The inevitable emergence of domain events

Even if you’ve read the original Domain Driven Design book by Eric Evans, you’ve probably still not heard about domain events (or DDD Domain Events), as he didn’t include them in the book. He talked about it a lot since then, for example in this talk in 2009 in the first 30 minutes.

Domain Events

In short, domain events are occurrences of “something that domain experts care about”. You should always be on the lookout for these events, because they are integral parts of the interface between the technical world and the domain world. In your source code, both worlds condense as the same things, so it isn’t easy (or downright impossible) to tell them apart. But if you are familiar with the concept of “pure fabrication”, you probably know that a single line of code can clearly belong to the technical fabric and still be relevant for the domain. Domain events are one possibility to separate the belongings again. But you have to listen to your domain experts, and they probably still don’t tell you the full story about what they care about.

Revealed by Refactoring

To underline my point, I want to tell you a story about a software project in a big organization. The software is already in production when my consulting job brings me into contact with the source code. We talked about a specific part of the code that screamed “pure fabrication” with just a few lines of domain code in between. Our goal was to refactor the code into two parts, one for the domain code and the other, bigger one for the technical part. In the technical part, some texts get logged into the logfile, like “item successfully written to the database” and “database connection closed”. It were clearly technical aspects of the code that got logged.

One of the texts had a spelling error in it and I reached out to correct it. A developer stopped me: “Don’t do that! They filter for that exact phrase.”. That surprised me. Nothing in the code indicated the relevance of that log statement, least of all the necessity of that typo. And I didn’t know who “they” were and that the logfiles got searched. So I asked a lot of questions and finally understood the situation:

Implicit Domain Events

The developers implemented the requirements of the domain experts as given in the specification documents. Nothing in there specified the exact text or even presence of logfile entries. But after the software was done and in production, the business side (including the domain experts) needed to know how many items were added to the system in a given period. And because they needed the information right away and couldn’t wait for the next development cycle, they contacted the operation department (that is separated from the development department) and got copies of the logfiles. They scanned the logfiles with some crude regular expression magic for the entries (like “item written to the database”) and got their result. The question was answered, the problem solved and the solution even worked a second time – and a third time, and so on. The one-time makeshift script was used permanently and repeatedly, in fact, it ran every hour and scanned for new items, because it became apparent that the business not only needed the statistics, but wanted to start a business process for each new item (like an editorial review of sorts) in a timely manner.

Pinned Code

Over the course of a few weeks, the purely technical logfile entry line in the source code got pinned and converted to a crucial domain requirement without any formal specification or even notification. Nothing in the source code hinted at the importance of this line or its typo. No test, no comment, no code structure. The line looked exactly the same as before, but suddenly played in another league. Every modification at this place or its surrounding code could hamper the business. Performing a well-intended refactoring could be seen as direct sabotage. The code was sacred, but in the unspoken kind. The code became a minefield.

Extracting the Domain Event

The whole hostage situation could be resolved by revealing the domain event and making it explicit. Let’s say that we model an “item added” domain event and post it in addition to the logfile entry. How we post it is up to the requirement or capabilities of the business department. An HTTP request to another service for every new item is a simple and viable solution. A line of text in a dedicated event log file would be another option. Even an e-mail sent to an human recipient could be discussed. Anything that separates the technical logfile from the business view on the system is better than forbidden code. After the separation, we can refactor the technical parts to our liking and still have tests in place that ensure that the domain event gets posted.

Domain Events are important

These domain events are important parts of your system, because they represent things (or actions) that the business cares about. Even if the business only remembers them after the fact, try to incorporate them in an explicit manner into your code. Not only will you be able to tell domain code and technical code apart easily, but you’ll also get this precious interface between business and tech. Make a list of all your domain events and you’ll know how your system is seen in the domain expert world. Everything else is more or less just details.

What story about implicit domain events comes to your mind? Tell us in a comment or write a blog entry about it. We want to hear from you!