Add agent debug logging for feature flag onboarding

products/feature-flagging/feature-flagging-agent/src/main/java/com/datadog/featureflag/FeatureFlaggingSystem.java

@@ -33,10 +33,13 @@ public static synchronized void start(final SharedCommunicationObjects sco) {
       LOGGER.debug("Feature Flagging system disabled");
       return;
     }
+    LOGGER.debug("Feature Flagging enabled; env={}", config.getEnv());
     final FeatureFlagsBackend backend = new FeatureFlagsBackend(sco, config);
     try {
       backend.start();
     } catch (final RuntimeException | Error e) {
+      LOGGER.debug(
+          "Feature Flagging system initialization failed: {}", e.getClass().getSimpleName());
       backend.close();
       throw e;
     }

products/feature-flagging/feature-flagging-lib/src/main/java/com/datadog/featureflag/RemoteConfigServiceImpl.java

@@ -39,6 +39,7 @@ public synchronized void start() {
       return;
     }
     this.started = true;
+    LOGGER.debug("Feature Flagging starting remote_config configuration source");
     this.configurationPoller.addCapabilities(CAPABILITY_FFE_FLAG_CONFIGURATION_RULES);
     // Listen as serialized event to prevent depending on the deserializer from agent
     this.configurationPoller.addListener(FFE_FLAGS, content -> content, this);
@@ -70,6 +71,7 @@ public synchronized void close() {
       return;
     }
     this.started = false;
+    LOGGER.debug("Feature Flagging remote_config configuration source stopped");
     this.configurationPoller.removeCapabilities(CAPABILITY_FFE_FLAG_CONFIGURATION_RULES);
     this.configurationPoller.removeListeners(FFE_FLAGS);
     this.subscribers.clear();
@@ -94,5 +96,14 @@ public void accept(
         LOGGER.debug("Feature Flags configuration subscriber failed", e);
       }
     }
+    // Logged last: a diagnostic must not get in the way of the delivery.
+    if (content == null) {
+      LOGGER.debug("Feature Flagging remote_config configuration removed: key={}", configKey);
+    } else {
+      LOGGER.debug(
+          "Feature Flagging remote_config configuration received: key={}, bytes={}",
+          configKey,
+          content.length);
+    }
   }
 }

products/feature-flagging/feature-flagging-lib/src/test/java/com/datadog/featureflag/RemoteConfigServiceImplTest.java

@@ -5,6 +5,7 @@
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -15,6 +16,10 @@
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
 import datadog.remoteconfig.Capabilities;
 import datadog.remoteconfig.ConfigurationDeserializer;
 import datadog.remoteconfig.ConfigurationPoller;
@@ -28,8 +33,13 @@
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicBoolean;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 import org.mockito.ArgumentCaptor;
+import org.slf4j.LoggerFactory;
 
 class RemoteConfigServiceImplTest {
   private static final byte[] DOCUMENT = "{\"flags\":{}}".getBytes(UTF_8);
@@ -178,4 +188,72 @@ void closeUnregistersTheProductAndDropsSubscribers() {
     verify(poller, never()).stop();
     assertEquals(0, received.size());
   }
+
+  @Nested
+  class Diagnostics {
+    private final Logger logger = (Logger) LoggerFactory.getLogger(RemoteConfigServiceImpl.class);
+    private final ListAppender<ILoggingEvent> appender = new ListAppender<>();
+    private Level previousLevel;
+    private RemoteConfigServiceImpl service;
+
+    @BeforeEach
+    void captureLogs() {
+      previousLevel = logger.getLevel();
+      logger.setLevel(Level.DEBUG);
+      appender.start();
+      logger.addAppender(appender);
+      service = new RemoteConfigServiceImpl(mock(ConfigurationPoller.class));
+    }
+
+    @AfterEach
+    void restoreLogging() {
+      logger.detachAppender(appender);
+      appender.stop();
+      logger.setLevel(previousLevel);
+    }
+
+    @Test
+    void reportsLifecycle() {
+      service.start();
+      service.close();
+
+      assertLogged("starting remote_config configuration source");
+      assertLogged("remote_config configuration source stopped");
+    }
+
+    @Test
+    void reportsReceivedConfiguration() {
+      service.accept("config", DOCUMENT, mock(PollingRateHinter.class));
+
+      assertLogged("configuration received: key=config, bytes=" + DOCUMENT.length);
+    }
+
+    @Test
+    void reportsRemovedConfiguration() {
+      service.accept("config", null, mock(PollingRateHinter.class));
+
+      assertLogged("configuration removed: key=config");
+    }
+
+    @Test
+    void reportsTheDocumentOnlyOnceItIsDelivered() {
+      final AtomicBoolean loggedBeforeDelivery = new AtomicBoolean(true);
+      service.subscribe(
+          content ->
+              loggedBeforeDelivery.set(
+                  appender.list.stream()
+                      .anyMatch(e -> e.getFormattedMessage().contains("configuration received"))));
+
+      service.accept("config", DOCUMENT, mock(PollingRateHinter.class));
+
+      assertFalse(loggedBeforeDelivery.get(), "The log must not delay the delivery");
+      assertLogged("configuration received: key=config, bytes=" + DOCUMENT.length);
+    }
+
+    private void assertLogged(final String text) {
+      assertTrue(
+          appender.list.stream().anyMatch(e -> e.getFormattedMessage().contains(text)),
+          "Missing log: " + text);
+    }
+  }
 }