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Why Events?

Events are central to modeling because they reflect real things that happen in a business domain. They are factual, meaningful and naturally align with how business people think.

1. Events Represent Business Truth

Events are things that have happened in the domain (e.g. OrderPlaced, PaymentReceived, InvoiceGenerated). They are past-tense, factual and undeniable truths from the business perspective.

  • ✅ "OrderPlaced" happened
  • ✅ "EmailSent" happened
  • ❌ "ShouldSendEmail" is not a fact, it’s an intent

Events are also immutable: once they occur, they cannot be changed. This immutability makes them a reliable record of what actually happened, supporting auditing, debugging and replayable workflows.

Focusing on what has happened aligns everyone with concrete, observable business behavior.

2. Events Are Language Everyone Understands

Everyone in an organization understands business events. They talk about them naturally in meetings:

“A customer placed an order”
“The payment failed”
“A reminder was sent”

This makes events a shared language between tech and non-tech people, critical for closing the communication gap.

3. Events Drive System Behavior

In event-driven systems, events trigger workflows, reactions and decisions. They’re not just outcomes, they're inputs for other parts of the system.

For example:

  • Event: OrderPlaced
  • Reaction: Send confirmation email, update inventory, start shipping process

4. Events Help Map the Real Workflow

By laying out events on a timeline, you can see the actual flow of a business process: what happens first, what follows and what causes what.

This helps:

  • Discover hidden logic
  • Spot race conditions
  • Understand dependencies
  • Reveal time-based behaviors
  • Find knowledge gaps

5. Events Are Stable Over Time

Requirements and processes change but past events remain valid. Designing systems around events often leads to more maintainable and resilient systems because:

  • Events don’t change often (e.g. OrderPlaced is always meaningful)
  • New behaviors can react to existing events without breaking existing functionality

6. Events Help Discover Domain Insights

When people start talking in terms of events, domain knowledge surfaces more easily. Participants begin to ask:

  • What caused this event?
  • What should happen next?
  • Who or what handled it?
  • Is this event always followed by another?

This leads to better understanding of cause-effect relationships and underlying business logic.

7. Events Contribute to Scalability

Events decouple producers and consumers, allowing systems to scale independently and asynchronously. This pattern supports high-throughput architectures where:

  • Services can process events at their own pace.
  • Workloads can be distributed across consumers.
  • New services can subscribe to existing events without modifying the core system.

Because events are immutable, they can be safely stored, replicated and replayed across distributed systems without coordination or risk of data inconsistency. Immutability makes it possible to:

  • Rebuild system state by replaying events.
  • Scale horizontally by sharing event streams.
  • Audit and debug with confidence in historical accuracy.

This makes event-based systems easier to scale, more reliable under load and more resilient to failure.