Most of the web sites we've developed are database driven. We build low - cost database solutions using the MySQL web database. Enterprise-level database solutions can use commercial database systems such as MS SQL server or Oracle. We always construct our database applications in such a way that the database migration is a simple process. So even if a web site had originally been developed with MySQL database, we could easily upgrade it to a MS SQL Server or other commercial database system if required.

In the last few years, we've seen the rise of a new breed of software methodologies, the agile methodologies. These make some new and significant demands on database design. One of the most central of these demands is the idea of evolutionary design. On an agile project you assume that you cannot fix the requirements of the system up-front. As a result having a detailed design phase at the beginning of a project becomes impractical. The design of the system has to evolve through the various iterations of the software. Agile methods, in particular extreme programming (XP), have a number of practices that make this evolutionary design practical.

Many people have questioned whether evolutionary design can be applied to a system with a large database component. Indeed many people told us that it was impossible - a troubling thought as ThoughtWorks embarked on a large database-oriented project using many agile and XP techniques.

This article describes the practices that we've used to allow us to do this impossible thing. We won't say that we've completely solved the database evolution problem, but we do think we've demonstrated a set of techniques that many people will find useful.

Dealing with Change

One of the primary features of agile methods is their attitude towards change. Most of the thinking about software process is about understanding requirements early, signing off on these requirements, using the requirements as a basis for design, signing off on that, and then proceeding with construction. This is a plan-driven cycle, often referred to (usually with derision) as the waterfall approach

Such approaches look to minimize changes by doing extensive up-front work. Once the early work is done, changes cause significant problems. As a result such approaches run into trouble if requirements are changing, and requirements churn is a big problem for such processes.

Agile processes approach change differently. They seek to embrace change, allowing changes to occur even late in a development project. Changes are controlled, but the attitude of the process is to enable change as much as possible. Partly this is in response to the inherent instability of requirements in many projects, partly it is to better support dynamic business environments by helping them change with the competitive pressures.

In order to make this work, you need a different attitude to design. Instead of thinking of design as a phase, which is mostly completed before you begin construction, you look at design as an on-going process that is interleaved with construction, testing, and even delivery. This is the contrast between planned and evolutionary design. One of the vital contributions of agile methods is that they have come up with practices that allow evolutionary design to work in a controlled manner. So instead of the common chaos that often happens when design isn't planned up-front, these methods provide techniques to control evolutionary design and make them practical.

An important part of this approach is iterative development, where you run the entire software life-cycle many times during the life of a project. Agile processes run complete life cycles in each iteration, completing the iteration with working, tested, integrated code for a small subset of the requirements of the final product. These iterations are short, usually running between a week and a couple of months, with a preference towards shorter iterations.

While these techniques have grown in use and interest, one of the biggest questions is how to make evolutionary design work for databases. Most people consider that database design is something that absolutely needs up-front planning. Changing the database schema late in the development tends to cause wide-spread breakages in application software. Furthermore changing a schema after deployment result in painful data migration problems.

Over the course of the last three years we've been involved in a large project (called Atlas) that has used evolutionary database design and made it work. The project involved almost 100 people in multiple sites world-wide (US, Australia, and India). It is around half a million lines of code and has over 200 tables. The database evolved during a year and a half of initial development and continues to evolve even though it's in production for multiple customers. During this project we started with iterations of a month, but after a few months changed to two week iterations which worked better. The techniques we describe here are the ones that we (or more accurately Pramod) used to make this work.

Since that project got going we've spread these techniques over more of our projects, gaining more experience from more cases. We've also found inspiration, ideas, and experience from other agile projects.