diff --git a/src/repository.ts b/src/repository.ts
index 8cc3210..ba58c11 100644
--- a/src/repository.ts
+++ b/src/repository.ts
@@ -55,24 +55,37 @@ export interface Repository {
   // Getter methods
 
   getSystems(): Promise<ISystem[]>;
-  getSystemById(systemId: string): Promise<ISystem>;
+  getSystemById(systemId: string): Promise<ISystem | null>;
 
   getStopsBySystemId(systemId: string): Promise<IStop[]>;
-  getStopById(stopId: string): Promise<IStop[]>;
+  getStopById(stopId: string): Promise<IStop | null>;
 
   getRoutesBySystemId(systemId: string): Promise<IRoute[]>;
-  getRouteById(routeId: string): Promise<IRoute>;
+  getRouteById(routeId: string): Promise<IRoute | null>;
 
   getShuttlesBySystemId(systemId: string): Promise<IShuttle[]>;
-  getShuttleById(shuttleId: string): Promise<IShuttle[]>;
+  getShuttleById(shuttleId: string): Promise<IShuttle | null>;
 
   getEtasForShuttleId(shuttleId: string): Promise<IEta[]>;
   getEtasForStopId(stopId: string): Promise<IEta[]>;
-  getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta[]>;
+  getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta | null>;
 
-  getOrderedStopByRouteAndStopId(routeId: string, stopId: string): Promise<IOrderedStop[]>;
+  getOrderedStopByRouteAndStopId(routeId: string, stopId: string): Promise<IOrderedStop | null>;
+
+  /**
+   * Get ordered stops with the given stop ID.
+   * Returns an empty array if no ordered stops found.
+   * @param stopId
+   */
   getOrderedStopsByStopId(stopId: string): Promise<IOrderedStop[]>;
+
+  /**
+   * Get ordered stops with the given route ID.
+   * Returns an empty array if no ordered stops found.
+   * @param routeId
+   */
   getOrderedStopsByRouteId(routeId: string): Promise<IOrderedStop[]>;
+
   getNextOrderedStopFromOrderedStopId(stopId: string): Promise<IOrderedStop | null>;
   getPreviousOrderedStopFromOrderedStopId(stopId: string): Promise<IOrderedStop | null>;