Agile Design

There is one aspect of Agile which people have trouble coming to grips with. I talked a little about this recently when I mentioned JIT design (see JIT). You may have heard it called other names like evolutionary design, incremental design, emergent design, continuous design, etc. I will use the term Agile Design or JIT Design.

Some people are adamant that designing like this is fraught with danger or just doesn't work. However, there are many, many projects where it has worked better than anyone expected and certainly better than the Waterfall approach of Big Design Up Front (BDUF). For example, see Continuous Design by Jim Shore.

The opposition is simply due to not really understanding how design is done in Agile development. In particular, using JIT design without using other aspects of Agile methodologies such as regular feedback, collaboration, continuous integration, collective code ownership, etc, will doom it to failure. And as usual I will mention that Unit Tests are crucial (see Unit Tests).

How it works as part of an Agile methodology is hard to understand so I will use an analogy. Hopefully, this allows you to understand my point.

An Analogy

I will use the analogy of designing a house. I chose this analogy as you probably already have an idea on how that's done.

Houses are built from a plan. Often an architect has spent many weeks or months creating a design on paper. Even the worst houses are built from some sort of floor plan. (In fact, in my country, government regulations preclude building without a detailed plan that has been approved by the local council.) The point is that for any house, a lot of time is spent in design and planning before construction can even begin.

Over many years (even millenia) this has been refined to be the best way to build a house. In fact this is essentially the same way that all complex physical devices and structures are built - some sort of designer, architect, or engineer (or team thereof) creates a blueprint which is carefully perused, verified, and approved before the final product is actually built.

If you just went and bought some materials and started building a house do you think it would be anything other than a disaster? Of course not!

So why do I think that designing and building software should be any different? The simple fact is that you can't build a house or software in the conventional manner without a fairly detailed design. However, if you change the way you build the software things are different. This is why Agile software development is considered to be a methodology - it only works when you understand and embrace it, in toto.

Let's see how we might build a house in the Agile manner. Note that, in the discussion below, D stands for developer (the house creator) and C stands for client (the house user).

From this ...

D: What is your most basic requirement for the house?
C: I want something that stops me getting wet when it rains.
D: Try this. [A piece of corrugated iron on four posts.]
C: I keep bumping my head on it.
D: How's this? [The corner posts have been extended.]
C: Better. What about something to stop the wind.
D: Try this. [Four walls attached to the posts.]
C: It's less breezy but I can't get out.
D: I could add a door. ...

to this!

This continues until C has their dream house (or they can't afford to pay for any more changes).

Software is different

I hope you realize that I am not advocating this way of building houses! This would be a particularly slow and expensive way to do so. However, when building software this can be a better way (and I would argue almost always is). Building software is different for these reasons:

1. Modifying physical objects (like extending the above-mentioned corner posts) can be time-consuming and expensive. Software is far more malleable. To make something larger may be a simple matter of changing a constant and recompiling.

2. Continually changing the design of anything of any complexity quickly becomes a nightmare of managing and controlling the interactions. With software this is a little easier because you do not have to worry about physical interactions (such as the location of pipes and wiring). However, it is still important to decouple different parts of the software whenever there are no major drawbacks to doing so (see DIRE). Crucially, Unit Tests can be used to ensure nothing is broken as the software is continuously redesigned and refactored.

3. House design is well-understood and there are many standards and conventions that are well-known, proven and followed. Software design is far more variable. Moreover, the client does not have a very good idea of what they want. Even if they did the developers do not have a very good idea of how to give it to them until they try to build it.

4. For practical reasons design constraints for houses do not change much if at all. For example, the government is not going to one day say that all electrical wiring in all houses is to be changed immediately. (Even if there were an urgent safety issue this would be logistically impossible.) However, these sort of major changes are regularly thrown at software developers. BDUF cannot easily cope with change the way continuous design does.

The Argument For BDUF

I have often read the blog of Joel Spolsky, whom I respect and admire greatly. However, many years ago I read a post which saddened and disappointed me. Part of Joel's blog is even quoted on the BDUF page on Wikipedia as an argument in favour of BDUF.

