diff --git a/src/repositories/GetterRepository.ts b/src/repositories/GetterRepository.ts
index c9f473c..8ab048d 100644
--- a/src/repositories/GetterRepository.ts
+++ b/src/repositories/GetterRepository.ts
@@ -17,7 +17,23 @@ export interface GetterRepository {
   getEtasForShuttleId(shuttleId: string): Promise<IEta[]>;
   getEtasForStopId(stopId: string): Promise<IEta[]>;
   getEtaForShuttleAndStopId(shuttleId: string, stopId: string): Promise<IEta | null>;
-  subscribeToEtaChanges(listener: (eta: IEta) => void): Promise<void>;
+
+  /**
+   * Subscribe to all updates in ETA data.
+   * The subscriber persists even if the ETA data does not
+   * exist within the repository, and may fire again
+   * if ETA data is restored.
+   * @param listener
+   */
+  subscribeToEtaChanges(
+    listener: (eta: IEta) => void,
+  ): Promise<void>;
+
+  /**
+   * Unsubscribe from all ETA updates for the given callback.
+   * Callback must be passed by reference.
+   * @param listener
+   */
   unsubscribeFromEtaChanges(listener: (eta: IEta) => void): Promise<void>;
 
   getOrderedStopByRouteAndStopId(routeId: string, stopId: string): Promise<IOrderedStop | null>;
diff --git a/test/repositories/UnoptimizedInMemoryRepositoryTests.test.ts b/test/repositories/UnoptimizedInMemoryRepositoryTests.test.ts
index f130ba5..5f1beca 100644
--- a/test/repositories/UnoptimizedInMemoryRepositoryTests.test.ts
+++ b/test/repositories/UnoptimizedInMemoryRepositoryTests.test.ts
@@ -225,6 +225,14 @@ describe("UnoptimizedInMemoryRepository", () => {
     });
   });
 
+  describe("subscribeToEtaChanges", () => {
+    test("notifies listener if the eta has changed")
+  });
+
+  describe("unsubscribeFromEtaChanges", () => {
+
+  });
+
   describe("getOrderedStopByRouteAndStopId", () => {
     test("gets an ordered stop by route ID and stop ID", async () => {
       const mockOrderedStops = generateMockOrderedStops();
@@ -711,4 +719,4 @@ describe("UnoptimizedInMemoryRepository", () => {
       expect(result).toEqual([]);
     });
   });
-});
\ No newline at end of file
+});