Add event versioning validation

- Add ErrVersionConflict error type and VersionConflictError for detailed
  conflict information
- Implement version validation in InMemoryEventStore.SaveEvent that rejects
  events with version <= current latest version
- Implement version validation in JetStreamEventStore.SaveEvent with version
  caching for performance
- Add comprehensive tests for version conflict detection including concurrent
  writes to same actor
- Document versioning semantics in EventStore interface and CLAUDE.md

This ensures events have monotonically increasing versions per actor and
provides clear error messages for version conflicts, enabling optimistic
concurrency control patterns.

Closes #6

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -79,6 +79,49 @@ store.SaveEvent(event)
 events, _ := store.GetEvents("order-123", 0)
 ```
 
+### Event Versioning
+
+Events for each actor must have **monotonically increasing versions**. This ensures event stream integrity and enables optimistic concurrency control.
+
+#### Version Semantics
+
+- Each actor has an independent version sequence
+- Version must be strictly greater than the current latest version
+- For new actors (no events), the first event must have version > 0
+- Non-consecutive versions are allowed (gaps are permitted)
+
+#### Optimistic Concurrency Pattern
+
+```go
+// 1. Get current version
+currentVersion, _ := store.GetLatestVersion("order-123")
+
+// 2. Create event with next version
+event := &aether.Event{
+    ID:        uuid.New().String(),
+    EventType: "OrderUpdated",
+    ActorID:   "order-123",
+    Version:   currentVersion + 1,
+    Data:      map[string]interface{}{"status": "shipped"},
+    Timestamp: time.Now(),
+}
+
+// 3. Attempt to save
+err := store.SaveEvent(event)
+if errors.Is(err, aether.ErrVersionConflict) {
+    // Another writer won - reload and retry if appropriate
+    var versionErr *aether.VersionConflictError
+    errors.As(err, &versionErr)
+    log.Printf("Conflict: actor %s has version %d, attempted %d",
+        versionErr.ActorID, versionErr.CurrentVersion, versionErr.AttemptedVersion)
+}
+```
+
+#### Error Types
+
+- `ErrVersionConflict` - Sentinel error for version conflicts (use with `errors.Is`)
+- `VersionConflictError` - Detailed error with ActorID, CurrentVersion, and AttemptedVersion
+
 ### Namespace Isolation
 
 Namespaces provide logical boundaries for events and subscriptions:
@@ -111,6 +154,7 @@ if manager.IsLeader() {
 ## Key Patterns
 
 - **Events are immutable** - Never modify, only append
+- **Versions are monotonic** - Each event must have version > previous for same actor
 - **Snapshots for performance** - Periodically snapshot state to avoid full replay
 - **Namespaces for isolation** - Not multi-tenancy, just logical boundaries
 - **NATS for everything** - Events, pub/sub, clustering all use NATS

event.go

@@ -1,9 +1,33 @@
 package aether
 
 import (
+	"errors"
+	"fmt"
 	"time"
 )
 
+// ErrVersionConflict is returned when attempting to save an event with a version
+// that is not strictly greater than the current latest version for an actor.
+// This ensures events have monotonically increasing versions per actor.
+var ErrVersionConflict = errors.New("version conflict")
+
+// VersionConflictError provides details about a version conflict.
+// It is returned when SaveEvent is called with a version <= the current latest version.
+type VersionConflictError struct {
+	ActorID          string
+	AttemptedVersion int64
+	CurrentVersion   int64
+}
+
+func (e *VersionConflictError) Error() string {
+	return fmt.Sprintf("%s: actor %q has version %d, cannot save version %d",
+		ErrVersionConflict, e.ActorID, e.CurrentVersion, e.AttemptedVersion)
+}
+
+func (e *VersionConflictError) Unwrap() error {
+	return ErrVersionConflict
+}
+
 // Event represents a domain event in the system
 type Event struct {
 	ID        string                 `json:"id"`
@@ -23,10 +47,36 @@ type ActorSnapshot struct {
 	Timestamp time.Time              `json:"timestamp"`
 }
 
-// EventStore defines the interface for event persistence
+// EventStore defines the interface for event persistence.
+//
+// # Version Semantics
+//
+// Events for an actor must have monotonically increasing versions. When SaveEvent
+// is called, the implementation must validate that the event's version is strictly
+// greater than the current latest version for that actor. If the version is less
+// than or equal to the current version, SaveEvent must return a VersionConflictError
+// (which wraps ErrVersionConflict).
