Domain-Driven Design

It's been a while since a colleague (thanks, Nolan) recommended this book, but I finally read it: Domain-Driven Design by Eric Evans. The author talks about a kind of code debt I'd intuited a long time ago, but could never describe in words--a semantical mismatch between the mental model used to explain the code and the language used by domain experts. This book is important because with an alignment between the code and the domain, we can automate at higher and higher levels of abstraction, and therefore reap the benefits of rapidly increasing productivity. This is the greatest value of custom software--the domain-specific objects that reflect a team's understanding.
In Philadelphia, our team focused our code by naming projects, classes, and methods in ways that business stakeholders would recognize. This made it so we shared a ubiquitous language, as Evans calls it, a clear set of names that make sense to both developers and stakeholders. Something we didn't do enough of, though, is refactoring as the terminology evolved. We did practice merciless refactoring, but it was driven primarily by technical needs. Evans also mentions that the natural language of domain experts will be imprecise and self-contradictory, so it's important to be able to create, together, a new language that is unambiguous. Humans are especially adept at creating language, and the process of using our new shared vocabulary will propel us to simplification--we naturally find easier ways to say things. By implementing these verbal shortcuts in the code, it is simplified as well. The resulting deep model will constantly evolve as our understanding of the domain improves, and our communication about the domain will become more and more precise. In fact, the communication can become so precise that Evans reports an end of the "mystifyingly unexpected requirement changes". The new language used to describe the domain model can be so powerful that it's a selling point for marketing purposes, it's a market differentiator!

The following are points I underlined in the book, but may not make much sense to anyone else:

When we're working on a domain model, name things to reflect intention, rather than implementation. Evans also likes "closure of operations" because they perform operations that bring along no other dependencies (closure means input/output are of the same type).
Evans presents many ways to simplify the domain model, for example:

• by constraining many-to-many associations with a clarifying attribute
• minimizing the number of associations
• making associations uni-directional

If the code is in need of significant refactoring, prioritize by clarifying the core domain first.
He also discusses entity objects, value objects, and services, the last should optimally be an operation that does not fit in either an entity or object, it takes some entity or object as a parameter, and it's stateless.
The Repository pattern is a way to keep caching/data store and retrieval from polluting the domain model. Essentially, the repository objects will act as if they are a permanent in-memory collection of persist-able objects, with even some helper methods for quick retrieval of often-run queries. Repositories are made to retrieve existing objects, while factories create new.