Eventmodeling Validating Event Models Checklist

Event Model Validation Checklist Skill

Purpose: Validate any event-sourced CQRS event model against 16 architectural checks across 7 phases. Identifies anti-patterns and confirms compliance with event sourcing principles.

Applies To: Any domain - e-commerce, banking, SaaS, marketplace, healthcare, etc.

When to Use:

• After completing Step 2 (Event Plot) of 7-step event modeling
• After completing Step 7 (Scenarios) before declaring model complete
• When reviewing an existing event model for production readiness
• When suspicious of architectural issues in event design

What It Does:

1. Systematically applies 16 validation checks across 7 phases
2. Identifies violations of event sourcing principles (domain-agnostic)
3. Flags anti-patterns (calculations as events, non-entity streams, etc.)
4. Verifies read model/event distinction
5. Confirms stream independence and business rule enforcement
6. Returns pass/fail verdict with evidence

Validation Phases (Domain-Agnostic)

Phase 1: Event Stream & Command Handler State Validation (3 checks)

• Check 1.1: Each event belongs to exactly one stream
• Check 1.2: Each command handler owns its own [CommandHandler]State class
• Check 1.3: No hard dependencies between command handlers (orchestrated via events only)

Anti-pattern to catch: Sharing state across handlers or treating state as persistent aggregate

Phase 2: Event Quality Validation (3 checks)

• Check 2.1: Events represent domain facts, not calculations
• Check 2.2: Event data is immutable after creation
• Check 2.3: Event names use past tense (what actually happened)

Anti-pattern to catch: Storing computed/aggregated data as events

Phase 3: Read Model vs Event Distinction (2 checks)

• Check 3.1: Each read model is NOT an event stream
• Check 3.2: Read model has natural query pattern

Anti-pattern to catch: Confusing projections/calculations with domain facts

Phase 4: Business Rules Validation (2 checks)

• Check 4.1: Constraints enforced in command handler decision logic (encapsulated in [CommandHandler]State)
• Check 4.2: Event preconditions are explicit (what state must exist before command is valid)

Anti-pattern to catch: Business rules scattered across handlers or encoded in event stream structure

Phase 5: Data Traceability (1 check)

• Check 5.1: Input → Event → Read Model traceability is complete

Anti-pattern to catch: Command inputs that disappear or read model fields without source

Phase 6: Event Flow Validation (1 check)

• Check 6.1: No impossible event sequences (state machine is sound)

Anti-pattern to catch: Events that can occur in invalid state combinations

Phase 7: Stream Independence (1 check)

• Check 7.1: Each stream can be versioned/restored independently

Anti-pattern to catch: Hard dependencies between streams

Final Questions (3 checks)

• Question 1: Could an architect unfamiliar with this domain understand the model in 15 minutes?
• Question 2: Could you change your core algorithm/calculation without changing event history?
• Question 3: Could this be implemented in your target technology stack (e.g., TypeScript + PostgreSQL)?

Output Format

The skill returns a validation report with:

For Each Check

 Check 1.2: Stream Root is Business Entity, Not Calculation
Status: PASS
Evidence: [Specific examples from your model]

Anti-Patterns Identified (if any)

CRITICAL: [Anti-pattern description]
Problem: [Why it violates event sourcing]
Violates: [Which checks fail]
Fix: [Recommended action]

Final Verdict

Status:  PASS (or  PASS WITH WARNINGS or  FAIL)
Implementation Ready: YES (or NO - fix issues first)
Confidence: [percentage]

Common Anti-Patterns (Domain-Agnostic)

1. Calculation Events

 ANTI-PATTERN:
CalculationPerformed {
  metric: 4.5  ← Mutable/recalculated
  timestamp: T
}

 CORRECT:
- Event: SomeActionOccurred (immutable fact)
- ReadModel: MetricView (recalculated from events)

Why: Calculations change multiple times as source data changes. Events are immutable.

2. Shared vs Handler-Owned State

 ANTI-PATTERN:
One shared "OrderAggregate" class used by all handlers
- ConfirmOrderHandler shares OrderAggregate state
- ShipOrderHandler modifies same OrderAggregate
- Result: Tight coupling, hard to parallelize

 CORRECT:
Each handler owns its own [CommandHandler]State class
- ConfirmOrderState (only ConfirmOrderHandler uses)
- ShipOrderState (only ShipOrderHandler uses)
- CancelOrderState (only CancelOrderHandler uses)
- All reconstruct state from same events, but independently

Why: Each handler is a micro-slice. Separate state classes maintain isolation, enable parallel teams, prevent merge conflicts.

3. Circular Dependencies

 ANTI-PATTERN:
StreamA → EventA → affects
StreamB → EventB → affects
StreamA (circular!)

 CORRECT:
StreamA → EventA → Event Bus
StreamB → EventB → Event Bus
ReadModels ← consume (one-way only)
No feedback loops

Why: Circular dependencies make the system hard to reason about and test.

4. Persistent State vs Ephemeral State

 ANTI-PATTERN:
Treat [CommandHandler]State as persistent entity
- Save SubmitReviewState to database after command
- Load it again next time
- Result: Duplicates event sourcing, loses audit trail

 CORRECT:
Reconstruct [CommandHandler]State on-demand
- Load events from stream
- Replay via evolve() to rebuild state
- Process command, emit outcome events
- Discard state (it's ephemeral, not persisted)

Why: State is derived from events, never stored. Events are source of truth. This enables consistent replay, audit trails, and time-travel debugging.

