starts the process by marking all stock as outstanding. One by one items are ticked off from the list until the remainder must be considered shrinkage. This means that only one stocktake can be going on at a time, and that the work of stocktake might be undone if someone starts the procedure again. On the other hand, it means that a crash in the stocktake GUI won't lose us our current stocktake state. Does this division of responsibility strike the right balance? That's what this design process is intended to make us ask ourselves.

Shares Scenario: Enter buy or sell

We decomposed the Shares function more than most others, and this gives us an opportunity to see how a diagram with many components might work. Note that I have more arrows now than I have labels against those arrows. I have been using a convention that continuing arrows carry the same data as the last one unless indicated otherwise. In this diagram we have the Buy or Sell Record moving about essentially untouched until it gets to General Ledger Export. Only then is it converted into a Financial Transaction for recording in the General Ledger.

I find it interesting to pinpoint components that have little to do with altering the content that passes through them. From a REST/web perspective we might apply the End to End principle here. Not only should the interim components not modify the data passing through them, they should generally not try to understand the data more than necessary either. If communication is in the form of documents, they should ignore but pass on attributes and elements even if they don't understand them. Take care to clean documents up at system boundaries in order to be able to assign blame, but otherwise try to keep things free and easy. Even the document clean-up should ideally be done by free standing document firewalls that are easy to reconfigure as the architecture evolves.

I haven't included all of the images this time around. If you would like to look at the whole (but still limited) set of diagrams I put together, point your StarUML here.

The Rest

Clearly 4+1 is not the whole story. There are levels of detail beyond what I have covered in my examples this time around, including interface control documentation and module design specifications. There are also a great deal of additional details that may accompany an architectural description to support design decisions made or to provide more detail.

Perhaps the most obvious gap in the 4+1 as applied at this level is a lack of data models for logical entities, for configuration data, and for interfaces. These data models can be extremely useful, and the earlier you have a stab at them the less likely you'll have multiple teams misunderstanding each other.

Other gaps include objectives for the decomposition, rationale for choices made, descriptions of how the architecture would change if current assumptions and constraints were to change, and a host of other detail. Any one of these could lead you down the path of discussing something to the point where you stop and say to yourself "Hang on. That doesn't make sense. What if...". Those moments are half the reason to put an architectural description together, and some of the value of the document can be measured in how many "oops"es are caught as compared to the effort you put into the document.

The other side of the document is obviously to steer the ship and record consensus decisions. Communicating the reasons for the current set of design decisions and the set of alternatives considered can therefore be crucial in the maintenance of the document over the project lifetime.

In the end, everything on the project is linked. From schedule to team structure to components to the list of change requests against those components. If you start modelling you might just end up modelling the whole world. Depending on the scale of the project it might be worth going that far. If you are working on something smaller you should be doing the minimum and assessing the rewards for taking any further steps with the document set.


I come from a fairly heavy-weight process end of the industry. It involves a Systems Engineering approach that has foundations in things like defense projects. The kinds of descriptions I have been talking about may not suit everyone, or perhaps even anyone. However, I hope they will make some of us take a step back and be aware of what we as a whole are building from time to time.

Again I am not an expert, and I hope my lack of expertise is not a hindrance or false signpost to the gentle reader. I am a software guy who has been walking a path lately between traditional software experience and the experience of those around me working on various projects.

I'll now return you to your usual programming of REST Architecture, and everything that goes with it.