Add warning if not emitting metrics and raiseOnEmptyMetrics is false.

Author: phipag

powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLogger.java

@@ -22,6 +22,9 @@
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import com.amazonaws.services.lambda.runtime.Context;
 
 import software.amazon.cloudwatchlogs.emf.environment.EnvironmentProvider;
@@ -38,6 +41,7 @@
  * library {@link software.amazon.cloudwatchlogs.emf.logger.MetricsLogger}.
  */
 public class EmfMetricsLogger implements MetricsLogger {
+    private static final Logger LOGGER = LoggerFactory.getLogger(EmfMetricsLogger.class);
     private static final String TRACE_ID_PROPERTY = "xray_trace_id";
     private static final String REQUEST_ID_PROPERTY = "function_request_id";
     private static final String COLD_START_METRIC = "ColdStart";
@@ -136,8 +140,12 @@ public void clearDefaultDimensions() {
 
     @Override
     public void flush() {
-        if (raiseOnEmptyMetrics && !hasMetrics.get()) {
-            throw new IllegalStateException("No metrics were emitted");
+        if (!hasMetrics.get()) {
+            if (raiseOnEmptyMetrics) {
+                throw new IllegalStateException("No metrics were emitted");
+            } else {
+                LOGGER.warn("No metrics were emitted");
+            }
         }
         emfLogger.flush();
     }

powertools-metrics/src/test/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLoggerTest.java

@@ -18,8 +18,11 @@
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import java.io.ByteArrayOutputStream;
+import java.io.File;
 import java.io.PrintStream;
 import java.lang.reflect.Method;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
 import java.util.stream.Stream;
 
 import org.junit.jupiter.api.AfterEach;
@@ -310,6 +313,21 @@ void shouldRaiseExceptionOnEmptyMetrics() {
                 .hasMessage("No metrics were emitted");
     }
 
+    @Test
+    void shouldLogWarningOnEmptyMetrics() throws Exception {
+        // Given
+        File logFile = new File("target/metrics-test.log");
+
+        // When
+        // Flushing without adding metrics
+        metricsLogger.flush();
+
+        // Then
+        // Read the log file and check for the warning
+        String logContent = new String(Files.readAllBytes(logFile.toPath()), StandardCharsets.UTF_8);
+        assertThat(logContent).contains("No metrics were emitted");
+    }
+
     @Test
     void shouldClearDefaultDimensions() throws Exception {
         // Given

powertools-metrics/src/test/resources/simplelogger.properties

@@ -0,0 +1,7 @@
+org.slf4j.simpleLogger.logFile=target/metrics-test.log
+org.slf4j.simpleLogger.defaultLogLevel=warn
+org.slf4j.simpleLogger.showDateTime=true
+org.slf4j.simpleLogger.dateTimeFormat=yyyy-MM-dd HH:mm:ss:SSS
+org.slf4j.simpleLogger.showThreadName=false
+org.slf4j.simpleLogger.showLogName=true
+org.slf4j.simpleLogger.showShortLogName=false