Axoniq Conference 2025 Day 2 | 215M Events, 5 Timelines: Event Sourcing Government Real Estate |

Maarten-Jan shares a decade-long journey at the Dutch Kadaster (land registry), where event sourcing tracks who owns what real estate in the Netherlands. Discover how they manage 215 million events across five different timelines, handle corrections from notaries and bugs, and why rewriting history is sometimes necessary - and sometimes forbidden.

🎯 Speaker:

Maarten-Jan van Gool - Software Engineer, Craftsman Consultancy (working on KORE system for 6-7 years of 10-year project)🏛️ The Dutch Kadaster:

💡 Key Insights:

"We got the left side down" - Event timeline is perfect, append-only works beautifully for technical timeline
"Follow the deeds" - By law, deeds are always right even when wrong. Kadaster must comply.
"115-200 types of deeds" - Including improvement deeds and cadastral corrections
"Seven years of history" - Only migrated October 2018 state, not full 200-year history

🎓 Lessons Learned:

Append-only perfect for technical timeline
Multiple timelines create dimension explosion
Clients need events, not just state
Projection bugs invisible without new events
Build projections for specific needs, not universal databases
Event-driven future beats unified relational DB

🚫 What Doesn't Work:

❌ On-the-fly projections (too slow, incomplete)
❌ One database to rule them all (17 timeline fields!)
❌ Forcing data into models that don't fit
❌ Solving all problems simultaneously
❌ Hiding events from clients

✅ What Works:

✅ Append-only event store
✅ Technical timeline preservation
✅ Correction events (not deletion)
✅ Domain-driven design (deeds as commands)