Painless Functional Specifications – Part 2: What’s a Spec?

(Have you already read part one? If not, that’s here .)

This series of articles is about functional specifications, not technical specifications. People get these mixed up. I don’t know if there’s any standard terminology, but here’s what I mean when I use these terms.

  1. A functional specification describes how a product will work entirely from the user’s perspective. It doesn’t care how the thing is implemented. It talks about features. It specifies screens, menus, dialogs, and so on.
  2. A technical specification describes the internal implementation of the program. It talks about data structures, relational database models, choice of programming languages and tools, algorithms, etc.

When you design a product, inside and out, the most important thing is to nail down the user experience. What are the screens, how do they work, what do they do. Later, you worry about how to get from here to there. There’s no use arguing about what programming language to use before you’ve decided what your product is going to do. In this series, I’m only talking about functional specifications.

I’ve written a short sample spec which should give you an idea for what a good functional specification looks like. Before we go further, please read the sample spec.

Did you read it?

No you didn’t. Go read it now and then come back, so we can talk more about what a good spec should and shouldn’t have in it. I’ll wait here for you. Thanks.

(waiting patiently…)


Ah, good. You’re back.

Here are some of the things I put in every spec.

A disclaimer. Pure self defense. If you put a paragraph saying something like “This spec is not complete”, people won’t come into your office to bite your head off. As time goes on, when the spec starts to be complete, you can change it to say “this spec is complete, to the best of my knowledge, but if I forgot something, please tell me.” Which reminds me, every spec needs:

An author. One author. Some companies think that the spec should be written by a team. If you’ve ever tried group writing, you know that there is no worse torture. Leave the group writing to the management consulting firms with armies of newly minted Harvard-educated graduates who need to do a ton of busywork so that they can justify their huge fees. Your specs should be owned and written by one person. If you have a big product, split it up into areas and give each area to a different person to spec separately. Other companies think that it’s egotistic or not “good teamwork” for a person to “take credit” for a spec by putting their name on it. Nonsense. People should take responsibility and ownership of the things that they specify. If something’s wrong with the spec, there should be a designated spec owner, with their name printed right there on the spec, who is responsible for fixing it. 

Scenarios. When you’re designing a product, you need to have some real live scenarios in mind for how people are going to use it. Otherwise you end up designing a product that doesn’t correspond to any real-world usage (like the Cue?Cat). Pick your product’s audiences and imagine a fictitious, totally imaginary but totally stereotypical user from each audience who uses the product in a totally typical way. Chapter 9 of my UI design book (available online for free) talks about creating fictional users and scenarios. This is where you put them. The more vivid and realistic the scenario, the better a job you will do designing a product for your real or imagined users, which is why I tend to put in lots of made-up details. 

Nongoals. When you’re building a product with a team, everybody tends to have their favorite, real or imagined pet features that they just can’t live without. If you do them all, it will take infinite time and cost too much money. You have to start culling features right away, and the best way to do this is with a “nongoals” section of the spec. Things we are just not going to do. A nongoal might be a feature you won’t have (“no telepathic user interface!”) or it might be something more general (“We don’t care about performance in this release. The product can be slow, as long as it works. If we have time in version 2, we’ll optimize the slow bits.”) These nongoals are likely to cause some debate, but it’s important to get it out in the open as soon as possible. “Not gonna do it!” as George Sr. puts it. 

An Overview. This is like the table of contents for your spec. It might be a simple flowchart, or it might be an extensive architectural discussion. Everybody will read this to get the big picture, then the details will make more sense.

Details, details, details. Finally you go into the details. Most people will skim this until they need to know a particular detail. When you’re designing a web-type service, a good way to do this is to give every possible screen a canonical name, and provide a chapter describing each one in utter and mind-numbing detail.

Details are the most important thing in a functional spec. You’ll notice in the sample spec how I go into outrageous detail talking about all the error cases for the login page. What if the email address isn’t valid? What if the password is wrong? All of these cases correspond to real code that’s going to be written, but, more importantly, these cases correspond to decisions that somebody is going to have to make. Somebody has to decide what the policy is going to be for a forgotten password. If you don’t decide, you can’t write the code. The spec needs to document the decision.

