Le projet Legacy, quelles stratégies pour s'en sortir ?

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What is Legacy Code ? Deﬁnition

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A code you want to change that no one understands but still provides value. Deﬁnition

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A code without predictability Deﬁnition

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A code without predictability Deﬁnition Developers have no idea how long a change can take 01.

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A code without predictability Deﬁnition Developers have no idea how long a change can take 01. 02. There is no correlation between the business and the technical complexity

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A code without predictability Deﬁnition 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 Deﬁnition

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A code with no test Deﬁnition « 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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Hello, I work in the ﬁnance industry at Lgo.group. 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

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A natural process Why does it appear? Add lines of code More complexity

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

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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* * part of your software 01. A natural process that cannot be stopped

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

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You need to let your software die* * 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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But that can be slowed down A natural process that cannot be stopped

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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.

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Always leave the campground cleaner than you found it. When to refactor This is the « boy scout rule » 01. Developing a new feature and refactoring doesn’t mix well! 02.

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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, ﬁrst write a test that triggers it, then ﬁx it. Developing a new feature and refactoring doesn’t mix well! 02.

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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 ﬁx it later » syndrome 01.

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Stop the weekly technical debt repayment session When to refactor The « We’ll ﬁx it later » syndrome 01. 02. The same cause always produces the same effect

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Stop the weekly technical debt repayment session When to refactor The « We’ll ﬁx 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 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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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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Ask your business to help you map your (business) domain 01. Invoice Generation Invoicing Car rental

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Starting from your existing system Don’t start from your existing system

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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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Starting with your core business Don’t start by your core business Invoice Generation Invoicing Car rental

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Starting with your core business Don’t start by your core business Invoice Generation Invoicing Car rental !

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Starting with your core business Don’t start by your core business Invoice Generation Invoicing Car rental !

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Don’t do horizontal slicing ! Don’t do horizontal slicing UI Persistence Service !

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Don’t do horizontal slicing ! UI Persistence Service Don’t do horizontal slicing

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

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Replace the old legacy system 06. Invoicing 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.

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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

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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. Optimise your new system for replacement!

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• The Mikado method (Ellnestam & Brolund) • Software Design X-Rays (Thornhill) • Working Eﬀectively with Legacy Code (Feather) • Refactoring (Fowler) 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 ﬁnance industry at Lgo.group. My name is Arnaud LEMAIRE, And my twitter handle is @lilobase