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What is Legacy Code ? De fi nition

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A code you want to change, that no one understands but still provides value. De fi nition

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A code without predictability De fi nition

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A code without predictability De fi nition Developers have no idea how long a change can take 01. 02. There is no correlation between the business and the technical complexity 03. You don’t know when something will break

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A code with no test De fi nition

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A code with no test De fi nition « Most of the fear involved in making changes to large code bases is fear of introducing subtle bugs; fear of changing things inadvertently. Without test, you just don’t know whether things are getting better or worse. » — Michael Feathers

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An inverse problem De fi nition

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An inverse problem De fi nition Observations Causes Forward problem Inverse problem

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An inverse problem De fi nition

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Hello, I work in the fi nance industry at Sunday.app. My name is Arnaud LEMAIRE, And my twitter handle is @lilobase Who am I?

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This talk is focused on strategic considerations and not tactical ones Disclaimer

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A natural process Why does it appear?

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A natural process Why does it appear? Add lines of code More complexity Increase in software entropy Legacy Code

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You need to let your software die* A natural process that cannot be stopped

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You need to let your software die* * a part of your software 01. 02. Without bringing the remaining parts with it 03. Optimize your software for replacement A natural process that cannot be stopped

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That can be speeded up or slowed down Why does it appear?

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That can be speeded up or slowed down Why does it appear? Refactoring helps to keep complexity under control 01. 02. A good software design is critical in the long term 03. Technical debt ≠ legacy software, but it can accelerate the process

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What do we do now ?

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You need to have empathy, and take the time to understand the project's history What do we do now ?

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Strategies at the code level Legacy Code

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Would you rather go to the dentist twice a year or brush your teeth every day? When to refactor

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When to refactor Always leave the campground cleaner than you found it.

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Always leave the campground cleaner than you found it. When to refactor This is the « boy scout rule » 01. 03. When you found a bug, fi rst write a test that triggers it, then fi x it. 04. Beware of the broken windows syndrome Developing a new feature and refactoring doesn’t mix well! 02.

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Culture emerge from practices, not the other way around. When to refactor

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Stop the weekly technical debt repayment session When to refactor

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Stop the weekly technical debt repayment session When to refactor The « We’ll fi x it later » syndrome 01. 02. The same cause always produces the same effect Instead provide a learning time, so you stop writing legacy code 03.

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Avoid the big bang refactoring

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The mikado™ method How to refactor

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The mikado™ method How to refactor Undo when an experiment fail

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The golden master Testing How to refactor

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The golden master Testing « In nearly every legacy system, what the system does is more important that what it’s supposed to do. » — Michael Feathers How to refactor

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The golden master Testing « A Gold Master test is a regression test for complex, untested systems that asserts a consistent macro-level behavior. » — Jake Worth How to refactor

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The Hotspot™ method How to refactor

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The Hotspot™ method How to refactor

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The Hotspot™ method How to refactor

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The Hotspot™ method How to refactor

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Strategies at the system level Legacy system

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Refactoring ? How to tackle it?

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Refactoring ? How to tackle it?

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Rewrite ? How to tackle it?

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Rewrite ? How to tackle it?

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The Ship of Theseus strategy How to tackle it?

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The Ship of Theseus strategy How to tackle it?

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Ask your business to help you mapping your (business) domain 01. Invoice Generation Invoicing Car rental

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Ask them to tag where there is room for improvement. 02. Invoice Generation Invoicing Car rental

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Select a tagged support service with few dependencies 03. Invoice Generation Invoicing

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Rewrite it… 04. Invoice Generation Invoicing New invoice Generation

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And put an anti-corruption layer to prevent legacy leakage 05. Invoicing New invoice Generation Legacy API Legacy Database New Api ACL

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The Anti-Corruption Layer Anti-Corrupution Layer The Green fi eld code must not call directly the Legacy code 01. 02. The Legacy code must not call directly the Green fi eld code 03. Even through the persistence system: database or event/message bus

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Invoicing New invoice Generation Legacy DB Event bus Hidden dependencies

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Invoicing New invoice Generation Legacy DB Event bus

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Our system Vendor API ACL An anti-corruption layer can also protect us from the outside world

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Replace the old legacy system 06. Invoicing Invoice Generation New invoice Generation

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Repeat 07. Invoice Generation Invoicing Car rental

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Until you can reach the core 08. Invoice Generation Invoicing Car rental

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The Ship of Theseus strategy also known as the Strangler pattern How to tackle it?

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The Ship of Theseus strategy also known as the Strangler pattern How to tackle it? You’ll need to coordinate your actions at the system level 01. 02. Beware of functional debt 03. Optimize your new system for replacement!

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What you shouldn’t do Some mistakes to avoid

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Starting from your existing system Mistakes to avoid

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Starting from your existing system Mistakes to avoid

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Starting by your core business Mistakes to avoid

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Starting by your core business Mistakes to avoid Invoice Generation Invoicing Car rental !

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Starting by your core business Mistakes to avoid Too many legacy dependencies 01. 02. The developers will need some training…

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Don’t do horizontal slicing ! Mistakes to avoid

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Don’t do horizontal slicing ! Mistakes to avoid UI Persistence Service !

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Don’t do horizontal slicing ! Mistakes to avoid UI Persistence Service

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• The Mikado method • Software Design X-Rays • Working E ff ectively with Legacy Code • Refactoring Going further Thanks to Michael C. Feather, Adam Thornhill, Cyrille Martraire, Thomas Pierrain, Martin Fowler, Jean-Baptiste Dusseaut, Ola Ellnestam, Daniel Brolund, Adrian Bolboaca

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Thank you! I work in the fi nance industry at Sunday.app. My name is Arnaud LEMAIRE, And my twitter handle is @lilobase

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