However, if you carefully read the full paragraph from which the quote is taken, Joel's example is actually an excellent argument for Agile Design. Here is the full paragraph:

"Many times, thinking things out in advance saved us serious development headaches later on. When I wrote the first draft of this spec, I had a more complicated flowchart that assumed that either party (helper or victim) could initiate the process. This became extremely complicated and convoluted for various reasons. As I worked through the screens that would be needed to allow either party to initiate the process, I realized that Aardvark would be just as useful, and radically simpler, if the helper was required to start the whole process. Making this change in the spec took an hour or two. If we had made this change in code, it would have added weeks to the schedule. I can’t tell you how strongly I believe in Big Design Up Front, which the proponents of Extreme Programming consider anathema. I have consistently saved time and made better products by using BDUF and I’m proud to use it, no matter what the XP fanatics claim. They’re just wrong on this point and I can’t be any clearer than that."

- Joel Spolsky, 2006

The first thing that hit me is the mention of the first draft of the spec with the "complicated flowchart". It is obvious that the first draft is already suffering from the problems commonly associated with BDUF. In particular, it is clearly making unnecessary assumptions.

Flowcharts      

Does anyone use flowcharts anymore? Back in 1979 I was told by my lecturer to avoid flow charts (probably due to their association with the evil goto). Personally, I have found UML activity diagrams (and sequence diagrams) to be more useful.

But using Agile Design obviates the need for lots of diagrams.

Luckily (for Joel) he had the experience and insight to be able to identify a problem in the spec and fix it before it made it into the code. Apparently in his development environment code was hard to modify since a change to the spec that takes an hour or two would require weeks of code changes. (Note that this is another symptom of not using Agile techniques such as Unit Tests.)

Now imagine if the designer(s) of the software had less time, less experience or less insight than Joel. They may have gone with the first draft of the spec with the consequent later weeks of recoding that this entailed. This is plausible as I doubt many designers are as experienced and insightful as Joel. So rather than demonstrating why BDUF is better than Agile, Joel's example clearly shows its weakness.

Now consider the Agile approach, which is to first create a minimal working version of the software. In this initial version it might not have even been obvious who (helper or victim) could initiate the process. Users would try the software and provide feedback which is used to evolve it into what they need. In this way the conundrum of who should initiate the process would probably not even have arisen.

Another thing to note is how Joel simulated the use of the software in his own mind by "working through the screens". Not many people can do this effectively. It's always far more effective to have working software than try to imagine how the software would work. (Note that this is another advantage of Agile element 2: Favour working software over comprehensive documentation.)

Finally there is one thing I need to be absolutely clear on: thinking things out in advance is not the only way to avoid serious headaches later on. And most of the time it's not even the best way.


Problems of BDUF

The plain fact is that for more than 50 years BDUF has been tried and it consistently fails. Even for simple, well-understood projects, no matter how hard you try, it's impossible to produce anything approaching a good design without attempting to implement that design. I have seen this time and again. Trust me on this.

"What?" I hear you say. "You can't say that without an explanation." Well, I can if I want to, so there! Actually, I don't need to, since it has been discussed elsewhere. For example, see the Wikipedia page.

I have also touched on the subject many times in previous posts in this blog. For example, it is the principle cause of the Gas Factory Anti-pattern. More recently, I have talked about it in the series of posts on Unit Tests and specifically in the Cost of Rework section in JIT.

But just because you asked so nicely, here is a summary of recognized problems with creating a detailed specification up-front:

• the customer often does not really know what they want, at least till they try it
• it does not adapt to changing requirements
• it tries to anticipate future changes, adding unnecessary complexity
• it tries to document every known "fact" (of which many are usually incorrect)
• it makes unnecessary assumptions about how the design will be implemented
• it is very hard to foresee problems
• many things (often major things) are overlooked
• it takes too long to create - stakeholders become worried when there is little sign of progress
• it separates design from coding, even though they are very closely related (see DIRE)
• it results in a contorted implementation - it's often easier to change the code than fix the spec
• the document is not updated when the inevitable changes are made

There are probably more, but those are the ones that occur to me now.

