Monday, July 21, 2008

Agile Bridge Analogy

It is quite common to make analogies between the IT industry and Civil engineering. Developers often compare software development and design with construction projects; for example, the importance of having a blueprint and following known successful practices. I have some hesitations about using analogies between these industries. I find the resources used by both industries fairly different and for that reason the analogies can create false expectations and reasoning. Nevertheless, I have been successfully using the analogy of building a bridge for explaining Agile development.

When I was a teenager, I spent my summer vacations in a small beach town near Rio de Janeiro. In that city (Rio das Ostras) there was a little river which separated two parts of the town. Back then there was no bridge to cross the river in a specific section of the town.

Over a few years, I was able to experience the bridge being built. When teaching Agile software development practices, I have been using the construction process for that bridge as an example of how Agile delivers business value incrementally and iteratively.

To better explain the analogy, I will take a look into the traditional approach for building bridges and then I will explain the Agile counterpart.

The Traditional development

Traditionally, a bridge construction will be planned in detail. The blueprint will be produced, the budget allocated and the schedule created (perhaps several months of construction).

'Big-bang release' is the term used to describe a common software development release. At the very end the software is released — one big (bang) release. But what happens if it is not successful?

Below are some photos from unfinished bridges.

Half-built bridges or long running construction does not deliver any business value: nobody is able to cross the river!

The Agile Development

Let’s now look into the Agile approach for building a bridge.

The figure below shows a first version view of the bridge (maybe after a first iteration). Even being very simple (and fast to plan and build) the bridge delivers business value: one person at a time can cross the river.

At the next iteration, another piece of wood is added to the bridge. Now, the bridge handles more load and two people can cross the bridge at a time, as well as bicycles and motorbikes.

Perhaps, for the next iteration(s), the bridge is reinforced and now people, bikes, motorbikes and light vehicles are able to cross the river.

Following an iterative and incremental development style, extra reinforcements are added to the bridge during the next iteration(s) and now people, bicycles, motorbikes, light and heavier vehicles can cross the river.

Even though it took a while to reach the current stage of the bridge, business value was delivered all along.

In this blog entry, I used an analogy between building bridges and developing software. I wrote about an Agile approach for building bridges. In fact, in this approach I have been a satisfied end user (someone wanting to cross a river). Furthermore, I used this analogy to illustrate how Agile focuses on delivering business value incrementally (and in shorter release cycles) through an iterative development process.

Even though in this example I compared software development to building a small bridge, Agile has been proven to work for large projects as well. I have witnessed large successful Agile projects for which business value was iteratively and incrementally delivered.


Unknown said...

Nice reading Paulo.

Mario León said...

Excellent analogy.

Max said...

Except each bridge was a complete recreation of the first!

DCB said...

And none of these bridges would actually support high volume traffic in a city such as New York.
Nor does it appear that this would work well over any large sized body of water.

In general this would work great for a small river with relatively little traffic.

Just like agile, getting it to work well with it works great for small groups, getting it to work in a large enterprise isn't quite the same thing :)

Ricky said...

nah man, i dont agree with this analogy at all. if i was an end user i'd be pretty annoyed.

like first i'd have to put up with a bare bone minimum implementation that's hard to use (balance on a log).

then i'd have to put up with sharing that log, until the next log is implemented. Which again is another bare bone implementation. (now there's more traffic too)

and repeat.

until we have cars travelling on an infrastructure never intended to be used this way.

that just sounds like windows.

Anonymous said...

Good analogy. The real reason for chossing Agile is to give constant business value.

It is true that it is a small town and the bridge doesn't serve good for cities like Newyork. Applying principles of Civil engineering in which all the projects are failure leads to loss of life to enterprise (not mission critical) software development where failure is not so serious may not be entirely apt.. But still the example is a good one.

Jon Strayer said...

Software isn't hardware. You don't have to recreate each iteration from scratch.

Inside every large project is a small project (or a set of small projects) struggling to get out.

No matter if you eventually want to drive across the bridge, being able to walk across today is valuable.

John said...

I love your analogy here... Perhaps you will enjoy my blog entry comparing the construction of a house with the implementation of a managed business process: The best way to build a house is the worst way to implement a process

Anonymous said...

We can't forget we've been building bridges for thousands of years. Software development is a less than 50 year old science. We still don't know exactly how to build software. Here's a reference:

Anonymous said...

At first glance I found the analogy was nice. It delivers the main message that is to start simple and add complexity afterward when you have a better knowledge of user needs, business opportunities and technical feasibility. But it turned out that there are more counter-analogies between the two industries. What's about building a big city bridge by starting with a piece of wood or changing its pillars after it starts being used.
It's difficult to compare an industry that produces real objects with an other one that produces only abstract artifacts. The latter has strong constraints and can only deliver one version to its customers. The former has more freedom and can deliver a weak "1.0" in most of the cases (not all of them).

Holger said...

I agree, it's a nice little example, at first glance but does not really work for following reason:
Besides typical constraints like difficult local facilities and logistics, minimized constraints to ongoing business, traffic, etc., civil engineering project often have to deal with high costs and limited time to delivery (of the complete scope!). Therefore, depending on the type of construction, I cannot believe that a customer will agree to a solution, that require more additional costs and time than necessary by providing intermediate solutions, that will be partly or completely removed for the next stage.
Thus the example shows mainly, that Agile is not the all purpose approach and applicable to all kinds or projects.

Jon Strayer said...

Guys, in the long run no analogy works.

He had an experience where he observed that building a bridge in an incremental fashion deliver value for the users fast than trying to build the final form would have.

That's what incremental development of software does.

That's all. Don't try to read several books worth of details into a simple analogy.

Jonathan Harley said...

I don't think it's a fair criticism to say you couldn't build a city bridge this way. You couldn't build it in an agile way, but the design of such a bridge takes many months and you could do that in an agile, iterative way, releasing your first drawings to the rest of the company straight away, and each later set of drawings as you refine the design. It doesn't deliver business value but it would certainly lower the risk that the finished bridge would fail to deliver business value.

Lots more thoughts about this on our blog at

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