refactor: introduce isyncstore interface (#660)

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## This PR
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- decouples the sync store from sync server startup, allowing for
alternative sync store implementations to be introduced

### Related Issues
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### Notes
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### Follow-up Tasks
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---------

Signed-off-by: James Milligan <[email protected]>

core/pkg/service/sync/handler.go

@@ -12,7 +12,7 @@ import (
 
 type handler struct {
 	rpc.UnimplementedFlagSyncServiceServer
-	syncStore *syncStore.SyncStore
+	syncStore syncStore.ISyncStore
 	logger    *logger.Logger
 }
 

core/pkg/service/sync/server.go

@@ -25,12 +25,11 @@ type Server struct {
 	config        iservice.Configuration
 }
 
-func NewServer(ctx context.Context, logger *logger.Logger) *Server {
-	syncStore := syncStore.NewSyncStore(ctx, logger)
+func NewServer(logger *logger.Logger, store syncStore.ISyncStore) *Server {
 	return &Server{
 		handler: &handler{
 			logger:    logger,
-			syncStore: syncStore,
+			syncStore: store,
 		},
 		Logger: logger,
 	}

core/pkg/sync-store/interface.go

@@ -0,0 +1,26 @@
+package store
+
+import (
+	"context"
+
+	isync "github.com/open-feature/flagd/core/pkg/sync"
+)
+
+// ISyncStore defines the interface for the sync store
+type ISyncStore interface {
+	FetchAllFlags(
+		ctx context.Context,
+		key interface{},
+		target string,
+	) (isync.DataSync, error)
+	RegisterSubscription(
+		ctx context.Context,
+		target string,
+		key interface{},
+		dataSync chan isync.DataSync,
+		errChan chan error,
+	)
+
+	// metrics hooks
+	GetActiveSubscriptionsInt64() int64
+}

flagd-proxy/cmd/start.go

@@ -13,6 +13,7 @@ import (
 	"github.com/open-feature/flagd/core/pkg/logger"
 	"github.com/open-feature/flagd/core/pkg/service"
 	syncServer "github.com/open-feature/flagd/core/pkg/service/sync"
+	syncStore "github.com/open-feature/flagd/core/pkg/sync-store"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
 	"go.uber.org/zap/zapcore"
@@ -61,7 +62,8 @@ var startCmd = &cobra.Command{
 
 		ctx, _ := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 
-		s := syncServer.NewServer(ctx, logger)
+		syncStore := syncStore.NewSyncStore(ctx, logger)
+		s := syncServer.NewServer(logger, syncStore)
 		cfg := service.Configuration{
 			ReadinessProbe: func() bool { return true },
 			Port:           viper.GetUint16(portFlagName),