However, now that I think about it there is one major drawback that I have not seen discussed. This is that developers don't really come to a full understanding of what the software is supposed to do until they actually try to implement it. It's only then that logical inconsistencies, omissions and other mistakes in the design become obvious. Further, limitations (unknown to the designer and often even to the developers) only become apparent when you attempt to write the code.

Possibly most significantly, only when the developers implement the design do better ways to do things occur to them. I have lost count of the times with BDUF when I have finally completed the code, only to then think of a much better solution (simpler, more efficient, more usable, more maintainable, etc).

Agile Design

The Agile approach is only to design what is needed for the next feature or iteration. This is usually the smallest possible addition to the system. The designer should only concentrate on the current iteration and never consider any possible future changes. This is a lot harder than it sounds. (I do concede that occasionally there are exceptions to this rule which I will discuss later.)

Since past iterations did not look to the future, a particular new feature may completely invalidate a large part of the existing design. Don't panic! This is normal. Traditionally such frequent refactoring would create chaos, since every iteration would have the distinct possibility of breaking existing functionality. Without complete regression testing after each iteration bugs would creep in and not be found till some time later

This is why Agile Design requires use of Unit Tests. Unit Tests allow continual redesign and refactoring to be performed without fear of breaking existing functionality.

The advantage of the Agile approach is that the closely related activities of design and coding are done consecutively and, to some extent, simultaneously. As I discussed previously in DIRE separating the design and coding leads to the major problems of Waterfall development.

The one huge benefit that I have found is that, after developing earlier features, the developers are very familiar with how the software works. This greatly improves the chances of a better design being developed for the next iteration. Mistakes and omissions are far less likely and a good (simple, DRY, etc) solution is much more likely.

B2DB Syndrome

Every programmer experiences the B2DB (Back To Drawing Board) syndrome when they are first learning to program. This is where one tries to write some code but in doing so obtains a better understanding of how to do it. The consequence is one has to go back to the drawing board, often many times. (I discussed this in more detail in JIT Testing).

For many this is a very painful experience and they believe there must be a better way, especially (as is often the case) they have already been trained in another field (such as a branch of engineering) where it is essential to do up-front design.

I believe this is the real reason for much opposition to Agile Design - it feels like the bad old B2DB syndrome.

It's true that there is a lot of refactoring going on in Agile Design, but this is actually an advantage. Of course, more experienced developers are going to have a better initial design, but even the best will see better ways to do things once they fully understand what they are trying to do. Also (as all the Agile experts emphasize) it makes the process much more amenable to changing requirements.

Finally, at the risk of sounding like a broken record, I will mention that Unit Tests make the refactoring safer and easier. Rather than having the feel of dread of B2DB, you can actually learn to enjoy refactoring and the process of creating the best design you can find.

Summary

• only make the simplest possible enhancement that will work
• forget the large design document with lots of UML diagrams
• use the code (especially Unit Tests) as the "documentation"
• don't design for the future, concentrate on what is needed now
• don't make the code flexible, just write for the specific task at hand
• try to forget all the extra (irrelevant) information that has been given to you
• don't be afraid to refactor existing code
• use Unit Tests to allow changes to be made without fear of breaking anything

DIRE

The most useful ideas in software design can be applied in different ways and in different settings. They can save a lot of time and trouble if you can learn to recognize when they are appropriate. Otherwise, you can try to keep them in mind when designing and building software.

One such idea is the DRY principle (see the DRY section in Software Design). It simply says that keeping an eye out for duplication (in its many subtle forms), and trying to eliminate it, will save you a lot of time and worry in the long run. The main advantage is that it is easy to remember and, with practice, easy to identify and apply. The DRY acronym was invented by Thomas and Hunt (see The Pragmatic Programmer). Of course, the idea behind it has been around a long time but Thomas and Hunt elucidated and promoted it well.

Unfortunately DIRE is a 4 letter acronym. I know the fashion nowadays is for TLA's but this was the best I could do.