+//
+// This validation ensures event stream integrity and enables optimistic concurrency
+// control. Clients should:
+//  1. Call GetLatestVersion to get the current version for an actor
+//  2. Set the new event's version to currentVersion + 1
+//  3. Call SaveEvent - if it returns ErrVersionConflict, another writer won
+//  4. On conflict, reload the latest version and retry if appropriate
+//
+// For new actors (no existing events), version 1 is expected for the first event.
 type EventStore interface {
+	// SaveEvent persists an event to the store. The event's Version must be
+	// strictly greater than the current latest version for the actor.
+	// Returns VersionConflictError if version <= current latest version.
 	SaveEvent(event *Event) error
+
+	// GetEvents retrieves events for an actor from a specific version (inclusive).
+	// Returns an empty slice if no events exist for the actor.
 	GetEvents(actorID string, fromVersion int64) ([]*Event, error)
+
+	// GetLatestVersion returns the latest version for an actor.
+	// Returns 0 if no events exist for the actor.
 	GetLatestVersion(actorID string) (int64, error)
 }
 

store/jetstream.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"strings"
+	"sync"
 	"time"
 
 	"git.flowmade.one/flowmade-one/aether"
@@ -14,6 +15,8 @@ import (
 type JetStreamEventStore struct {
 	js         nats.JetStreamContext
 	streamName string
+	mu         sync.Mutex                // Protects version checks during SaveEvent
+	versions   map[string]int64          // actorID -> latest version cache
 }
 
 // NewJetStreamEventStore creates a new JetStream-based event store
@@ -41,11 +44,32 @@ func NewJetStreamEventStore(natsConn *nats.Conn, streamName string) (*JetStreamE
 	return &JetStreamEventStore{
 		js:         js,
 		streamName: streamName,
+		versions:   make(map[string]int64),
 	}, nil
 }
 
-// SaveEvent persists an event to JetStream
+// SaveEvent persists an event to JetStream.
+// Returns VersionConflictError if the event's version is not strictly greater
+// than the current latest version for the actor.
 func (jes *JetStreamEventStore) SaveEvent(event *aether.Event) error {
+	jes.mu.Lock()
+	defer jes.mu.Unlock()
+
+	// Get current latest version for this actor
+	currentVersion, err := jes.getLatestVersionLocked(event.ActorID)
+	if err != nil {
+		return fmt.Errorf("failed to get latest version: %w", err)
+	}
+
+	// Validate version is strictly greater than current
+	if event.Version <= currentVersion {
+		return &aether.VersionConflictError{
+			ActorID:          event.ActorID,
+			AttemptedVersion: event.Version,
+			CurrentVersion:   currentVersion,
+		}
+	}
+
 	// Serialize event to JSON
 	data, err := json.Marshal(event)
 	if err != nil {
@@ -64,11 +88,50 @@ func (jes *JetStreamEventStore) SaveEvent(event *aether.Event) error {
 		return fmt.Errorf("failed to publish event to JetStream: %w", err)
 	}
 
+	// Update version cache
+	jes.versions[event.ActorID] = event.Version
+
 	return nil
 }
 
+// getLatestVersionLocked returns the latest version for an actor.
+// Caller must hold jes.mu.
+func (jes *JetStreamEventStore) getLatestVersionLocked(actorID string) (int64, error) {
+	// Check cache first
+	if version, ok := jes.versions[actorID]; ok {
+		return version, nil
+	}
+
+	// Fetch from JetStream
+	events, err := jes.getEventsInternal(actorID, 0)
+	if err != nil {
+		return 0, err
+	}
+
+	if len(events) == 0 {
+		return 0, nil
+	}
+
+	latestVersion := int64(0)
+	for _, event := range events {
+		if event.Version > latestVersion {
+			latestVersion = event.Version
+		}
+	}
+
+	// Update cache
+	jes.versions[actorID] = latestVersion
+
+	return latestVersion, nil
+}
+
 // GetEvents retrieves all events for an actor since a version
 func (jes *JetStreamEventStore) GetEvents(actorID string, fromVersion int64) ([]*aether.Event, error) {
+	return jes.getEventsInternal(actorID, fromVersion)
+}
+
+// getEventsInternal is the internal implementation of GetEvents
+func (jes *JetStreamEventStore) getEventsInternal(actorID string, fromVersion int64) ([]*aether.Event, error) {
 	// Create subject filter for this actor
 	subject := fmt.Sprintf("%s.events.%s.%s",
 		jes.streamName,

store/memory.go

@@ -1,7 +1,6 @@
 package store
 
 import (
-	"fmt"
 	"sync"
 
 	"git.flowmade.one/flowmade-one/aether"
@@ -22,11 +21,32 @@ func NewInMemoryEventStore() *InMemoryEventStore {
 	}
 }
 
-// SaveEvent saves an event to the in-memory store
+// SaveEvent saves an event to the in-memory store.
