Every engineer finds themselves, at some point, in a system they want to rewrite. Often we need to take a large legacy monolith and move it to a distributed architecture, for purposes of scaling. This talk will discuss some of the challenges encountered when attempting rewrite a complex system, the key strategies for success, and the potential unexpected outcomes of such a project.

1. Rewrites in Real Life
Camille Fournier @skamille
Two Sigma Investments, LP

2. About me.

3. @skamille
So I wrote a book…

4. We were failing to support our growth,
our customers, and our technology.
//failing

5. Everything’s not fine.
//failing

6. A rewrite will solve all my problems!
//failing
By Mstyslav Chernov/Unframe/
www.unframe.com (Own work)

7. “There’s no such thing as a
successful rewrite.
The sustainable rewrite
looks like firefighting.”
Cliff Moon
//failing

8. @skamille
Overview
page
08
outcomes
What happens next? How to succeed
principles
Why you might fail
dangers

9. THE PATH TO REWRITING IS
FRAUGHT WITH DANGER.

10. You’re failing now.

11. You can’t scale.

12. You can’t meet customer demand.

13. You’re crushed
under the weight of
technical debt.

14. It is easy to fail on the
unknowns

15. How well do you know what the
system is doing now?
This doesn’t look THAT big…

16. What about the data?

17. How does the team need to change
to make this successful?

18. The sirens will tempt you off
course.

19. Trying to do too much.

20. Counting on a “big
bang” release.

21. Choosing the wrong software.

22. Choosing the wrong software.

23. WHAT ARE THE PRINCIPLES TO
MAKE THIS SUCCESSFUL?

24. 1. Change as little as possible.

25. Don’t rewrite at all,
modify in place!

26. 2. Rewrite but keep the
language the same.

27. 3. Decompose the
system, and
change one thing
at a time

28. You need to sell this.

29. Sell it to yourself (and
your boss) first.

30. Sell it to your partners and “the
business” with Big Scary Graphs.

31. Sell it to your team.

32. You need a detailed
definition of done.

33. A test suite that
acts as a safety
harness.

34. What is the quality we’re
measuring to improve?

35. What is our data
migration plan?

36. What is our data
migration plan?
Celebrate!

37. WHAT DOES THE OUTCOME LOOK
LIKE?

38. Your culture will change
in the process.

39. Long-time
employees
may feel
threatened.By Jay Reed from Winslow
Township, USA (ConducJng)

40. Workflows will have to change.

41. The structure of your
team changes.
CTO
Dev
Architect
//outcomes
Dev Dev Dev Dev Dev

42. The structure of your
team changes.
CTO
Dev
Director
Dev Dev Dev
Director VP Ops
Front
End
QA Back
End
Front
End
QA Back
End
DevOps Syseng
DevOps DevOps

43. A new architecture brings
new challenges.

44. Tempting to make v2 everything
you ever wanted!

45. Overengineering can happen to
process, too.

47. You’re never really done.

48. You create a system that
will outlast its predecessor.

49. You have the flexibility to build
the things you will need.

50. You have standards around
that flexibility to mitigate
complexity.

51. Build with the needs of a
larger or smaller team in mind.

52. Everything isn’t fine.
//conclusion

53. It’s time to rewrite.
//conclusion

54. @skamille
Three Factors of
Successful Rewrites
page
054
dangers
You’re failing, it’s hard to know what the scope really is, and you’re tempted to do too much.
Be aware of the risks
principles for success
Sell it, change as liMle as possible, know what done looks like.
Have a plan
outcomes
Culture and process changes will happen, you will overdo some things, plan to manage flexibility
Prepare for the unpredictable future

55. Prepare for a brave new world.
//conclusion

56. Create a sustainable future
//conclusion
By Rawen Pasha (Own work)

57. Questions?
@skamille

58. credits
• www.mccord-museum.qc.ca/fr/collection/artefacts/MP-0000.2265
• Zombie mob: https://www.flickr.com/photos/aheram/219515706/
• Fail whale: Rob Friedman / playerx / @px
• National Ocean Service Image Gallery
• https://www.flickr.com/photos/matt_gibson/3281131319
• https://www.flickr.com/photos/philliecasablanca/3344142642
• https://www.flickr.com/photos/johnragai/12601518074
• https://www.flickr.com/photos/katiew/311380970
• https://www.flickr.com/photos/lucianvenutian/1279760885
• https://www.flickr.com/photos/jesse_sneed/2383953694
• https://commons.wikimedia.org/wiki/
File:New_Olympian_heavyweight_train_celebration_1929.JPG