A first example of rules - introducing Archichect

It's high time for a few examples. I'll draw them from two programs:

• First, the medium-sized "Archichect" tool that I wrote for checking and exploration purposes (with substantial input some years ago from Thomas Freudenberg; and requests and ideas by colleagues at Pharmatechnik in Germany, my employer). Archichect is right now a proof-of-concept software, which means, among other things, that I change anything quite freely on a daily basis (if you nevertheless want to take a look, you can search it on GitHub and look into it).
• Second, the large flagship IXOS product of our company, which I will use later to demonstrate large-scale architectural aspects.

Let me start with a small example from the prescriptive architecture of Archichect. Here is a rough sketch of some part of its intended (therefore, prescriptive) architecture:

Obviously, this is a "type 1 sketch": It is a mixture of definitive information ("there are three subpackages called Reading, Transforming, and Rendering") and illustrative, but incomplete information ("There is a class called DependencyChecker implementing interface ITransformer, but probably there are more implementors of this interface"). There is nothing at all wrong with such diagrams—except that one must be extremely careful to draw conclusions from them: The "existential assumption" is usually ok ("All the things in the diagram will be there in reality"), but even this is sometimes risky ("oh, I just meant that as an example").

So let us write some rules for this part of Archichect that lift the information from the diagram to a "type 3 declaration". Here they are, in Archichect syntax:

$ DOTNETITEM ---> DOTNETITEM

(**)                    ---> ::\1
Archichect.Reading      ---> ::Archichect
Archichect.Transforming ---> ::Archichect
Archichect.Rendering    ---> ::Archichect

(You are not happy with the notation?—you would like to express this in code? Well, as I said, I am not too stubborn about notation, so I will show how to write the same rules with code in one of the next postings).

One can actually run these rules over Archichect itself and get a nice and very long result telling us that Archichect (in its current version) has 35505 dependencies, 26690 of which violate the rules above: "Sad", as some well-known guy would tweet. Obviously, transferring an architectural diagram to strict rules requires a little more work than just more or less faithfully copy it to text—but it's only a little more, I promise (and will show you).

Before that, however, let me explain the rules above, and the assumptions behind them, a little more.

First of all, the diagram did not spell out how the concept of a UML package is mapped to the language. Most modern languages have at least one concept of nestable groups for naming things; for example, Java has packages, and the .Net languages have namespaces. In addition to these logical nestable constructs, the physical units of runtime environments, e.g. JAR files or .NET assemblies, are typically named with filenames, which can again use a hierarchical naming concept. For example, in .Net, there are assemblies named System.dll, System.Threading.dll, System.Threading.Tasks.dll, System.Threading.Tasks.Dataflow and System.Threading.Tasks.Parallel.dll etc.

In Archichect, I chose the standard approach of mapping packages to the naming concept of the implementation language, i.e., to .Net's (and C#'s) namespaces. Thus, I would put

• Program, Item, Dependency as well as the three base interfaces into namespace Archichect;
• DotNetAssemblyReader in namespace Archichect.Reading,
• DependencyChecker in namespace Archichect.Transforming, and
• ViolationsWriter in namespace Archichect.Rendering.

The type DOTNETITEM defined by Archichect's DotNetAssemblyReader nicely defines the fields Namespace, Class, Assembly.Name, Assembly.Version, Assembly.Culture, and Member.Name, and therefore the rules above refer to the Namespace field without any further syntactical ado.

Still, an alternative architecture decision could be to distribute the classes and interfaces into multiple assemblies, and then, the rules would have to be written differently, e.g. (there are more possibilities) as follows:

$ DOTNETITEM ---> DOTNETITEM

::(**)                    ---> ::\1
::Archichect.Reading      ---> ::Archichect
::Archichect.Transforming ---> ::Archichect
::Archichect.Rendering    ---> ::Archichect

The two colons indicate that the strings are assumed to refer to the third field of DOTNETITEM, i.e., the Assembly.Name field. Assembly rules collect, at least in .Net, also important architectural information and should therefore be part of the "rule set"; but I will ignore them for the moment and continue with namespace-based rules.

Secondly, regarding the diagram, it does not say anything about any dependency restrictions inside each package. A typical implicit assumption is that, on this level of granularity, each item inside a package may use any other item in the same package. To allow such dependencies, the rule

(**) ---> \1

is added. The \1 notation here is borrowed from regular expressions (and actually, internally, the rule checking is mostly done by creating regexes from the rules and then matching the read-in dependencies against them).
This Archichect rule says that items from some namespace can use items from exactly the same namespace, but not from a child or parent namespace. We will see in a later posting that this would, in some cases, prevent the useful organization of namespaces, and therefore the actual architecture rules of Archichect are somewhat different. For the moment, we leave it at that.