+// Returns VersionConflictError if the event's version is not strictly greater
+// than the current latest version for the actor.
 func (es *InMemoryEventStore) SaveEvent(event *aether.Event) error {
 	es.mu.Lock()
 	defer es.mu.Unlock()
 
+	// Get current latest version for this actor
+	currentVersion := int64(0)
+	if events, exists := es.events[event.ActorID]; exists {
+		for _, e := range events {
+			if e.Version > currentVersion {
+				currentVersion = e.Version
+			}
+		}
+	}
+
+	// Validate version is strictly greater than current
+	if event.Version <= currentVersion {
+		return &aether.VersionConflictError{
+			ActorID:          event.ActorID,
+			AttemptedVersion: event.Version,
+			CurrentVersion:   currentVersion,
+		}
+	}
+
 	if _, exists := es.events[event.ActorID]; !exists {
 		es.events[event.ActorID] = make([]*aether.Event, 0)
 	}
@@ -77,7 +97,7 @@ func (es *InMemoryEventStore) GetLatestVersion(actorID string) (int64, error) {
 // SaveSnapshot saves a snapshot to the in-memory store
 func (es *InMemoryEventStore) SaveSnapshot(snapshot *aether.ActorSnapshot) error {
 	if snapshot == nil {
-		return fmt.Errorf("snapshot cannot be nil")
+		return &snapshotNilError{}
 	}
 
 	es.mu.Lock()
@@ -109,3 +129,10 @@ func (es *InMemoryEventStore) GetLatestSnapshot(actorID string) (*aether.ActorSn
 
 	return latest, nil
 }
+
+// snapshotNilError is returned when attempting to save a nil snapshot
+type snapshotNilError struct{}
+
+func (e *snapshotNilError) Error() string {
+	return "snapshot cannot be nil"
+}

store/memory_test.go

@@ -1,15 +1,17 @@
 package store
 
 import (
+	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
 	"git.flowmade.one/flowmade-one/aether"
 )
 
-// === Event Store Tests (from main branch) ===
+// === Event Store Tests ===
 
 func TestNewInMemoryEventStore(t *testing.T) {
 	store := NewInMemoryEventStore()
@@ -292,8 +294,8 @@ func TestGetEvents_FromVersionZero(t *testing.T) {
 func TestGetLatestVersion_ReturnsCorrectVersion(t *testing.T) {
 	store := NewInMemoryEventStore()
 
-	// Save events with various versions
+	// Save events with strictly increasing versions
-	versions := []int64{1, 3, 2, 5, 4} // Out of order
+	versions := []int64{1, 2, 3, 4, 5}
 	for i, version := range versions {
 		event := &aether.Event{
 			ID:        fmt.Sprintf("evt-%d", i),
@@ -553,17 +555,18 @@ func TestConcurrentSaveAndGet(t *testing.T) {
 		}()
 	}
 
-	// Start writers
+	// Start writers - each writer gets their own actor to avoid version conflicts
 	wg.Add(numWriters)
 	for w := 0; w < numWriters; w++ {
 		go func(writerID int) {
 			defer wg.Done()
+			actorID := fmt.Sprintf("writer-actor-%d", writerID)
 			for i := 0; i < writesPerWriter; i++ {
 				event := &aether.Event{
 					ID:        fmt.Sprintf("evt-writer-%d-%d", writerID, i),
 					EventType: "TestEvent",
-					ActorID:   "actor-123",
+					ActorID:   actorID,
-					Version:   int64(100 + writerID*writesPerWriter + i),
+					Version:   int64(i + 1),
 					Data:      map[string]interface{}{},
 					Timestamp: time.Now(),
 				}
@@ -576,15 +579,27 @@ func TestConcurrentSaveAndGet(t *testing.T) {
 
 	wg.Wait()
 
-	// Verify final state
+	// Verify actor-123 still has its original events
 	events, err := store.GetEvents("actor-123", 0)
 	if err != nil {
 		t.Fatalf("GetEvents failed: %v", err)
 	}
 
-	expectedTotal := 10 + numWriters*writesPerWriter