Open Issues. It’s OK for the first version of the spec to leave open issues. When I write a first draft, I always have lots of open issues, but I flag them (using a special style so I can search for them) and, if appropriate, discuss the alternatives. By the time the programmers start work, all of these need to be stomped out. (You might think it’s OK to just let the programmers start on all the easy stuff, and you’ll solve the open issues later. Bad idea. You will have enough problems resolving the new issues that come up when the programmers try to implement the code, without having old open issues around that you knew about in advance and could have solved then. Besides, the way you resolve anything non-trivial may have a major impact on how the code should be written.)

Side notes. While you’re writing a spec, remember your various audiences: programmers, testers, marketing, tech writers, etc. As you write the spec you may think of useful factoids that will be helpful to just one of those groups. For example, I flag messages to the programmer, which usually describe some technical implementation detail, as “Technical Notes”. Marketing people ignore those. Programmers devour them. My specs are often chock full of “Testing Notes,” “Marketing Notes,” and “Documentation Notes.”

Specs Need To Stay Alive. Some programming teams adopt a “waterfall” mentality: we will design the program all at once, write a spec, print it, and throw it over the wall at the programmers and go home. All I have to say is: “Ha ha ha ha ha ha ha ha!”

This approach is why specs have such a bad reputation. A lot of people have said to me, “specs are useless, because nobody follows them, they’re always out of date, and they never reflect the product.”

Excuse me. Maybe your specs are out of date and don’t reflect the product. My specs are updated frequently. The updating continues as the product is developed and new decisions are made. The spec always reflects our best collective understanding of how the product is going to work. The spec is only frozen when the product is code complete (that is, when all functionality is complete, but there’s still testing and debugging work.)

To make people’s life easier, I don’t rerelease the spec daily. I usually keep an up to date version on a server somewhere where the team can use it as a reference. On occasional milestones, I print a copy of the spec with revision marks so that people don’t have to reread the whole thing — they can scan the revision marks to see what changes have been made.

Who should write the specs? Read all about it in Part 3.


So… what’s a spec?

When you design a product, inside and out, the most important thing is to nail down the user experience. What are the screens, how do they work, what do they do.

Part two of my four-part series on writing functional specifications is now available.

If you subscribe to my low-volume email list, I’ll send you an email as soon as the next parts are posted.

Painless Functional Specifications – Part 1: Why Bother?

When The Joel Test first appeared, one of the biggest sore points readers reported had to do with writing specs. It seems that specs are like flossing: everybody knows they should be writing them, but nobody does.

Why won’t people write specs? People claim that it’s because they’re saving time by skipping the spec-writing phase. They act as if spec-writing was a luxury reserved for NASA space shuttle engineers, or people who work for giant, established insurance companies. Balderdash. First of all, failing to write a spec is the single biggest unnecessary risk you take in a software project. It’s as stupid as setting off to cross the Mojave desert with just the clothes on your back, hoping to “wing it.” Programmers and software engineers who dive into code without writing a spec tend to think they’re cool gunslingers, shooting from the hip. They’re not. They are terribly unproductive. They write bad code and produce shoddy software, and they threaten their projects by taking giant risks which are completely uncalled for.

I believe that on any non-trivial project (more than about 1 week of coding or more than 1 programmer), if you don’t have a spec, you will always spend more time and create lower quality code. Here’s why.

The most important function of a spec is to design the program. Even if you are working on code all by yourself, and you write a spec solely for your own benefit, the act of writing the spec — describing how the program works in minute detail — will force you to actually design the program.

Let’s visit two imaginary programmers at two companies. Speedy, at Hasty Bananas Software, never writes specs. “Specs? We don’t need no stinkin’ specs!” At the same time, Mr. Rogers, over at The Well-Tempered Software Company, refuses to write code until the spec is completely nailed down. These are only two of my many imaginary friends.

Speedy and Mr. Rogers have one thing in common: they are both in charge of backwards compatibility for version 2.0 of their respective products.

Speedy decides that the best way to provide backwards compatibility is to write a converter which simply converts 1.0 version files into 2.0 version files. She starts banging that out. Type, type, type. Clickety clickety clack. Hard drives spin. Dust flies. After about 2 weeks, she has a reasonable converter. But Speedy’s customers are unhappy. Speedy’s code will force them to upgrade everyone in the company at once to the new version. Speedy’s biggest customer, Nanner Splits Unlimited, refuses to buy the new software. Nanner Splits needs to know that version 2.0 will still be able to work on version 1.0 files without converting them. Speedy decides to write a backwards converter and then hook it into the “save” function. It’s a bit of a mess, because when you use a version 2.0 feature, it seems to work, until you go to save the file in 1.0 format. Only then are you told that the feature you used half an hour ago doesn’t work in the old file format. So the backwards converter took another two weeks to write, and it don’t work so nice. Elapsed time, 4 weeks.

