The Pragmatic Programmer – The Evils of Duplication

Released Monday, 13th May 2019
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imageimage We take a deep dive into the various forms of duplication and jump aboard the complain train as Allen complains about Confluent’s documentation, Michael complains about Docker’s documentation, and Joe complains about curl.
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The Evils of Duplication

  • Change is constant.
  • We spend a large part of our time in maintenance mode.
    • Maintenance is a part of the entire process – it happens daily, even while we’re writing the initial code and get new requirements.
  • It’s easy to duplicate knowledge: in the specifications, the code, etc.
  • DRY Principle (Don’t Repeat Yourself).
    • Every piece of knowledge should have a single, unambiguous representation within a system.
  • Repeating knowledge in your code means … it’s not a question of if you’ll forget about the multiple places to maintain, it’s when.
  • The authors make the point that DRY is one of the most important tools/principals in the book.

How does duplication happen?

  • Imposed – Programmers feel like they don’t have a choice.
  • Inadvertent – Programmers didn’t realize they were doing it.
  • Impatient – Laziness.
  • Interdeveloper – Multiple people doing it and don’t realize it.

Imposed Duplication

  • Using a code generator to build code from some common set of metadata can help protect you from this form of duplication.
  • Some examples:
    • Documentation in code.
      • Bad code requires a lot of documentation.
        • A lot of documentation often becomes stale as the code changes, leading to lies!
      • Possibly generate code from your documentation.
    • Problems with the language itself
      • C, C++, Pascal require separate header files with declarations and then the implementation files with the code for those declarations. No way around this really.
        • Just make sure you’re not putting the same comments in each file. Doing so would be a waste and likely a place where something will be forgotten over time.

Inadvertent Duplication

  • Some examples:
    • Accidental denormalization of data – same attribute in multiple layers that are related.
    • This can also happen with mutually dependent items.
      • The authors point out that sometimes this is necessary due to expensive calculations that you want to cache for performance reasons.
        • If you need to do this, encapsulate that information. Don’t expose the property directly, but instead use accessor methods. That way your storage and retrieval of the data is transparent to the consumer of the data and you can implement it however you like.

Impatient Duplication

  • “… short cuts make for long delays.”
  • Some examples:
    • Using the same literal in multiple spots.
    • Copying and pasting source code from a standard implementation and tweaking just a bit.
  • It takes discipline to avoid this type of duplications but doing so will save a lot of time in the long run.

Interdeveloper Duplication

  • It’s very common for this to happen when multiple developers are working on multiple applications.
  • How to solve?
    • Clear overall vision.
    • Strong project leader.
    • Division of responsibilities.
  • That still doesn’t solve all of it. When there’s functionality that doesn’t “live” in one of those core responsibilities, it often has a good chance of being duplicated.
    • Methods that help:
      • Open and frequent communication between developers.
      • Appoint a project librarian (or group?).
      • Have a central place in the source tree where utility scripts live.
        • Take care that this doesn’t become a dumping ground.
      • Continually explore other’s source code and be a part of PR’s and code reviews.


  • Make it easy to reuse.
    • If you don’t make it easy, people won’t reuse it.

Resources We Like

Tip of the Week

  • Use multiple mediums to learn, such as, podcasts, YouTube, online courses (Udacity, Udemy, Pluralsight, etc.), books, writing (blogs, wikis, etc.), and side-projects.
  • Use to host your next JavaScript project for free! (
  • Google Docs have version history! You can find the history options from within the document’s File menu (File -> Version history) or by right clicking on the file and selecting View details.
  • Use flowchart.js to create a SVG flow chart diagram from a textual representation. (flowchart.js)
  • Use SHIFT+Right Click on a file in File Explorer to select “Copy as path” as well as other options. Who knew Microsoft took a page out of Apple’s playbook and started hiding options in the UI.

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