+	if len(events) != 10 {
-	if len(events) != expectedTotal {
+		t.Errorf("expected 10 events for actor-123, got %d", len(events))
-		t.Errorf("expected %d total events, got %d", expectedTotal, len(events))
+	}
+
+	// Verify each writer's actor has the correct events
+	for w := 0; w < numWriters; w++ {
+		actorID := fmt.Sprintf("writer-actor-%d", w)
+		events, err := store.GetEvents(actorID, 0)
+		if err != nil {
+			t.Errorf("GetEvents failed for %s: %v", actorID, err)
+			continue
+		}
+		if len(events) != writesPerWriter {
+			t.Errorf("expected %d events for %s, got %d", writesPerWriter, actorID, len(events))
+		}
 	}
 }
 
@@ -686,8 +701,8 @@ func TestSaveEvent_EmptyData(t *testing.T) {
 func TestGetEvents_VersionEdgeCases(t *testing.T) {
 	store := NewInMemoryEventStore()
 
-	// Save events with edge case versions
+	// Save events with edge case versions (strictly increasing)
-	versions := []int64{0, 1, 9223372036854775807} // Zero, one, MaxInt64
+	versions := []int64{1, 100, 9223372036854775807} // one, 100, MaxInt64
 	for i, version := range versions {
 		event := &aether.Event{
 			ID:        fmt.Sprintf("evt-%d", i),
@@ -698,7 +713,7 @@ func TestGetEvents_VersionEdgeCases(t *testing.T) {
 			Timestamp: time.Now(),
 		}
 		if err := store.SaveEvent(event); err != nil {
-			t.Fatalf("SaveEvent failed: %v", err)
+			t.Fatalf("SaveEvent failed for version %d: %v", version, err)
 		}
 	}
 
@@ -787,6 +802,189 @@ func TestSaveEvent_SpecialActorIDs(t *testing.T) {
 	}
 }
 
+// === Version Validation Tests ===
+
+func TestSaveEvent_RejectsLowerVersion(t *testing.T) {
+	store := NewInMemoryEventStore()
+
+	// Save first event with version 5
+	event1 := &aether.Event{
+		ID:        "evt-1",
+		EventType: "TestEvent",
+		ActorID:   "actor-123",
+		Version:   5,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	if err := store.SaveEvent(event1); err != nil {
+		t.Fatalf("SaveEvent failed for first event: %v", err)
+	}
+
+	// Attempt to save event with lower version (should fail)
+	event2 := &aether.Event{
+		ID:        "evt-2",
+		EventType: "TestEvent",
+		ActorID:   "actor-123",
+		Version:   3,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	err := store.SaveEvent(event2)
+	if err == nil {
+		t.Fatal("expected error when saving event with lower version, got nil")
+	}
+
+	// Verify it's a VersionConflictError
+	if !errors.Is(err, aether.ErrVersionConflict) {
+		t.Errorf("expected ErrVersionConflict, got %v", err)
+	}
+
+	var versionErr *aether.VersionConflictError
+	if !errors.As(err, &versionErr) {
+		t.Fatalf("expected VersionConflictError, got %T", err)
+	}
+
+	if versionErr.ActorID != "actor-123" {
+		t.Errorf("ActorID mismatch: got %q, want %q", versionErr.ActorID, "actor-123")
+	}
+	if versionErr.CurrentVersion != 5 {
+		t.Errorf("CurrentVersion mismatch: got %d, want %d", versionErr.CurrentVersion, 5)
+	}
+	if versionErr.AttemptedVersion != 3 {
+		t.Errorf("AttemptedVersion mismatch: got %d, want %d", versionErr.AttemptedVersion, 3)
+	}
+}
+
+func TestSaveEvent_RejectsEqualVersion(t *testing.T) {
+	store := NewInMemoryEventStore()
+
+	// Save first event with version 5
+	event1 := &aether.Event{
+		ID:        "evt-1",
+		EventType: "TestEvent",
+		ActorID:   "actor-123",
+		Version:   5,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	if err := store.SaveEvent(event1); err != nil {
