Axoniq Conference 2025 Day 2: Consultant's Guide to Convincing Customers About Event Sourcing

🎯 Speakers:

• Florian Pfleiderer - Senior Consultant, Digital Frontiers

• Frank Steimle - Senior Consultant, Digital Frontiers

💡 The Real Title:

"A Consultant's Guide to Convincing Your Customer They Actually Want Event Sourcing"🔑 Four Hidden Strengths:

1️⃣ Building Blocks as First-Class Citizens

• Event modeling blueprint → Code (1:1 translation)

• Skip UML layer entirely

• No abstraction overhead

• Direct from business requirements to implementation

2️⃣ Well-Defined Interfaces & Boundaries

• Avoid "octopus code" (changes everywhere)

• Add features without touching existing code

• Premium subscription example: Just add events

• Ports & adapters naturally enforced

3️⃣ Testability Without Mocking

• Given-When-Then = natural language in code

• No repository mocking hell

• No constructor mocking complexity

• Isolated tests that don't interfere

• New requirements = just add one event to test

4️⃣ Day One Data Features

• Build features on historical data TODAY

• 5-year badge? Available immediately via replay

• New read models from existing events

• Mobile optimization without new data capture

🚫 Five Misconceptions Debunked:#1: Immutability = Can't Change

• ❌ Wrong: Upcasting exists (like adding DB columns)

• ✅ Right: Transform old events to new format#2: Eventual Consistency Unsuitable for Users

• ❌ Wrong: Users need immediate feedback

• ✅ Right: UX design solves this (Adam: "Non-issue")#3: CQRS Only for Analytics

• ❌ Wrong: Just for reporting/scalability

• ✅ Right: Flexibility for any use case#4: GDPR Incompatible

• ❌ Wrong: Can't delete immutable events

• ✅ Right: Encrypt & delete keys, or external PII store#5: Kafka = Event Sourcing

• ❌ WRONG: Kafka is NOT an event store

• ✅ Right: Event-driven (between services) ≠ Event sourcing (inside services)

📊 Example: Online Auction HouseTraditional Architecture Pain:

• Seller entity across 3 layers

• Add second address line = touch everywhere

• Tests need mocking: ShipmentRepo, PaymentRepo, ReviewRepo

• New requirement = change 100+ test locations

Event Sourcing Solution:

• SellerRegistered event

• PremiumSubscriptionStarted event

• Add badge requirement? Replay UserRegistered events

• New mobile UI? New read model from same events