Eric Evans: Keynote (“Language in Context”)

Starting out with the basics (words have meaning within a context; when we make the boundary of this context explicit we end up with a bounded context), Eric discussed two main topics: the Big Ball of Mud, and bounded contexts in the context (no pun intended) of microservices.

Legacy applications are always framed in a negative way, as if it’s something to get away from. Personally, I’ve come to enjoy them a lot. However, there will always be the urge to work around legacy code. The Bubble Context (PDF) can be a great way of creating a new model that works well next to the already existing models. To keep a safe buffer between the new and the old model, you could build an Anti-Corruption Layer (ACL). A fun thing Eric mentioned is that the buffer works in two directions. The ACL also allows the old model to keep functioning without being disturbed by all the new things that are going on in the Bubble Context.

All of a sudden it became book-writing season. It began in August when I started revising my second book, “Principles of Package Design”. Apress had contacted me about adopting it, and they didn’t want to change a lot about it. However, the book was from 2015 and although I had aimed for it to be “timeless”, some parts needed an update. Furthermore, I had happily pressed the “Release” button back then, but it’s the same as with software development: the code you wrote last year, you wouldn’t approve of today.

In part 1 of this short series (it’s going to end with this article) we covered how you can test-drive the queries in a repository class. Returning query results is only part of the job of a repository though. The other part is to store objects (entities), retrieve them using something like a save() and a getById() method, and possibly delete them. Some people will implement these two jobs in one repository class, some like to use two or even many repositories for this. When you have a separate write and read model (CQRS), the read model repositories will have the querying functionality (e.g. find me all the active products), the write model repositories will have the store/retrieve/delete functionality.

There’s something I’ve only been doing since a year or so, and I’ve come to like it a lot. My previous testing experiences were mostly at the level of unit tests or functional/system tests. What was left out of the equation was integration tests. The perfect example of which is a test that proves that your custom repository class works well with the type of database that you use for the project, and possibly the ORM or database abstraction library you use to talk with that database. A test for a repository can’t be a unit test; that wouldn’t make sense. You’d leave a lot of assumptions untested. So, no mocking is allowed.

When you’re looking at a function (an actual function or a method), you can usually identify several blocks of code in there. There are pre-conditions, there’s the function body, and there may be post-conditions. The pre-conditions are there to verify that the function can safely proceed to do its real job. Post-conditions may be there to verify that you’re going to give something back to the caller that will make sense to them.

I recently wrote about when to add an interface to a class. After explaining good reasons for adding an interface, I claim that if none of those reasons apply in your situation, you should just use a class and declare it “final”.

PHP 5 introduces the final keyword, which prevents child classes from overriding a method by prefixing the definition with final. If the class itself is being defined final then it cannot be extended.

Last week I wrote about when to add an interface to a class. The article finishes with the claim that classes from the application’s domain don’t usually need an interface. The reason is that domain code isn’t going to be swapped out with something else. This code is the result of careful modelling work that’s done based on the business domain that you work with. And even if you’d work on, say, two financial software projects in a row, you’ll find that the models you produce for each of them will be different in many subtle (if not radical) ways. Paradoxically you’ll find that in practice a domain model can sometimes be reused after all. There are some great examples out there. In this article I explain different scenarios of where and how reuse could work.

I’m currently revising my book “Principles of Package Design”. It covers lots of design principles, like the SOLID principles and the lesser known Package (or Component) Design Principles. When discussing these principles in the book, I regularly encourage the reader to add more interfaces to their classes, to make the overall design of the package or application more flexible. However, not every class needs an interface, and not every interface makes sense. I thought it would be useful to enumerate some good reasons for adding an interface to a class. At the end of this post I’ll make sure to mention a few good reasons for not adding an interface too.

During the holiday I read a book mentioned to me by Pim Elshoff: “Skin in the game”, by Nassim Nicholas Taleb. Discussing this concept of “skin in the game” with Pim had made me curious about the book. It’s not such a fat book as one of Taleb’s other books, which is possibly more famous, called “Antifragile”. “Skin in the game” is a much lighter book, and also quite polemic, making it an often uncomfortable, but fun reading experience. I can easily see how people could get mad about this book or aggressive towards its author (not that I’m encouraging or approving of that aggression!). While reading it, it reminded me of Nietzsche, and his despise of the “common man”, who Taleb calls “the intellectual” - someone who has no “skin in the game”, let alone “soul in the game”. Taleb’s ideas are interesting, just like Nietzsche’s are, but they could easily be abused, and probably so by misinterpreting them.

Recently I read a comment on Twitter by Nikola Poša:

I guess the discussion on my thread is going in the wrong direction because I left out a crucial hashtag: #NoCommentsInCode - avoid comments in code, write descriptive classes, methods, variables.https://t.co/MuHoOFXCvV

— Nikola Poša (@nikolaposa) July 13, 2018

He was providing us with a useful suggestion, one that I myself have been following ever since reading “Clean Code” by Robert Martin. The paraphrased suggestion in that book, as well as in the tweet, is to consider a comment to be a naming issue in disguise, and to solve that issue, instead of keeping the comment. By the way, the book has some very nice examples of how comments should and should not be used.