+		t.Fatalf("SaveEvent failed for first event: %v", err)
+	}
+
+	// Attempt to save event with equal version (should fail)
+	event2 := &aether.Event{
+		ID:        "evt-2",
+		EventType: "TestEvent",
+		ActorID:   "actor-123",
+		Version:   5,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	err := store.SaveEvent(event2)
+	if err == nil {
+		t.Fatal("expected error when saving event with equal version, got nil")
+	}
+
+	if !errors.Is(err, aether.ErrVersionConflict) {
+		t.Errorf("expected ErrVersionConflict, got %v", err)
+	}
+}
+
+func TestSaveEvent_RejectsZeroVersion(t *testing.T) {
+	store := NewInMemoryEventStore()
+
+	// Version 0 should be rejected (not strictly greater than initial 0)
+	event := &aether.Event{
+		ID:        "evt-1",
+		EventType: "TestEvent",
+		ActorID:   "actor-new",
+		Version:   0,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	err := store.SaveEvent(event)
+	if err == nil {
+		t.Fatal("expected error when saving event with version 0, got nil")
+	}
+
+	if !errors.Is(err, aether.ErrVersionConflict) {
+		t.Errorf("expected ErrVersionConflict, got %v", err)
+	}
+}
+
+func TestSaveEvent_RejectsNegativeVersion(t *testing.T) {
+	store := NewInMemoryEventStore()
+
+	event := &aether.Event{
+		ID:        "evt-neg",
+		EventType: "TestEvent",
+		ActorID:   "actor-123",
+		Version:   -1,
+		Data:      map[string]interface{}{},
+		Timestamp: time.Now(),
+	}
+	err := store.SaveEvent(event)
+	if err == nil {
+		t.Fatal("expected error when saving event with negative version, got nil")
+	}
+
+	if !errors.Is(err, aether.ErrVersionConflict) {
+		t.Errorf("expected ErrVersionConflict, got %v", err)
+	}
+}
+
+func TestSaveEvent_ConcurrentWritesToSameActor(t *testing.T) {
+	store := NewInMemoryEventStore()
+
+	numGoroutines := 100
+	var successCount int64
+	var conflictCount int64
+	var wg sync.WaitGroup
+
+	// All goroutines try to save version 1
+	wg.Add(numGoroutines)
+	for i := 0; i < numGoroutines; i++ {
+		go func(id int) {
+			defer wg.Done()
+			event := &aether.Event{
+				ID:        fmt.Sprintf("evt-%d", id),
+				EventType: "TestEvent",
+				ActorID:   "actor-contested",
+				Version:   1,
+				Data:      map[string]interface{}{"goroutine": id},
+				Timestamp: time.Now(),
+			}
+			err := store.SaveEvent(event)
+			if err == nil {
+				atomic.AddInt64(&successCount, 1)
+			} else if errors.Is(err, aether.ErrVersionConflict) {
+				atomic.AddInt64(&conflictCount, 1)
+			} else {
+				t.Errorf("unexpected error: %v", err)
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	// Exactly one should succeed, rest should conflict
+	if successCount != 1 {
+		t.Errorf("expected exactly 1 success, got %d", successCount)
+	}
+	if conflictCount != int64(numGoroutines-1) {
+		t.Errorf("expected %d conflicts, got %d", numGoroutines-1, conflictCount)
+	}
+
+	// Verify only one event was stored
+	events, err := store.GetEvents("actor-contested", 0)
+	if err != nil {
+		t.Fatalf("GetEvents failed: %v", err)
+	}
+	if len(events) != 1 {
+		t.Errorf("expected 1 event, got %d", len(events))
+	}
+}
+
 func BenchmarkSaveEvent(b *testing.B) {
 	store := NewInMemoryEventStore()
 
@@ -848,7 +1046,7 @@ func BenchmarkGetLatestVersion(b *testing.B) {
 	}
 }
 
-// === Snapshot Store Tests (from PR branch) ===
+// === Snapshot Store Tests ===
 
 func TestSaveSnapshot_PersistsCorrectly(t *testing.T) {
 	store := NewInMemoryEventStore()