Still, we should understand why our apparently so useful rules gave us so many invalid dependencies, shouldn't we? That's some stuff for another posting.

Models, notations, and languages

In previous postings, I used the terms "notation" and "language" too sloppily. Here is a short explanation how I intend to use them in subsequent postings—I hope that this is in line with common usage:

• Whenever we want to work with some real world things, we need a model of it. The model is a more or less rigid (mathematical) abstraction of the object(s) under consideration (which are called the "universe"). For example, a real CPU might be modelled via an abstract processor, which only considers its assembly-level commands, but not e.g. its heat emission. A model of an SQL database might only consider tables, columns and views, but "abstract away" triggers, stored procedures and everything else the vendor might have added as a feature.
• A notation is some sort of symbols that adhere to some syntax. Many notations are linear text notations (all programming languages I know of), but there are graphical notations like UML's notation.
• A language is a combination of a model and a notation, where the notation is mapped to the model or modifications of it.

The last definition implies that one can have many languages for the same model. Here is a simple example of this: Let our model (and also universe) be expressions of integral numbers, with e.g. subtraction, multiplication, and evaluation. Three possible notations are

• parenthesized infix expressions, e.g. "(5 – 3) * (5 – 2)"
• postfix expressions, e.g. "5 3 – 5 2 – *"
• and a tree notation that shows the expression tree.

All three (and many other) notations can be mapped to the model in a way that they "compute the result" of an expression correctly.

Having a good notation is important, but getting notation right is better done by a series of experiments with real people than a conceptual process. I will, therefore, not be very stubborn about notations for architectural problems. On the other hand, I will try to find a single small representation and manipulation model for architectural problems; and then try to argue that the chosen model is sufficient and practical and, well, good.

In spite of my laissez-faire approach to notation, I do hold a few beliefs about notation that I will try to argue more or less emphatically.

The most important is that any notation must scale to large descriptions. Thus, it must be possible to describe, in a manageable and legible way, a system that consists of a "flat 1000 different parts". By "flat 1000 different parts", I mean that the notation must not force the writer and the reader to introduce any sort of abstractions solely because the notation becomes unwieldy. I call this the "telephone directory property": A useful notation must be capable of practically notating a large, boring list of slightly different things "just so".

As a special case, I will not consider any diagrammatic notations for the moment (later, I'll come back to diagrams). For almost the complete history of software engineering, people—intelligent people—have tried to come up with a graphical replacement for formal textual languages like programming languages. There is a complete theory and much practical experience with two-dimensional diagram languages—but on the whole, they have never replaced textual languages in anything but small, and often not-too critical, software systems. The reason is exactly that diagram notations do not have the "telephone directory property"—diagrams describing a 1000-part system are, for all purposes, unusable: They cannot be viewed easily (especially if they contain longer, winding line paths), cannot be printed easily, and they cannot be manipulated easily. The morale: Designing and maintaining diagrams that are not useless from the outset is very hard.

(If you think "UML", and especially "UML according to all the software architecture textbooks out there", I remind you that my focus is not the use of diagrams for informal or "semi-formal"—whatever that means—purposes. For this, many diagram notations are perfectly fine. But I consider only what I called "use case no.3", i.e., languages for describing and maintaining architectures that have a strict semantics that can be used to prove or maintain something interesting in a software system).

There are a few more aspects—important aspects— that will influence all the many parts I want to assemble for useful "rule-based architecturing", but in order to keep the suspense low, I will now immediately give away what my proposed model is: (Finite) directed graphs with labelled edges and nodes. I will not restrict this quite general model much more, except that I have to define the allowed labels. They are:

• A node is identified by a label that is a tuple of strings. In addition, a node can have informational tags, each of which consist of a name and a real number.
• An edge is only identified by the nodes at its ends. It has three counts that are called the overall count, the questionable count, and the bad count. In addition, it can also have name+number tags like nodes.
• Both nodes and edges can have an arbitrary source information that is intended to find the object from which the node or edge were derived at some time.

For "historical reasons" (we invented the basics of this model some 10 years ago), I use the following terms:

• Nodes are called items.
• Edges are called dependencies, and the two items at the end of a dependency are called the using item and the used item.
• The identifying strings of items are called values, and the non-identifying tags of items and dependencies are called markers.

I hope that these terms do confer a rough meaning of the purposes for which they are used.

But—for which purposes are they used?

What made xunit testing successful?

The xunit revolution introduced

• a very simple notation (actually, two notations);
• a reasonable benefit for every developer;
• and, later, a culture that extended "mere xunit testing" to various "development philosophies" like TDD, TDD with baby steps, or BDD.