DIRE is an acronym that I invented for another idea. It stands for Don't Isolate Related Entities. It basically means that you should make every effort to keep things together that belong together. There is a close relationship between DRY and DIRE -- DRY is really a special case of DIRE where the "related entities" are so closely related that they can be partially or totally combined.

DIRE may seem obvious but many things conspire to defeat it. It is not always apparent how or why "entities" are closely related. Often related entities are separated for technical (and even non-technical) reasons, or simply by accident. I give examples below to show the broad range of areas and how you might apply it.

Origins

I guess everyone has had the experience of being interrupted in a sequence of tasks. When you come back later you've forgotten most of the context. Continuing where you left off is tedious and error-prone. In particular, interrupting developers a lot is a great way to generate bugs.

Like everyone I quickly discovered this when I started programming professionally (about 3 decades ago). However, it was not until about 2 decades ago that I began to think of this as an example of a more general principle. This was precipitated by my first encounter with JIT.

I discussed JIT in detail last week but in brief JIT is a QA technique where things are done "Just In Time". JIT is particularly useful in software development to:

1. Increase visibility making it easier to find problems in the development process
2. Reduce costs of delay (such as interruptions as discussed above)
3. Reduce costs of rework (such as redoing design and testing)

SPACE vs TIME

JIT is an example of not isolating related activities in time. I guess in that case DIRE actually stands for Don't Isolate Related Events.

There are also many examples of not isolating related things physically (ie in space) - from the order of lines in source code to the seating arrangements of developers.

The cost of rework has by far been the biggest problems in traditional software development. For example, a large up-front design can mean a lot of rework as the design needs to change. But typically, to avoid the cost, we somehow find a way to live with a bad design, with many undesirable consequences which I explained a few months ago (see Change).

JIT
is an example
of DIRE

Gradually developers are realizing the advantages of JIT, and hence DIRE, since JIT is an example of DIRE.  For example, many Agile practices (such as JIT-design and TDD) avoids the costs of rework by applying the idea of JIT, and performing related activities more closely in time.

Examples

Coding

A good example of DIRE as applied to code is the declaration of variables. Separating the declaration of a variable from its use is a common source of bugs (as I mentioned in point 8 of this Code Project Article). This is why C++ improves on C by allowing variables to be declared anywhere not just at the start of a compound statement.

Many Coding Standards work against the advantages of DIRE, by prescribing:

• order of declarations in a header file
• order of functions in a source file
• order of members in a class

Design

Most design practices and patterns do not intentionally contravene DIRE. However, they almost invariably promote decoupling in some form or another. Decoupling by it's nature tends to separate things. Though essential in almost every design, decoupling can be overused or misapplied.

Often a better design can decouple in a different way and not isolate closely related things - a good example is provided by the Layer Anti-Pattern. Sometimes a particular design pattern is appropriate but a cumbersome interface can make communication difficult between two closely related objects. Finally, sometimes complicated design patterns are simply not necessary but are used due to a misunderstanding or zealous desire to try something new.

Management

Managers essentially do two things:

1. help employees to achieve the goals of the organization
2. show that the work is being done

The first of these is very much about using the DIRE principle; but the second can work against DIRE.

First, helping employees is mostly about removing roadblocks. Roadblocks take many forms but are essentially things that prevent people doing their job efficiently and effectively. Removing roadblocks simply means getting rid of something that isolates related entities - such as a developer and the information they require to do their job.

Another part of helping employees is to get them working towards a common goal. This is achieved by fostering communication between them - another good example of DIRE where the related entities are the team members.

The problems occur when a manager allocates tasks and tries to monitor the work being performed. To do this a project must be divided into smaller and smaller tasks. The problem is that dividing tasks, by definition, creates divisions. If the way that tasks are divided is badly done then related tasks are separated in time or done by different people when they should not be. The classic example of this is the Waterfall development methodology as discussed in the next section.

Finally, some managers directly contravene DIRE for their own self-interest. This is why developers are not allowed to communicate directly with users or why they are only given just enough information to do their job. By controlling access and knowledge managers retain power which reduces the likelihood that it will be discovered that they are unnecessary, or not as important, to getting the work done as they like to believe. Of course, this sort of behavior is bad (but still common in the software industry) since it has DIRE consequences.

