Warn when flushed messages will be lost due to ALC

Author: ConnorKirk

Diff for: packages/logger/src/Logger.ts

@@ -1451,6 +1451,14 @@ class Logger extends Utility implements LoggerInterface {
         )
       );
     }
+    const AlcLogLevel =
+      this.getEnvVarsService().getAwsLogLevel() as keyof typeof LogLevelThreshold;
+
+    if (LogLevelThreshold[AlcLogLevel] > this.#bufferConfig.bufferAtVerbosity) {
+      this.#warnOnce(
+        'Advanced Loggging Controls (ALC) Log Level is less verbose than Log Buffering Log Level. Some logs might be missing.'
+      );
+    }
 
     this.#buffer?.delete(traceId);
   }

Diff for: packages/logger/tests/unit/logBuffer.test.ts

@@ -92,7 +92,7 @@ describe('Buffer logs', () => {
     );
   });
 
-  it('outputs a warning when the Advanced Logging Configuration Log Level is higher than the Log Buffering Log Level', () => {
+  it('outputs a warning when the Advanced Logging Configuration Log Level is less verbose than the Log Buffering Log Level', () => {
     // Assemble
     process.env.AWS_LAMBDA_LOG_LEVEL = 'INFO';
     const logger = new Logger({
@@ -114,6 +114,29 @@ describe('Buffer logs', () => {
     );
   });
 
+  it('When the buffer is flushed it outputs a warning if the Advanced Logging Configuration Log Level is less verbose than the Log Buffering Log Level', () => {
+    // Assemble
+    process.env.AWS_LAMBDA_LOG_LEVEL = 'INFO';
+    const logger = new Logger({
+      logLevel: LogLevel.DEBUG,
+      logBufferOptions: { enabled: true, bufferAtVerbosity: 'DEBUG' },
+    });
+
+    // Act
+    logger.debug('This is a debug');
+    logger.flushBuffer();
+
+    // Assess
+    expect(console.warn).toHaveLogged(
+      expect.objectContaining({
+        message: expect.stringContaining(
+          'Advanced Loggging Controls (ALC) Log Level is less verbose than Log Buffering Log Level. Some logs might be missing.'
+        ),
+        level: LogLevel.WARN,
+      })
+    );
+  });
+
   it('outputs a warning when there is an error buffering the log', () => {
     // Prepare
     const logger = new Logger({ logBufferOptions: { maxBytes: 100 } });