This chapter is from the book
Conclusion
遵循这些简单的规则并不难, and it will save you a lot of headaches going forward. By separating the software into layers and conforming to the Dependency Rule, you will create a system that is intrinsically testable, with all the benefits that implies. When any of the external parts of the system become obsolete, such as the database, or the web framework, you can replace those obsolete elements with a minimum of fuss.
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