aether/event.go

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"`
	EventType string                 `json:"eventType"`
	ActorID   string                 `json:"actorId"`
	CommandID string                 `json:"commandId,omitempty"` // Correlation ID for command that triggered this event
	Version   int64                  `json:"version"`
	Data      map[string]interface{} `json:"data"`
	Metadata  map[string]string      `json:"metadata,omitempty"` // Optional metadata for tracing and auditing
	Timestamp time.Time              `json:"timestamp"`
}

// Common metadata keys for distributed tracing and auditing
const (
	// MetadataKeyCorrelationID identifies related events across services
	MetadataKeyCorrelationID = "correlationId"
	// MetadataKeyCausationID identifies the event that caused this event
	MetadataKeyCausationID = "causationId"
	// MetadataKeyUserID identifies the user who triggered this event
	MetadataKeyUserID = "userId"
	// MetadataKeyTraceID for distributed tracing integration (e.g., OpenTelemetry)
	MetadataKeyTraceID = "traceId"
	// MetadataKeySpanID for distributed tracing integration
	MetadataKeySpanID = "spanId"
)

// SetMetadata sets a metadata key-value pair, initializing the map if needed
func (e *Event) SetMetadata(key, value string) {
	if e.Metadata == nil {
		e.Metadata = make(map[string]string)
	}
	e.Metadata[key] = value
}

// GetMetadata returns the value for a metadata key, or empty string if not found
func (e *Event) GetMetadata(key string) string {
	if e.Metadata == nil {
		return ""
	}
	return e.Metadata[key]
}

// SetCorrelationID sets the correlation ID metadata
func (e *Event) SetCorrelationID(correlationID string) {
	e.SetMetadata(MetadataKeyCorrelationID, correlationID)
}

// GetCorrelationID returns the correlation ID metadata
func (e *Event) GetCorrelationID() string {
	return e.GetMetadata(MetadataKeyCorrelationID)
}

// SetCausationID sets the causation ID metadata
func (e *Event) SetCausationID(causationID string) {
	e.SetMetadata(MetadataKeyCausationID, causationID)
}

// GetCausationID returns the causation ID metadata
func (e *Event) GetCausationID() string {
	return e.GetMetadata(MetadataKeyCausationID)
}

// SetUserID sets the user ID metadata
func (e *Event) SetUserID(userID string) {
	e.SetMetadata(MetadataKeyUserID, userID)
}

// GetUserID returns the user ID metadata
func (e *Event) GetUserID() string {
	return e.GetMetadata(MetadataKeyUserID)
}

// SetTraceID sets the trace ID metadata for distributed tracing
func (e *Event) SetTraceID(traceID string) {
	e.SetMetadata(MetadataKeyTraceID, traceID)
}

// GetTraceID returns the trace ID metadata
func (e *Event) GetTraceID() string {
	return e.GetMetadata(MetadataKeyTraceID)
}

// SetSpanID sets the span ID metadata for distributed tracing
func (e *Event) SetSpanID(spanID string) {
	e.SetMetadata(MetadataKeySpanID, spanID)
}

// GetSpanID returns the span ID metadata
func (e *Event) GetSpanID() string {
	return e.GetMetadata(MetadataKeySpanID)
}

// WithMetadataFrom copies metadata from another event (useful for event chaining)
func (e *Event) WithMetadataFrom(source *Event) {
	if source == nil || source.Metadata == nil {
		return
	}
	if e.Metadata == nil {
		e.Metadata = make(map[string]string)
	}
	for k, v := range source.Metadata {
		e.Metadata[k] = v
	}
}

// ActorSnapshot represents a point-in-time state snapshot
type ActorSnapshot struct {
	ActorID   string                 `json:"actorId"`
	Version   int64                  `json:"version"`
	State     map[string]interface{} `json:"state"`
	Timestamp time.Time              `json:"timestamp"`
}

// 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)
}

// SnapshotStore extends EventStore with snapshot capabilities
type SnapshotStore interface {
	EventStore
	GetLatestSnapshot(actorID string) (*ActorSnapshot, error)
	SaveSnapshot(snapshot *ActorSnapshot) error
}