April 8, 2025

🧠 Understanding Axon Framework (CQRS & Event Sourcing) – In Simple Words

 For developers, architects, and curious minds – including your tech-loving uncle! 😄

🔍 What is Axon Framework?

Imagine you're managing a huge library. Every time someone borrows or returns a book, you log it in a diary. Later, if you want to know which books were borrowed most, or which user has never returned a book, you just flip through the diary.

That's event sourcing. Instead of storing the current state, you store every change (event). Axon Framework helps you do this with Java and Spring Boot in a clean and scalable way.

🛠️ Core Concepts (With Analogies)

1. CQRS – Command Query Responsibility Segregation

In normal apps, one class both updates and fetches data.

With CQRS, we split that into:

  • Command: "Please change something" (e.g., borrow a book)

  • Query: "Tell me something" (e.g., list all books borrowed by Jatin)

This separation helps us scale better and move faster.

2. Event Sourcing – Every Action Is Recorded

Instead of updating a database row, we append an event:

  • "Book borrowed by Jatin at 2 PM"

  • "Book returned by Jatin at 5 PM"

Want to know who had the book on Jan 1st? Just replay the events!

3. Aggregates

Think of these as mini-managers for each type of data.

  • A LibraryAggregate ensures no one borrows the same book twice.

4. Sagas

These are like long conversations.

  • "User borrowed book -> Notify system -> Send reminder -> Handle return"

Axon automates these flows with reliability.

📚 In-depth Topics to Know

🔄 1. Command Bus vs Event Bus

  • Command Bus is like a single delivery truck taking your message to the right person. Only ONE handler can process a command.

  • Event Bus is like a loudspeaker. When an event happens, everyone listening can respond.

Axon provides both out of the box and lets you plug in distributed versions.

📖 2. Snapshotting

Over time, an aggregate may have thousands of events. Replaying all of them might get slow.

With snapshotting, Axon stores a recent snapshot of the state, so it only replays newer events. Think of it like saving your progress in a video game.

🔍 3. Query Side with Projections

In CQRS, your read side often has its own database (like MongoDB or PostgreSQL).

  • Axon lets you build projections by reacting to events and updating read models.

  • You can have multiple projections for different use cases: dashboards, reports, etc.

🔁 4. Replay Events

Did your logic change? Want to rebuild your reports?

Axon allows event replay:

  • Clears the projection DB

  • Replays all events to rebuild data accurately

You don't need to mess with old code or data — just replay and regenerate.

🔐 5. Security in CQRS

With commands and queries separated, security must be enforced separately:

  • Use Spring Security to protect REST endpoints

  • Inside Axon, use interceptors to validate commands or restrict queries

This fine-grained control improves robustness.

🚀 Why Use Axon?

✅ Scales well – easy to split across microservices
✅ Maintains audit logs – every change is recorded
✅ Fits into Spring Boot easily
✅ Built-in tools for commands, events, queries, sagas
✅ Comes with Axon Server (a native event store & router)

🆚 Axon vs Others – Who Are Its Competitors?

1. Eventuate

  • 🔹 Java + Microservices

  • 🔹 Event sourcing + distributed sagas

  • 🔸 Less tooling and documentation compared to Axon

2. Lagom (by Lightbend)

  • 🔹 Scala-first, supports Java

  • 🔹 Reactive + event-sourced

  • 🔸 Complex for beginners

3. JHipster + Blueprints

  • 🔹 Quick scaffolding with optional CQRS support

  • 🔸 Not true event sourcing

4. Kafka / RabbitMQ (Custom builds)

  • 🔹 DIY event-driven systems

  • 🔸 Requires heavy lifting to get to Axon's level

🧾 Summary Table

Feature Axon Eventuate Lagom JHipster Kafka
CQRS Support ✅ Full ✅ Full ✅ Full ⚠️ Partial ❌
Event Sourcing ✅ Yes ✅ Yes ✅ Yes ⚠️ Basic ⚠️ Custom
Spring Boot Ready ✅ Yes ✅ Yes ⚠️ Limited ✅ Yes ✅
UI Tools ✅ Axon Server ⚠️ Basic ⚠️ Basic ✅ Dev UI ⚠️ Plugins
Learning Curve ⚠️ Moderate ⚠️ High ⚠️ High ✅ Easy ⚠️ Medium

🎯 Should You Use Axon?

Use Axon if:

  • You’re building a complex Java system (microservices or monolith)

  • You want event history, audit trails, and saga flows

  • You use Spring Boot and want out-of-the-box support

Avoid if:

  • You prefer very simple CRUD apps

  • You need ultra-low latency (CQRS adds slight delay)

👵 A Word for Non-Techies

Think of Axon as a really smart notebook where:

  • You record everything

  • You don’t lose any data

  • You can always replay events to see what happened

  • And it has a brain that makes sure everything happens correctly!

📦 Bonus: What’s Axon Server?

It’s a free server by the Axon team.

  • Stores events

  • Routes commands and queries

  • Has a nice dashboard to monitor everything

Optional enterprise version adds clustering, scaling, and backup.

📚 Final Thoughts

Axon Framework isn’t just a tool — it’s a well-thought-out platform for building reliable, event-driven Java applications.

If you’re an architect or backend developer and you haven’t tried Axon yet — now’s the time.

Happy coding! 💻

Was this blog helpful? Let me know — or share with someone who’s exploring CQRS/Event Sourcing! 🧡

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