How we rebuild a legacy monolithic platform at CERN and still sleep at night
summary
A 30-year Java monolith that runs HR, finance and operations migrates by forking the repo and incrementally extracting a fork-and-refactor core into versioned libraries consumed by product-specific backends, adding a React frontend and an event-driven workflow layer. Spring 5→6 upgrade balloons to more than two quarters, 80 developers touch the code, and coverage sits at ~40%. Governance uses semantic-versioned core libraries, downstream compatibility pipelines and QA; pilots run successfully but the full migration is ongoing.
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chapters (13)
- 0:00Introduction
- 1:05EDH system at CERN
- 3:57Legacy monolith problems
- 8:11Modernizing constraints
- 11:20Modernization strategies
- 13:47Forking the monolith
- 16:38New architecture overview
- 21:36Trade-offs of new architecture
- 24:04Core library governance
- 28:36Team accountability shift
- 30:43Implementation plan
- 33:36Lessons learned
- 36:47Conclusion and key takeaways
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