P.5 Seven Rules for Sound Documentation
Architecture documentation is much like the documentation we write in other facets of our software development projects. As such, it obeys the same fundamental rules for what distinguishes good, usable documentation from poor, ignored documentation. We close the prologue with seven rules for sound software documentation. Use this checklist when you write technical documentation. (You can also use it when you read technical documentation: the rules provide objective criteria for judging a document’s quality, and they let you say something constructive in a critical review.)
Rule 1: Write Documentation from the Reader’s Point of View
This rule simply reminds us to keep the end game in mind as we produce our documentation: Make your document serve its stakeholders and their intended uses of it. It is surprisingly easy to forget that rule in the midst of looming deadlines, an overflowing e-mail queue, and a cell phone that won’t shut up.
The great computing scientist Edsger Dijkstra (1930–2002), the inventor of many of the software engineering principles we now take for granted, once said that he would happily spend two hours pondering how to make a single sentence clearer. He reasoned that if the paper were read by a couple of hundred people—a decidedly modest estimate for someone of Dijkstra’s caliber—and he could save each reader a minute or two of confusion, it was well worth the effort. Professor Dijkstra’s consideration for the reader reflects his classic manners, but it also gives us a new and useful concept of the effort associated with a document. Usually we just count how long it takes to write. Dijkstra taught us to be concerned with how long it takes to use. Writing a document that a reader finds easy to use will help tilt the economics of documentation in our favor, as defined in the formula in Section P.2.4.
Writing for the reader is just plain polite, but it has a practical advantage as well. A reader who feels that the document was written with him or her in mind appreciates the effort but, more to the point, will come back to the document again and again in the future. Documents written for the reader will be read; documents written for the convenience of the writer will not. All of us like to shop at stores that seem to want our business, and we avoid stores that do not. This is no different.
Tips on how to write for the reader include:
- Find out who your readers are, what they know, and what they expect of the document. Have an informal chat with some representatives of various kinds of readers and see what their expectations are. Don’t make uninformed assumptions about what your readers know.
- Avoid stream of consciousness writing. If you find yourself writing things down in the order they occur to you, without an overall organizational plan, stop. Work out where specific kinds of information should go and put them where they belong. Make sure that you know what question(s) are being answered by each section of a document.
- Avoid unnecessary insider jargon. The documentation may be read by someone new to the field or from a company that does not share the same jargon. Add a glossary to define specialized terms.
- Avoid overuse of acronyms. Resist using an acronym when the spelled-out phrase is short or it appears only a few times. Always provide a dictionary that decodes whatever acronyms you do use.
Rule 2: Avoid Unnecessary Repetition
Each kind of information should be recorded in exactly one place. This makes documentation easier to use and much easier to change as it evolves. It also avoids confusion: information that is repeated is likely to be in a slightly different form, and now the reader must wonder “Was the difference intentional? If so, what is the meaning of the difference? Did the author change one place and forget to update the other?”
It should be a goal that information never be repeated. However, at times the cost to the reader of not repeating information in the other places where it’s needed is high. Readers don’t like to flip pages or click hyperlinks unnecessarily. The information may be repeated in two or more different places for clarity or to make different points. Also, expressing the same idea in different forms is often useful for achieving a thorough understanding. If keeping the information separate comes at too high a cost to the reader, repeat the information.
In a document maintained and viewed online, hyperlinks make this rule easier to follow. For example, each term can be hyperlinked to its definition; a concept can be hyperlinked to an explanation or elaboration.
Rule 3: Avoid Ambiguity
Ambiguity occurs when documentation can be interpreted in more than one way and at least one of those ways is incorrect. The most dangerous kind of ambiguity is undetected ambiguity. Here, each reader will think he or she understands the document, but unwittingly each reader will come to different conclusions about what it is saying.
Following two of the other rules will help you avoid ambiguity:
- By avoiding needless repetition (rule 2), you avoid the “almost but not quite alike” form of ambiguity.
- Reviewing the document with members of its intended audience (rule 7) will help spot and weed out ambiguities.
A well-defined notation with precise semantics goes a long way toward eliminating whole classes of linguistic ambiguity from a document. This is one area where standard languages and notations help a great deal, but using a formal language isn’t always necessary. Simply adopting a set of notational conventions and then using them consistently and rigorously will help eliminate many sources of ambiguity. But if you do adopt a notation, then the following corollary applies:
Rule 3a: Explain Your Notation
The ubiquitous box-and-line diagrams that people always draw on whiteboards are one of the greatest sources of ambiguity in architecture documentation. Although not a bad starting point, these diagrams are certainly not good architecture documentation. First, most such diagrams suffer from ambiguity. Are the boxes supposed to be modules, objects, classes, services, clients, servers, databases, processes, functions, tiers, procedures, processors, or something else? Do the arrows mean calls, uses, data flow, I/O, inheritance, communication, processor migration, or something else?