Now, Mr. Rogers over at Well-Tempered Software Company (colloquially, “WellTemperSoft”) is one of those nerdy organized types who refuses to write code until he’s got a spec. He spends about 20 minutes designing the backwards compatibility feature the same way Speedy did, and comes up with a spec that basically says:

  • When opening a file created with an older version of the product, the file is converted to the new format.

The spec is shown to the customer, who says “wait a minute! We don’t want to switch everyone at once!” So Mr. Rogers thinks some more, and amends the spec to say:

  • When opening a file created with an older version of the product, the file is converted to the new format in memory. When saving this file, the user is given the option to convert it back.

Another 20 minutes have elapsed.

Mr. Rogers’ boss, an object nut, looks at this and thinks something might be amiss. He suggests a different architecture.

  • The code will be factored to use two interfaces: V1 and V2.  V1 contains all the version one features, and V2, which inherits from V1, adds all the new features. Now V1::Save can handle the backwards compatibility while V2::Save can be used to save all the new stuff. If you’ve opened a V1 file and try to use V2 functionality, the program can warn you right away, and you will have to either convert the file or give up the new functionality.

20 more minutes.

Mr. Rogers is grumpy. This refactoring will take 3 weeks, instead of the 2 weeks he originally estimated! But it does solve all the customer problems, in an elegant way, so he goes off and does it.

Total elapsed time for Mr. Rogers: 3 weeks and 1 hour. Elapsed time for Speedy: 4 weeks, but Speedy’s code is not as good.

The moral of the story is that with a contrived example, you can prove anything. Oops. No, that’s not what I meant to say. The moral of the story is that when you design your product in a human language, it only takes a few minutes to try thinking about several possibilities, revising, and improving your design. Nobody feels bad when they delete a paragraph in a word processor. But when you design your product in a programming language, it takes weeks to do iterative designs. What’s worse, a programmer who’s just spend 2 weeks writing some code is going to be quite attached to that code, no matter how wrong it is. Nothing Speedy’s boss or customers could say would convince her to throw away her beautiful converting code, even though that didn’t represent the best architecture. As a result, the final product tends to be a compromise between the initial, wrong design and the ideal design. It was “the best design we could get, given that we’d already written all this code and we just didn’t want to throw it away.” Not quite as good as “the best design we could get, period.”

So that’s giant reason number one to write a spec. Giant reason number two is to save time communicating. When you write a spec, you only have to communicate how the program is supposed to work once. Everybody on the team can just read the spec. The QA people read it so that they know how the program is supposed to work and they know what to test for. The marketing people use it to write their vague vaporware white papers to throw up on the web site about products that haven’t been created yet. The business development people misread it to spin weird fantasies about how the product will cure baldness and warts and stuff, but it gets investors, so that’s OK. The developers read it so that they know what code to write. The customers read it to make sure the developers are building a product that they would want to pay for. The technical writers read it and write a nice manual (that gets lost or thrown away, but that’s a different story). The managers read it so that they can look like they know what’s going on in management meetings. And so on.

When you don’t have a spec, all this communication still happens, because it has to, but it happens ad hoc. The QA people fool around with the program willy-nilly, and when something looks odd, they go and interrupt the programmers yet again to ask them another stupid question about how the thing is supposed to work. Besides the fact that this ruins the programmers’ productivity, the programmers tend to give the answer that corresponds to what they wrote in the code, rather than the “right answer.” So the QA people are really testing the program against the program rather than the program against the design, which would be, um, a little bit more useful.

When you don’t have a spec, what happens with the poor technical writers is the funniest (in a sad kind of way). Tech writers often don’t have the political clout to interrupt programmers. In many companies, if tech writers get in the habit of interrupting programmers to ask how something is supposed to work, the programmers go to their managers and cry about how they can’t get any work done because of these [expletive deleted] writers, and could they please keep them away, and the managers, trying to improve productivity, forbid the tech writers to waste any more of their precious programmers’ time. You can always tell these companies, because the help files and the manuals don’t give you any more information than you can figure out from the screen. When you see a message on a screen which says

  • Would you like to enable LRF-1914 support?