The notations have a set of important properties:

1. They define a small language of a few important concepts:
   • At the core, only testcases that run in a predefined test harness framework; and—almost unrelated to that framework—assertions;
   • for scalability, testfixtures and setup and teardown of test cases and fixtures.
2. The building blocks are very small: A single assertion is atomic; a single testcase can also be made atomic (i.e. just test a very tiny segment of the intended behavior).
3. There is a simple tool that efficiently does the mundane job of collecting and executing all notated items (test fixtures and test cases).
4. The tool can be easily run by any developer at any time.
5. The tool can also be easily integrated into existing automated build processes.
6. And, finally, the automatic execution can have a drastic feedback on the processes: Tests that do not pass halt the delivery process (by resulting in a "red" build).

The direct benefit for the developer is not that more quality assurance can be done during code development—even though later "xunit philosophies" are, one could argue, roughly founded on this belief (and delivered arguably better processes for direct support of development). On the contrary, more quality assurance (in the sense of "trying to find destructive input to check a program against the limits of a specification") during development would actually be an annoyance, because it disrupts the developer's constructive thought processes necessary for constructing code.

Rather, xunit testing helps to solve the problem of "later regression checks" occurring after code changes, when it is necessary to remember and run the simple as well as the tricky test cases that actually allow a developer (or a team) to hold the belief that the modified piece of code still behaves sanely.

The important experience is that that "later" is not only "much later", when a feature upgrade or bug fix requires changing the code, but that it can be right after the next (well or not so well thought out) modification during the initial development of some piece of code. That really helps developers.

Finally, xunit testing is open in multiple ways—how many tests one writes, how much behavior each one ascertains, when they are run in the development cycle, and when in the build cycle, and, last but not least, how writing and executing of xunit tests feeds back into design and code development. Because all this is not enforced by the tooling in any way, a host of "philosophies" could emerge on top of xunit testing, leading to a lively and sometimes heated debate with a huge effect on wide understanding and on "marketing" of xunit testing.

Great.

Could the same be accomplished for some parts of "architecting"?

We should try, at least, shouldn't we?

So, you and I and everyone should start to invent notations and tools for "architecting" along the lines of what made unit testing successful. I'll leave your ideas to you; in the next posting, I'll start to present mine.

Purposes of architectural documentation disentangled

I have been a little unfair in my last posting: The eight pages on UML 2.0 in Gorton's "Essential Software Architecture" are more than a mere advertisement for that (then) new UML version 2.0—they do actually contain some core advice about how to document architectural aspects of a program. I'll try to extract a compact view of what architecture documentation is, in Gorton's and, I think, the mainstream architecture's textbooks' view, from these pages and the case study in chapter 7.

First of all, architecture documentation is a collection of artifacts for human beings only. This is in contrast to code, which is targeted both at the "machine" and at human readers. In the background, there looms the idea of model-driven architecture, where an architecture model is used to create code—essentially, a compiler for a new language on some "higher" level than standard programming languages. However, like the book, I will disregard this aspect right now and return to it somewhat later.

The clear target of providing information to humans has lead most of us to the use of informal diagrams and standard prose to describe the architectural aspects of a software—"simple box-and-arrow diagrams", as Gorton calls them. He claims that there is "an appropriate diagram key to give a clear meaning to the notation used" in his examples, but most diagrams in his chapters 1 to 5 don't have such a key, and in any case, most people drawing such diagrams don't include one. The problem with this is that any plan to derive hard facts from such diagrams is then doomed.

Now, one purpose of architecture documentation is to give someone a "feeling of the interplay of things", and for this purpose, informal diagrams with textual or oral explanations are perfectly fine and, I am quite sure, even preferable: They appeal to our intuitive approach to most problems, which includes working with somewhat unclear terms and their relations in order to limit thinking about tricky consequences, so that our mind is free to "suck in the universe" of the problem area at hand.

Maybe it should be noted that formal clarity, precise meaning and even "simple" (mathematical) consistency entail, in almost all cases, "hard thought work", as the history of mathematics has shown:

• Geometry in the plane seems like an easy subject, until you start trying to understand its base and algorithms from Euclid's axioms and definitions, well over 2300 years old: There is nothing easy with concepts like parallels or ratios of line segment lengths! And later formalizations, mainly from about the 1800s onwards, are even more intricate.
• The other, apparently so "simple" basis of mathematics, namely the natural numbers, lost its simplicity also in ancient times with some prime number theory by the Greeks. It was and is by no means obvious what can emerge from simple addition and multiplication, let alone from the algebraic structures and formalizations extracted in the 19th century, leading to Gödel's mind-bending encodings and Turing's work.

Let me state this in my "Axiom 1": Mathematics, by and large, is not what we want in software documentation (and that from me, who majored in theoretical computer science ...).