Make it as easy as possible for your reader to determine the meaning of the notation. The best way to do this is always to include a key in your diagrams. If you’re using a standard visual language defined elsewhere, the key can simply name it or refer readers to the source of the language’s semantics. Even if the language is standard or widely used, different versions often exist. Let your reader know, by citation, which one you’re using. For example, “Key: UML 2.0” is a perfectly fine key, and it puts readers and authors on the same page. For a homegrown informal notation, include a key to the symbology. This is good practice because it compels you to understand what the pieces of your system are and how they relate to one another; it’s also courteous to your readers.
Rule 4: Use a Standard Organization
Establish a standard, planned organization scheme, make your documents adhere to it, and ensure that readers know about it. A standard organization, also called a template, offers many benefits.
- It helps the reader navigate the document and find specific information quickly. Thus, this benefit is also related to the write-for-the-reader rule.
- It also helps the document writer plan and organize the contents. The writer doesn’t have to start with a blank page when answering the question “What topics and in what order should I have in this document?” The template already provides an outline of the important topics to cover.
- It allows the writer to record information as soon as it’s known. For example, pieces of section 4 may be written before sections 1–3 are there.
- It reveals what work remains to be done by the number of sections labeled “TBD” (to be determined) or “To Do.”
- It embodies completeness rules for the information; the sections of the document constitute the set of important aspects that need to be conveyed. Hence, the standard organization can form the basis for a first-order validation check of the document at review time.
Corollaries to this rule are these:
Organize documentation for ease of reference. Software documentation may be read from cover to cover at most once, probably never. But a document is likely to be referenced hundreds or thousands of times. Do what you can to make it easy to find information quickly. Adding a table of contents, an index, a glossary, and an acronym list are all good ways to help readers look up specific information.
Don’t leave any section blank; mark as “TBD” what you don’t yet know or “NA” what you know is not applicable. Many times, we can’t fill in a document completely because we don’t yet know the information, or because decisions have not been made, or because we didn’t yet have time to do it. In that case, mark the document accordingly (for example, “TBD” or “To Do”). Templates are by nature generic and hence comprehensive. If a given section of the template does not apply for the document you’re creating, mark it as “NA.” If the section is blank, the reader will wonder whether the information is coming later or whether it is indeed supposed to be blank. Thus this advice is related to the rule about avoiding ambiguity.
Rule 5: Record Rationale
Architecture is the result of making a set of important design decisions, and architecture documentation records the outcomes of those decisions. For the most important decisions, you should record why you made them the way you did. You should also record the important or most likely alternatives you rejected and state why. Later, when those decisions come under scrutiny or pressure to change, you will find yourself revisiting the same arguments and wondering why you didn’t take another path. Recording your rationale will save you enormous time in the long run, although it requires discipline to record your rationale in the heat of the moment.
Of course, not every single design decision should have the rationale captured in the architecture documentation. If a design decision is key to achieve a quality requirement of the system, its rationale is probably worth capturing. If a design decision required a long meeting with stakeholders, that’s a good decision to capture. If you conducted technical experiments and studies or created prototypes to evaluate design alternatives, the conclusions of this effort should be captured as rationale for the chosen alternative. Keep in mind that one week, one month, or one year from now, you may not remember why you did things that way, and other people will not know either.
Rule 6: Keep Documentation Current but Not Too Current
Documentation that is incomplete or out of date does not reflect truth, does not obey its own rules for form and internal consistency, and is not used. Documentation that is kept current and accurate is used. Why? Because questions about the software can be most easily and most efficiently answered by referring to the appropriate document. Documentation that is somehow inadequate to answer the question needs to be fixed. Updating it and then referring the questioner to it will deliver a strong message that the documentation is the final, authoritative source for information.
During the design process, on the other hand, decisions are made and reconsidered with great frequency. Revising documentation to reflect decisions that will not persist is an unnecessary expense.
Your development plan should specify particular points at which the documentation is brought up to date or the process for keeping the documentation current. For example, the end of each iteration or sprint, or each incremental release, could be associated with providing revised documentation. Every design decision should not be recorded and distributed the instant it is made; rather, the document should be subject to version control and have a release strategy, just as every other artifact does.
Rule 7: Review Documentation for Fitness of Purpose
Only the intended users of a document will be able to tell you whether it contains the right information presented in the right way. Enlist their aid. Before a document is released, have it reviewed by representatives of the community or communities for which it was written.