… and you click “Help”, a tragicomic help topic comes up which says something like

  • Allows you to choose between LRF-1914 support (default) or no LRF-1914 support. If you want LRF-1914 support, choose “Yes” or press “Y”. If you don’t want  LRF-1914 support, choose “No” or press “N”.

Um, thanks. It’s pretty obvious here that the technical writer was trying to cover up the fact that they didn’t know what LRF-1914 support is. They couldn’t ask the programmer, because (a) they’re embarrassed, or (b) the programmer is in Hyderabad and they’re in London, or (c) they have been prohibited by management from interrupting the programmer, or any other number of corporate pathologies too numerous to mention, but the fundamental problem is that there wasn’t a spec.

Number three giant important reason to have a spec is that without a detailed spec, it’s impossible to make a schedule. Not having a schedule is OK if it’s your PhD and you plan to spend 14 years on the thing, or if you’re a programmer working on the next Duke Nukem and we’ll ship when we’re good and ready. But for almost any kind of real business, you just have to know how long things are going to take, because developing a product costs money. You wouldn’t buy a pair of jeans without knowing what the price is, so how can a responsible business decide whether to build a product without knowing how long it will take and, therefore, how much it will cost? For more on scheduling, read Painless Software Schedules.

A terribly common error is having a debate over how something should be designed, and then never resolving the debate. Brian Valentine, the lead developer on Windows 2000, was famous for his motto “Decisions in 10 minutes or less, or the next one is free.”

In too many programming organizations, every time there’s a design debate, nobody ever manages to make a decision, usually for political reasons. So the programmers only work on uncontroversial stuff. As time goes on, all the hard decisions are pushed to the end. These are the most likely projects to fail. If you are starting a new company around a new technology and you notice that your company is constitutionally incapable of making decisions, you might as well close down now and return the money to the investors, because you ain’t never gonna ship nothing.

Writing a spec is a great way to nail down all those irritating design decisions, large and small, that get covered up if you don’t have a spec. Even small decisions can get nailed down with a spec. For example, if you’re building a web site with membership, you might all agree that if the user forgets their password, you’ll mail it to them. Great. But that’s not enough to write the code. To write the code, you need to know the actual words in that email. At most companies, programmers aren’t trusted with words that a user might actually see (and for good reason, much of the time). So a marketing person or a PR person or some other English major is likely to be required to come up with the precise wording of the message. “Dear Shlub, Here’s the password you forgot. Try not to be so careless in the future.” When you force yourself to write a good, complete spec (and I’ll talk a lot more about that soon), you notice all these things and you either fix them or at least you mark them with a big red flag.

OK. We’re on the same page now. Specs are motherhood and apple pie. I suspect that most people understand this, and my rants, while amusing, aren’t teaching you anything new. So why don’t people write specs? It’s not to save time, because it doesn’t, and I think most coders recognize this. (In most organizations, the only “specs” that exist are staccato, one page text documents that a programmer banged out in Notepad after writing the code and after explaining that damn feature to the three hundredth person.)

I think it’s because so many people don’t like to write. Staring at a blank screen is horribly frustrating. Personally, I overcame my fear of writing by taking a class in college that required a 3-5 page essay once a week. Writing is a muscle. The more you write, the more you’ll be able to write. If you need to write specs and you can’t, start a journal, create a weblog, take a creative writing class, or just write a nice letter to every relative and college roommate you’ve blown off for the last 4 years. Anything that involves putting words down on paper will improve your spec writing skills. If you’re a software development manager and the people who are supposed to be writing specs aren’t, send them off for one of those two week creative writing classes in the mountains.

If you’ve never worked in a company that does functional specifications, you may never have seen one. In the next part of this series, I’ll show you a short, sample spec for you to check out, and we’ll talk about what a good spec needs to have. Read on!


When The Joel Test first appeared, one of the biggest sore points readers reported had to do with writing specs. It seems that specs are like flossing: everybody knows they should be writing them, but nobody does.

This week, I’ll be talking in depth about functional specifications: what they are, why you should write them, how you should write them, and so on. We’ll start today with Part One: Why Bother?