Still, it seems we all want something more than the informal box-and-arrow-diagrams.

Gorton, like many others, proposes the use of UML. I cannot help the feeling that he is not really happy about it. The summary of chapter 6 has the following two sentences:

I’m a bit of a supporter of using UML-based notations and tools for producing architecture documentation. The UML, especially with version 2.0, makes it pretty straightforward to document various structural and behavioral views of a design.

"A bit of a supporter", "pretty straightforward": This does not really sound like wholehearted endorsement.

So, what is the problem?

The problem is, in my humble opinion, that there is no clear picture of what a notation for architectural documentation should do. The described use-cases typically oscillate between a "better notation" for those informal, easily comprehensible overviews over some aspects of a software system, and a more formal notation that can help derive hard knowledge about a system, with that implied goal of "generating code" in model-driven approaches.

I am, after many years in the field, now certain that we have to structure the use cases for architectural documentation in a threefold classification, with different notations for each area:

1. Informal documentation, from which humans can learn easily and intuitively gather a common understanding and a useful overview about some aspects of the system. In the best case, such a documentation is part of a common culture about "how we name and see things." However, this documentation is not intended to derive any hard facts: Everything shown can be disputed and discussed and viewed differently, and the notation can be extended at will if it helps with that intuitive understanding. All must agree that formal arguments based on such documentation are futile and hence must be avoided.
2. Formally sound and precise documentation that can be used to derive invariants and definitive properties of the documented system. If such documentation is used as the basis for a tool-supported model-driven approach, then there is no difference between a descriptive and a prescriptive architectural documentation for the aspects covered by the process. However, such an approach is very expensive in more than one respect:
   • First, especially without full tool support, keeping such a documentation in line with the system is much work, as even tiny changes on one or both sides require precise updates.
   • Second, as software can exhibit very complex behavior, the notation must be capable of describing many and, usually, deep concepts, which makes it hard and "mathematical" to understand and even harder to write. Such documentation therefore blatantly contradicts "Axiom 1".
   • Last, on a conceptual level, it is not really clear that such a documentation is actually "documentation" in the sense of "humanly accessible information relevant for many decisions in the software life-cycle". Rather, it might be more of a formal specification or even—when used in a model-driven process with code generation—part of the implementation, albeit (maybe) on some higher or "more compact" level than standard programming languages.

Thus, rich informal and deep formal notations are not sufficient for documenting and arguing about architectural aspects of a software.

3. Therefore, we need notations that are somewhere in-between: Not informal, so that they can be used to derive and ensure hard facts. But equally, they must be easily usable so that they can be read and written by the average software engineer under average project circumstances. It should be obvious that this type of notation cannot be very rich and also not very abstract. Only then, it can on the one hand avoid requiring an extensive semantics for formal derivations, and on the other hand being too esoteric to be used for understandable documents. In other words, it must be a quite mundane notation. I'll show my preferred notation for this, and its uses, in later postings—just in case you think that this looks a little like the search for the holy grail.

UML, incidentally and unfortunately, does not work really well for any of these purposes if its complex semantics is taken seriously:

1. For an informal notation, it carries a too heavy backpack of that formal semantics which no-one wants to remember when drawing informative diagrams in a running text (as, e.g., in the case study in Gorton's book).
2. For a formal notation, it is too indirect: One needs to map UML propositions back to the underlying semantic model (like Petri nets or state machines), and only then one can formally draw conclusions; as far as I can oversee it, the number of publications that use UML as a formal base has declined quite a bit over the last years.
3. Finally, as a simple but yet strict notation, UML is much too baroque, because it was lobbied to include every useful diagram and icon. This large notational size would recommend it for many different informal diagrams—if it weren't for that formal semantics ballast ...

But even if  you think that UML does work well (or well enough) for one area, there is the danger of misinterpreting UML diagrams: Is a diagram which your team uses as a basis for a decision a "type 1." diagram?—then it conveys informal concepts, but does not limit the decision strictly or formally. A "type 2." or "type 3." diagram, on the other hand, would narrowly limit some choices you can make—and definitely require a formally (for "type 2.") or at least collectively (for "type 3.") approved update of the diagram for any change in the software or the architecture. But most diagrams do not spell out explicitly their "conformance level".

Nonetheless, our analysts and some of our developers and architects (including me) are happy enough to use UML as a pool of symbols for sketching explanatory diagrams that help us to keep our complex machinery at least somewhat documented. So yes, I am, and we are also "a bit of a supporter of using UML-based notations and tools", as Ian Gorton puts it.

But now, I feel, I am starting to owe you an explanation how to do architectural documentation better. The next posting ... well, after I wrote it, it turned out to still consider some general observations about software architecture and how we deal with it.