Process

Development methodologies are one of my pet topics (and the reason for this blog). Here also there are good examples of the benefits of DIRE.

As I mentioned above the best (or worst, depending on how you look at it) example of this is the Waterfall development methodology, which tries to control and monitor the development process, but in the process isolates closely related tasks. The processes of designing, coding and testing of a particular features or piece of software are very closely related. The Waterfall methodology separates them (and not just in time) by splitting the project into phases. This contravention of DIRE is the reason for all the problems with the Waterfall methodology and the main reason for the invention of Agile (as explained below).

the Layer Anti-Pattern in design is analogous to the Waterfall model of development

It's only when you think of these things in terms of DIRE that an interesting analogy between software design and the development process becomes apparent. Splitting development tasks in the wrong way causes DIRE problems. In software design, as we saw above, splitting the design into modules in a bad way causes similar problems. A specific example is the analogy between the Layer Anti-Pattern in software design and the Waterfall model of development - splitting a design into a stack of layers is like splitting the work into a series of phases. In both cases the separation causes problems by isolating closely related entities.

How Do Related Entities become Isolated?

The whole point of DIRE is to bring together things that should never have been separated. But was there a reason for this separation in the first place? Sometimes things are separated by accident but usually it is done for a reason. Often there is a trade-off, where DIRE is sacrificed for some other perceived benefit such as reduced complexity (as in the Layer anti-pattern) or improved monitoring/control (as in the Waterfall methodology).

Divide and Conquer

The most common reason for isolating things is to reduce complexity, using the principle of divide and conquer. This is fundamental to software design and expressed in such terms as modularity, information hiding, decoupling, etc (see Software Complexity).

Although important, it can be used improperly. For example, how software is divided into modules is often done poorly. On the other hand a split *may* be appropriate but the method with which a module communicates (ie, its interface) may be too limiting. And sometimes different parts of code are separated prematurely or for no good reason at all.

Planning

Talk to almost any manager and they will say "You can't achieve anything without a plan". In fact many will believe this with every ounce of their being. I guess this is natural as planning is their raison d'etre. However, it is an exaggeration at best. In fact, there have been many worthwhile achievements in human history which had no plan.

Don't get me wrong - I think a little bit of planning can be a good thing. However, the tendency is to divide a project into many small tasks. Each such task has a well-defined deliverable so that the manager can closely monitor progress to determine if the project is behind schedule.

Unfortunately, detailed planning can interfere with actually achieving team goals. By dividing the project into tasks the implication is that each task can be completed independently, which can result in closely related activities being isolated. Even at a high level dividing a project into stages means that work stops after each major milestone, then gradually builds up before the next deadline. This stop-start work-flow isolates related tasks and leads to poor continuity.

Playing Safe

A common reason for separating related entities is avoiding risk. JIT is a perfect example -- when JIT is used in manufacturing the process becomes crucially dependent on input parts arriving on time. Hoarding inventory is safer but introduces the problems I discussed last week.

In software development a good example of the disadvantages of playing safe is demonstrated in the traditional way of designing software. Here the design is supposed to be fully specified and signed off by the customer before coding is begun. This allows for the developers to protect themselves if  when things go wrong and the customer becomes unhappy. Their usual refrain is "It's not our fault, you signed off on the spec."


Much better results are invariably obtained using the Agile approach of JIT-design (see below). Doing things the right way usually involves more risk which is the reason XP promotes the value of courage (see Extreme Programming). 

Retaining Control

Another, more insidious, reason that managers like to isolate, is to retain control. If developers start talking directly to users then the work may begin to get done without even involving the manager. And since knowledge is power then information is doled out on a "need to know" basis. If you have a boss like this then my advice is simply to look for a new job.

Over-design

Similar to the problem of over-planning is the problem of over-design, exemplified by the Gas Factory Anti-Pattern. Martin Fowler gives a very reasoned account of this in Is Design Dead?

