Closes 1363

Add a local cache to AppConfigProvider to handle the scenario where the maxAge is expired, but the AppConfig session still has the latest configuration. In this case, AppConfig returns an empty value. We don't want to return empty values to our callers.

This uses the same data structure we have in place to track NextPollConfigurationToken.

This approach roughly the Python library's behavior.

Author: brnkrygs

Diff for: packages/parameters/src/appconfig/AppConfigProvider.ts

@@ -13,6 +13,7 @@ import type {
 class AppConfigProvider extends BaseProvider {
   public client: AppConfigDataClient;
   protected configurationTokenStore: Map<string, string> = new Map();
+  protected valueStore: Map<string, Uint8Array> = new Map();
   private application?: string;
   private environment: string;
 
@@ -79,6 +80,10 @@ class AppConfigProvider extends BaseProvider {
      * The new AppConfig APIs require two API calls to return the configuration
      * First we start the session and after that we retrieve the configuration
      * We need to store { name: token } pairs to use in the next execution
+     * We also need to store { name : value } pairs because AppConfig returns
+     * an empty value if the session already has the latest configuration
+     * but, we don't want to return an empty value to our callers.
+     * {@link https://docs.aws.amazon.com/appconfig/latest/userguide/appconfig-retrieving-the-configuration.html}
      **/
 
     if (!this.configurationTokenStore.has(name)) {
@@ -106,14 +111,25 @@ class AppConfigProvider extends BaseProvider {
     });
 
     const response = await this.client.send(getConfigurationCommand);
-    
+
     if (response.NextPollConfigurationToken) {
       this.configurationTokenStore.set(name, response.NextPollConfigurationToken);
     } else {
       this.configurationTokenStore.delete(name);
     }
 
-    return response.Configuration;
+    /** When the response is not empty, stash the result locally before returning
+     * See AppConfig docs: 
+     * {@link https://docs.aws.amazon.com/appconfig/latest/userguide/appconfig-retrieving-the-configuration.html}
+     **/
+    if (response.Configuration !== undefined && response.Configuration?.length > 0 ) {
+      this.valueStore.set(name, response.Configuration);
+
+      return response.Configuration;
+    }
+
+    // Otherwise, use a stashed value
+    return this.valueStore.get(name);
   }
 
   /**

Diff for: packages/parameters/tests/unit/AppConfigProvider.test.ts

@@ -225,6 +225,51 @@ describe('Class: AppConfigProvider', () => {
       // Act & Assess
       await expect(provider.get(name)).rejects.toThrow();
     });
+
+    test('when session returns an empty configuration on the second call, it returns the last value', async () => {
+
+      client.reset();
+
+      // Prepare
+      const options: AppConfigProviderOptions = {
+        application: 'MyApp',
+        environment: 'MyAppProdEnv',
+      };
+      const provider = new AppConfigProvider(options);
+      const name = 'MyAppFeatureFlag';
+
+      const fakeInitialToken = 'aW5pdGlhbFRva2Vu';
+      const fakeNextToken1 = 'bmV4dFRva2Vu';
+      const fakeNextToken2 = 'bmV4dFRva2Vq';
+      const mockData = encoder.encode('myAppConfiguration');
+
+      client
+        .on(StartConfigurationSessionCommand)
+        .resolves({
+          InitialConfigurationToken: fakeInitialToken,
+        })
+        .on(GetLatestConfigurationCommand)
+        .resolvesOnce({
+          Configuration: mockData,
+          NextPollConfigurationToken: fakeNextToken1,
+        })
+        .resolvesOnce({
+          Configuration: undefined,
+          NextPollConfigurationToken: fakeNextToken2,
+        });
+
+      // Act
+
+      // Load local cache
+      const result1 = await provider.get(name, { forceFetch: true });
+
+      // Read from local cache, given empty response from service
+      const result2 = await provider.get(name, { forceFetch: true });
+
+      // Assess
+      expect(result1).toBe(mockData);
+      expect(result2).toBe(mockData);
+    });
   });
 
   describe('Method: _getMultiple', () => {