Questions to Ask During Validation

For each event stream:

1. "Do all events in this stream share the same identity (streamId)?"
2. "Could these events occur in any order, or is sequence important?"
3. "Is every event in this stream an immutable fact that actually happened?"

For each command handler:

1. "Does this handler own its own [CommandHandler]State class?"
2. "Is the state ephemeral (reconstructed per command, not persisted)?"
3. "Can I trace the state reconstruction: events → evolve() → decision?"

For each read model:

1. "Is this calculated from events via a projection?"
2. "Does it answer a specific query need?"
3. "Could its data change due to new events or state changes?"

For system architecture:

1. "Does each command handler operate independently (communicate via events only)?"
2. "Could I run two handlers' code in parallel without merge conflicts?"
3. "Are the only shared artifacts the event definitions?"

Success Criteria

Model is validated when:

• All 16 checks pass (or have documented workarounds)
• No critical anti-patterns identified
• All 3 final questions answer YES
• Event sourcing principles clearly upheld
• Ready to proceed to code generation

Model needs fixes when:

• Any check fails with clear evidence
• Anti-patterns identified with specific violations
• Final questions have NO answers
• Fixes are straightforward and targeted

Model should be redesigned when:

• Multiple phases fail
• Architectural assumptions are fundamentally flawed
• Anti-patterns are systemic and pervasive
• Would require rewriting core event structure

Example Validation Patterns

Pattern: Calculation vs Event

When you see something like "CalculationDone" or "ReviewRatingUpdated":

• Ask: "Is this immutable and caused by a user/system action?"
• If NO → It's a read model, not an event
• Fix: Remove from events, create read model projection instead

Pattern: Shared vs Isolated State

When you see "ReviewAggregate" used by multiple handlers:

• Ask: "Could SubmitReviewHandler and ApproveReviewHandler work on separate files?"
• If NO → State classes aren't properly isolated
• Fix: Create SubmitReviewState and ApproveReviewState, each handler owns one

Pattern: Persistent vs Ephemeral State

When state is saved to database after a command:

• Ask: "Is this state only needed during command processing?"
• If YES → It's ephemeral, reconstruct from events instead
• Fix: Load events, replay via evolve(), process command, discard state

Pattern: Data That Doesn't Trace

When a read model field appears without source:

• Ask: "Where did this come from?"
• If no event source → Add event or remove field
• If sourced from calculation → Verify it's in projection, not event

Integration with Event Modeling Process

Recommended timing:

Step 1: Brainstorm Events
Step 2: The Plot (Sequence)
  ↓
→ RUN eventmodeling-validating-event-models-checklist (catch structural issues early)
  ↓
Fix any violations
  ↓
Step 3-7: Complete remaining steps
  ↓
→ RUN eventmodeling-validating-event-models-checklist again (final validation)
  ↓
PASS → Code generation
FAIL → Fix identified issues

Running the checklist after Step 2 prevents wasting time on later steps if core events are flawed.

Checklist Questions by Domain

The skill applies the same 16 checks regardless of domain. Here's how to think about it in different contexts:

E-commerce domain:

• Events: OrderCreated, OrderConfirmed, PaymentAuthorized, OrderShipped
• NOT events: OrderTotal, InventoryLevel, ShippingCost (these are read models)

Banking domain:

• Events: AccountOpened, DepositReceived, WithdrawalProcessed, FundsTransferred
• NOT events: AccountBalance, InterestCalculated (these are read models)

SaaS domain:

• Events: SubscriptionCreated, PaymentProcessed, PlanUpgraded, SubscriptionCancelled
• NOT events: MonthlyRecurringRevenue, ChurnRate (these are read models)

Healthcare domain:

• Events: PatientRegistered, AppointmentScheduled, ProcedureCompleted, BillGenerated
• NOT events: PatientAge, AverageCost (these are read models)

The principle is the same across all domains: immutable facts as events, calculated results as read models.

Tips for Best Results

1. Be specific: List actual event names, command handler names, and state classes from your model
2. Reference your documentation: Link to or quote from your step 1-7 documents and micro-slice plans
3. Provide context: Explain what your domain is and how handlers will be parallelized
4. Ask follow-ups: If a check flags an issue, ask "How do I fix this specifically?" or "Can this handler be isolated?"
5. Iterate: Run again after making fixes to confirm all checks pass and handlers are properly isolated

Quality Checklist

• All 16 checks evaluated — no check skipped without documented justification
• Every FAIL result includes the specific event, handler, or stream that violated the check
• Anti-patterns identified by name with the exact model element that triggered the flag
• Final verdict is one of: PASS / PASS WITH WARNINGS / FAIL — no ambiguous outcomes
• All 3 final architectural questions answered YES before declaring model ready for implementation
• Any FAIL result has a recommended fix, not just a problem statement

Related Skills

• eventmodeling-orchestrating-event-modeling: Main skill coordinating the 7-step event modeling process
• eventmodeling-brainstorming-events: Extract events from requirements (Step 1)
• eventmodeling-plotting-events: Sequence events chronologically (Step 2)
• eventmodeling-designing-event-models: Design your complete event model
• eventmodeling-validating-event-models: Detailed validator with deep analysis

Validation Checklist Reference

The 16-point checklist is defined in the Validation Phases section above. Each check includes the anti-pattern to catch and questions to ask when evaluating your model.