Accidental

Sometimes things are separated for no good, or apparent, reason. This is a problem to especially watch out for as a design evolves.

Agile

DIRE  is at the core of the Agile methodology. Consider the four elements of the Agile Manifesto and how they exemplify DIRE...

1. Favor Individuals and Interactions over processes and tools.

I have always found this rule a bit vague, but I think it is about getting the team working together, rather than following some safe, bureaucratic procedure. Breaking down barriers and encouraging communication and brainstorming is a great example of DIRE.

2. Favor working software over comprehensive documentation.

This is a critical part of Agile and nowhere is it better demonstrated than in the use of Unit Tests. Unit Tests are working software. Moreover, they can work as a substitute for massive amounts of design documentation. In fact they have several advantages over design documentation (see the Documentation section of Unit Test Advantages).

How does this relate to DIRE? Well the problem with a design document is that it is isolated from the "real" design - ie the actual implementation. The only link between the two is through the mind of the developer(s). This has forever been a major source of problems since people misinterpret and misunderstand.

Unit Tests avoid this problem. Using Unit Tests as a substitute for design documentation allows you to take advantage of the DIRE principle. As documentation Unit Tests are directly connected to the code they are designed to document and test.

3. Favor customer collaboration over contract negotiation.

This is similar to rule 1. It is about getting developers to communicate and work with the users. As I said above, contract negotiation is symptomatic of playing safe.

4. Favor responding to change over following a plan.

Responding to change is at the heart of Agile development. Most Agile principles and techniques use DIRE to quickly and more easily respond to change:

• JIT-design brings the design process and corresponding coding together
• continuous integration is about bringing different modules together early to detect problems earlier
• TDD brings coding and corresponding testing together to find bugs faster
• getting developers and users/customer to work more closely together allows quick response to change
• giving users working software earlier finds problems faster

Disadvantages of DIRE

There are no inherent disadvantages of DIRE that occur to me. The main problem is that it may be a matter of opinion the best way to split up a task or problem, such as the design of some software.

For a large problem, we necessarily have to split it up to reduce complexity. Using divide and conquer we can usually isolate and solve sub-problems one at a time until the whole problem is solved. This is the most fundamental principle in software design, and in most technical problems.  See my first post.

As an example, consider the Waterfall development methodology, which splits the process of creating software into phases (design, coding, etc). There are obvious advantages into doing the design of all the parts of the software at the same time, then all the coding together, etc. Doing all the testing at the end should avoid a lot of regression testing. However, experience (and awareness of DIRE) tells us that isolating the design of particular part of the software from the coding, and isolating the coding from the testing causes many problems.

Waterfall Methodology

The Agile approach is to divide the problem up in a very different way. First, quickly get a minimal working version of the software, then add to it over time. I have previously discussed the advantages of this evolutionary approach, ad nauseum, so I won't repeat them here.

Agile Methodology

The disadvantage of the Agile approach is the separation of related activities that occurs.

It's easier to design all the features at the same time since when you go back to design the next part you may have forgotten a lot of the original design. Similarly, most programmers will tell you that it is faster to implement Features 1a and 1b (of the above diagram) together. Finally, there will be a great deal of testing rework, since testing Feature 1b will probably entail retesting feature 1a and the core system again. 

Unit Testing

Fortunately Agile techniques, particularly Unit Testing, come to the rescue in alleviating all these disadvantages:

• Unit Tests act as design documentation making it easy to do design and re-design at any time
• Unit Tests make it is easy to add new features without fear of breaking existing behaviour
• Much testing is automated with Unit Tests which means the re-testing is automated

Summary

DIRE is a simple principle that helps to create better software. It may seem obvious but keeping it in mind allows you to find better approaches to many problems. For example, DIRE is behind most of the principles of Agile.

Using a divide and conquer approach is natural and essential to most tasks and problems, but sometimes how something is divided may cause undesirable consequences. However, by careful application of the DIRE principle it is often possible to reduce the "isolating" effects of the division. Further, looking out for things that should not be separated may allow you to see a different way to do the division. A good example of this is the Layer Anti-Pattern.