Introducing deliberate architectural practices into existing monolithic codebases can be daunting. Often the code itself can resist such attempts in subtle ways that negatively impact a team with little architectural experience. Drawing on a real-world project as an example, James Thompson explains how to implement bounded contexts into such applications in conjunction with normal feature development and maintenance. Emphasis will be given to the aspects of the application that made implementing better architectural practices more difficult, including matters of process and implementation. Along the way, James shares a feature that provides the practical vehicle for introducing architectural improvements to one area of the application and discusses how this feature was used to bring more deliberate architectural thinking and boundaries to the example project.

1. Bounded Contexts
For Legacy Code

2. Bounded Context

3. “a semantic contextual boundary”
— Domain-Driven Design Distilled byVaughnVernon
A meaningful collection of components that meets a
defined, limited, and useful purpose with defined
interfaces and means of interaction with external
actors.

4. Legacy Code

5. “source code inherited from someone
else”
— Legacy Code,Wikipedia
Code that because of its lack of tests, poor design, or
other factors is difficult to work with.

6. Goals
Maintain Functional State
AddValue & Reduce Debt
Permit Changeability

7. Resources

8. Resources

9. The Project

10. The Problems

11. The Approach

12. Account
User
Project
Story

13. Account
User
Project
Story
Account
User
Project
Existing System 3rd Party

14. Account
User
Project
Story
Account
User
Project
Existing System 3rd Party
AntiCorruptionLayer

15. AntiCorruption Layer
Web-hooks
Commands &
Queries

16. Account
User
Project
Story
Account
User
Project
Existing System 3rd Party
AntiCorruptionLayer
Mapping

17. Account
User
Project
Story
Existing System 3rd Party
AntiCorruptionLayer
Mapping
Account
User
Project
Proofs
Revisions
Proofs

18. Account
User
Project
Story
Existing System 3rd Party
AntiCorruptionLayer
Mapping
Account
User
Project
Proofs
Revisions
Proofs
Create Proof
Add Revision
Delete Proof
…

19. Recap

20. Difficulties

21. Achievements

22. Questions

23. James Thompson
Staff Software Engineer @ Nav
@plainprogrammer
theplainprogrammer.com