Start log lines with the Lambda request ID

README.md

@@ -36,11 +36,13 @@ $logger->error('This is an error');
 ```
 
 ```
-INFO	This is an info	{"message":"This is an info","level":"INFO"}
-WARNING	This is a warning	{"message":"This is a warning","level":"WARNING"}
-ERROR	This is an error	{"message":"This is an error","level":"ERROR"}
+8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2	INFO	This is an info	{"message":"This is an info","level":"INFO"}
+8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2	WARNING	This is a warning	{"message":"This is a warning","level":"WARNING"}
+8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2	ERROR	This is an error	{"message":"This is an error","level":"ERROR"}
 ```
 
+This follows the standard format used by official AWS Lambda runtimes. For example the initial request ID is recognized by CloudWatch so that `filter @requestId = '8f507cfc-...'` shows all the logs of a single request/invocation.
+
 Messages under `info` are not logged.
 
 ### Message placeholders

src/StderrLogger.php

@@ -82,6 +82,13 @@ public function log($level, $message, array $context = []): void
 
         $formattedMessage = sprintf("%s\t%s\t%s\n", strtoupper($level), $displayMessage, $this->toJson($this->normalize($data)));
 
+        // Bref sets the ID of the current Lambda invocation. Lambda's own runtimes start their lines with it:
+        // CloudWatch Logs Insights reads it as `@requestId`, like in Lambda's START, END and REPORT lines.
+        $requestId = $_SERVER['LAMBDA_REQUEST_ID'] ?? null;
+        if (is_string($requestId) && $requestId !== '') {
+            $formattedMessage = "$requestId\t$formattedMessage";
+        }
+
         fwrite($this->stream, $formattedMessage);
     }
 

tests/StderrLoggerTest.php

@@ -136,6 +136,22 @@ public function test_with_exception()
         $this->assertStringContainsString('"message":"Test error"', $logs);
     }
 
+    public function test_lines_start_with_the_lambda_request_id()
+    {
+        $_SERVER['LAMBDA_REQUEST_ID'] = '8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2';
+        try {
+            $this->logger->info('Test message');
+        } finally {
+            unset($_SERVER['LAMBDA_REQUEST_ID']);
+        }
+
+        $this->assertLogsMatch(<<<'LOGS'
+8f507cfc-8b35-4e7e-9f26-f2a3a6e7e1a2	INFO	Test message	{"message":"Test message","level":"INFO"}
+
+LOGS
+        );
+    }
+
     private function assertLogsMatch(string $expectedLog): void
     {
         rewind($this->stream);