Jef Claes

On software and life

23 Feb 2014

Strategic DDD in a nutshell

There are two big parts to Domain Driven Design; strategy and tactics. Strategy helps setting out a high-level grand design, while tactics enable us to execute on the strategy.

Practicing strategic design, we generally first try to list all of the different parts that make our business a whole; these are sub-domains. When you look at a supermarket chain, you would find sub-domains like real estate management, advertising, suppliers, stock, sales, human resources, finance, security and so on. Sub-domains will often relate to existing structures like departments and functions.

Once you’ve defined your sub-domains, it’s useful to determine how important of a role they play. First of all, you should figure out which sub-domain is most important to your business; the core domain. This is the sub-domain that differentiates you from other businesses, or more bluntly put; this is where the money is at. For our super market chain, this might not be that obvious for an outsider. A first uneducated guess would be sales, but if you gave it some more thought, you would realize that sales are very similar for most supermarkets. Digging deeper, we would find that supermarkets really compete with each other by squeezing the last bit of value out of suppliers and by collecting data to use for targeted advertising. Supplier management and advertising can’t stand on their own though; they need other sub-domains like stock and sales. These are supporting sub-domains; they are not core, but our business couldn’t do without them either - they still add a bunch of value. Other sub-domains like property management, human resources or security are generic sub-domains; these problems have been widely addressed and solving them yourself won’t make you any money.

Having a map of which areas are most important to your business makes it easy to distribute brain power accordingly. Make sure your core domain gets the most capable team assigned, before any other supporting sub-domain. Try to buy solutions of the shelve for generic sub-domains.

The concept of sub-domains lives in the problem space. The solution space on the other hand is where bounded contexts are at. Domain Driven Design tries to define natural boundaries between parts of your solution by putting the language first. These boundaries allow us to keep a language and model consistent inside of them, protecting conceptual integrity.
If you would ask a marketer what a product is, he would talk about images, campaigns, weekly promotions and so on. If you’d ask sales on the other hand, they would only mention price, quantity and loyalty points. The same concept can turn into something completely different depending on how you look at it. Bounded contexts enable us to build a ubiquitous understanding of concepts in a clearly defined context.

Mapping a bounded context to exactly one sub-domain would be DDD nirvana; addressing one problem with one focused solution. In the real world, things are more messy though. There will always be systems out of our control; for example legacy and third party software. As our understanding of the business grows, keeping our software aligned can be hard too. If we would lay out a map of sub-domains and bounded contexts we would see lots of overlap.

Bounded contexts will often be worthless on their own though; most useful systems exist of interconnected parts. If you have worked in the enterprise, you know how complex communication between teams and departments can be. This isn’t very different while integrating bounded contexts; you need to consider politics. This is where concepts like up-stream, down-stream, bandwidth, partnership, shared kernel, customer-supplier, conformist, anti-corruption layer etc come into play. The activity of thinking about and capturing how all these systems play together is called context mapping.
In our example, we notice that supplier- and stock management would fail or succeed together; they have a partnership where the bandwidth is very high - the teams sit across the hall from each other. Human resources and security on the other hand have a very different relationship. A product was bought for human resources, while a solution for security was outsourced. Security relies quite heavily on what the human resources’ open host service is exposing. If a product version bump changes those exposed contracts, security needs to comply as soon as possible; security is down-stream from human resources - shit floats down-stream.

For me, strategic DDD in one sentence, is the constant exercise of trying to see and understand your business at large, and aligning your software as efficiently as possible.