refactor: abstract billing operations behind a new domain.BillingProvider interface and update services to use it.

go-b2b-starter/src/app/billing/app/services/consume_invoice_quota_service.go

@@ -82,7 +82,7 @@ func (s *billingService) ingestMeterEventToPolar(ctx context.Context, organizati
 	// Filter: name equals "invoice.processed"
 	// Amount: 1 (one invoice processed)
 	meterSlug := invoicesProcessedMeterSlug // Event name MUST match meter filter exactly (with dot)
-	if err := s.polarAdapter.IngestMeterEvent(ctx, externalID, meterSlug, 1); err != nil {
+	if err := s.billingProvider.IngestMeterEvent(ctx, externalID, meterSlug, 1); err != nil {
 		s.logger.Error("Failed to ingest meter event to Polar", map[string]any{
 			"organization_id": organizationID,
 			"external_id":     externalID,

go-b2b-starter/src/app/billing/app/services/module.go

@@ -34,8 +34,8 @@ func (m *Module) Configure(container *dig.Container) error {
 		return err
 	}
 
-	// Register PolarAdapter
+	// Register BillingProvider (Polar implementation)
-	if err := container.Provide(func(client *polarpkg.Client) PolarAdapter {
+	if err := container.Provide(func(client *polarpkg.Client) domain.BillingProvider {
 		return polar.NewPolarAdapter(client)
 	}); err != nil {
 		return err
@@ -45,10 +45,10 @@ func (m *Module) Configure(container *dig.Container) error {
 	if err := container.Provide(func(
 		repo domain.SubscriptionRepository,
 		orgAdapter domain.OrganizationAdapter,
-		polarAdapter PolarAdapter,
+		billingProvider domain.BillingProvider,
 		logger logger.Logger,
 	) BillingService {
-		return NewBillingService(repo, orgAdapter, polarAdapter, logger)
+		return NewBillingService(repo, orgAdapter, billingProvider, logger)
 	}); err != nil {
 		return err
 	}

go-b2b-starter/src/app/billing/app/services/subscription_service_dec.go

@@ -71,30 +71,22 @@ type BillingService interface {
 }
 
 type billingService struct {
-	repo         domain.SubscriptionRepository
+	repo            domain.SubscriptionRepository
-	orgAdapter   domain.OrganizationAdapter
+	orgAdapter      domain.OrganizationAdapter
-	polarAdapter PolarAdapter
+	billingProvider domain.BillingProvider
-	logger       logger.Logger
+	logger          logger.Logger
 }
 
 func NewBillingService(
 	repo domain.SubscriptionRepository,
 	orgAdapter domain.OrganizationAdapter,
-	polarAdapter PolarAdapter,
+	billingProvider domain.BillingProvider,
 	logger logger.Logger,
 ) BillingService {
 	return &billingService{
-		repo:         repo,
+		repo:            repo,
-		orgAdapter:   orgAdapter,
+		orgAdapter:      orgAdapter,
-		polarAdapter: polarAdapter,
+		billingProvider: billingProvider,
-		logger:       logger,
+		logger:          logger,
 	}
 }
-
-// PolarAdapter defines the interface for Polar API operations
-type PolarAdapter interface {
-	GetSubscription(ctx context.Context, externalCustomerID string) (*domain.Subscription, error)
-	GetCheckoutSession(ctx context.Context, sessionID string) (*domain.CheckoutSessionResponse, error)
-	GetCheckoutSessionWithPolling(ctx context.Context, sessionID string) (*domain.CheckoutSessionResponse, error)
-	IngestMeterEvent(ctx context.Context, externalCustomerID string, meterSlug string, amount int32) error
-}

go-b2b-starter/src/app/billing/app/services/sync_subscription_service.go

@@ -17,7 +17,7 @@ func (s *billingService) SyncSubscriptionFromPolar(ctx context.Context, organiza
 	}
 
 	// Fetch subscription from Polar
-	subscription, err := s.polarAdapter.GetSubscription(ctx, externalID)
+	subscription, err := s.billingProvider.GetSubscription(ctx, externalID)
 	if err != nil {
 		return fmt.Errorf("failed to fetch subscription from Polar: %w", err)
 	}

go-b2b-starter/src/app/billing/app/services/verify_payment_service.go

@@ -11,7 +11,7 @@ import (
 
 func (s *billingService) VerifyPaymentFromCheckout(ctx context.Context, sessionID string) (*domain.BillingStatus, error) {
 	// Step 1: Get checkout session from Polar with polling
-	checkoutSession, err := s.polarAdapter.GetCheckoutSessionWithPolling(ctx, sessionID)
+	checkoutSession, err := s.billingProvider.GetCheckoutSessionWithPolling(ctx, sessionID)
 	if err != nil {
 		fmt.Printf("❌ [VerifyPayment] Failed to verify checkout session %s: %v\n", sessionID, err)
 		return nil, fmt.Errorf("failed to get checkout session: %w", err)
@@ -42,7 +42,7 @@ func (s *billingService) VerifyPaymentFromCheckout(ctx context.Context, sessionI
 	}
 
 	// Step 5: Fetch full subscription details from Polar
-	subscription, err := s.polarAdapter.GetSubscription(ctx, externalCustomerID)
+	subscription, err := s.billingProvider.GetSubscription(ctx, externalCustomerID)
 	if err != nil {
 		return nil, fmt.Errorf("failed to fetch subscription from Polar: %w", err)
 	}

go-b2b-starter/src/app/billing/domain/repository.go

@@ -23,3 +23,12 @@ type OrganizationAdapter interface {
 	GetStytchOrgID(ctx context.Context, organizationID int32) (string, error)
 	GetOrganizationIDByStytchOrgID(ctx context.Context, stytchOrgID string) (int32, error)
 }
+
+// BillingProvider defines operations for external billing providers
+// This interface abstracts the billing provider (e.g., Polar.sh) from the app layer
+type BillingProvider interface {
+	GetSubscription(ctx context.Context, externalCustomerID string) (*Subscription, error)
+	GetCheckoutSession(ctx context.Context, sessionID string) (*CheckoutSessionResponse, error)
+	GetCheckoutSessionWithPolling(ctx context.Context, sessionID string) (*CheckoutSessionResponse, error)
+	IngestMeterEvent(ctx context.Context, externalCustomerID string, meterSlug string, amount int32) error
+}

go-b2b-starter/src/app/billing/infra/polar/polar_adapter.go

@@ -13,11 +13,14 @@ import (
 	polarpkg "github.com/moasq/go-b2b-starter/pkg/polar"
 )
 
+// Ensure polarAdapter implements domain.BillingProvider at compile time
+var _ domain.BillingProvider = (*polarAdapter)(nil)
+
 type polarAdapter struct {
 	client *polarpkg.Client
 }
 
-func NewPolarAdapter(client *polarpkg.Client) *polarAdapter {
+func NewPolarAdapter(client *polarpkg.Client) domain.BillingProvider {
 	return &polarAdapter{
 		client: client,
 	}