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Ten Lessons from Rewriting Software

  1. It Will Take A Lot Longer Than Estimated

    • Its navie to actually think this but if a system has been in production for say five years, expecting to reproduce it in five weeks is not possible. You may be able to get 80% of the core functionality done, but the remaining 20% that was added to, iterated and stabilized over the remaining five years is what will destroy any form of schedule.
    • If your estimate exceeds three months, you need to reasses what you are doing by breaking down the work, or changing plan. The bigger the estimate, the bigger the risk.
  2. Deploy Incrementally Via CI

    • If you aren't deploying to a live environment as soon as possible, any future releases are destined to be failures, troublesome or just plain difficult.
    • Soft releases and feature toggles should be used to aid constant releases.
  3. Morale Will Drop The Longer It Goes On

    • Probably the biggest and most surprising realization is the drop in personal and team morale.
    • If you miss a "deadline" or keep failing to ship, then morale will tank.
    • While software is never complete, a rewrite has a definitive target. If this target continues to move, team morale will move too.
  4. Users Will Probably Hate It Anyway

    • Predominantly the UI, but your users will complain about change.
    • Big sweeping changes often receive the most hate. A website I frequent had a major change both in visuals and the underlying technology used. While there was warning, you were left to your own to figure out where features were. This caused a great deal of frustration and negative feedback.
    • Small, incremental changes allow your users to keep pace.
    • Alternatively some tutorial or hint system can help reduce user pain.
  5. Do What The Legacy System Does

    • As many of the original developers will likely have moved on, no one is really sure what the legacy system does.
    • Even with the source code available, it is likely going to be hard to figure out the intent, afterall that's one of the reasons for the rewrite.
    • If you are not careful you will end up simply reimplementing the same legacy in a new language or framework. Always weigh up preserving existing behaviour versus introducing technical debt.
  6. Be Cheap And Quick - Use Stubs

    • When implementing the new system, don't build a thing. At least at first.
    • Use stubs to build the simplest, dumbest thing you can to get feedback.
    • Without fully integrating the system in an end to end manner you'll end up throwing away a great deal of code.
  7. Feedback, Feedback, Feedback

    • Early and fast feedback is essential.
    • With a working end to end system gather as much as you can from any stakeholders.
    • Chances are as you begin you'll naturally incur some additions, removals or modifications.
    • Waiting months or longer for feedback is a guaranteed path to failure.
  8. Thin Vertical Slices Over Fat Technology Splits

    • Avoid the temptation to have a UI team, a backend team and a data team and so on.
    • Splitting at technology boundaries leads to systems that do not integrate well, or worse fail to handle the required use cases.
    • Your first iteration should consist of all parts of the technology stack, in the thinnest manner possible. Combine this with early feedback and the fast development speed of stubs.
  9. Strangle Existing Legacy Code

    • When rewriting in increments or by logical sections the technique of strangulation is useful.
    • Instead of releasing the new code as a standalone piece, integrate the new code into the existing legacy code base.
    • This may be tricky at first however over time the legacy system will form nothing but an empty shell that integrates with the new system.
    • The beauty of this approach is early feedback, and a guarantee that the new system behaves as intended.
    • The final step would be to replace the legacy shell with the new modern interface or frontend.
  10. Refactor Where Possible

    • Deciding to refactor or rewrite is never easy. Refactoring should be the default approach in many cases.
    • Old languages or unsupported frameworks are good reasons to adopt a rewrite, but this varies case by case.
    • If business agility is suffering such rewrites can be beneficial when using some of the techniques above.

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