feat: emit durable execution log to stdout at invocation start

Emits a structured JSON log to stdout at the start of each durable
function invocation, mapping the Lambda request_id to the durable
execution name and ID. The Lambda extension layer uses this to correlate
request IDs with durable executions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lym953

datadog_lambda/durable.py

@@ -2,11 +2,17 @@
 # under the Apache License Version 2.0.
 # This product includes software developed at Datadog (https://www.datadoghq.com/).
 # Copyright 2019 Datadog, Inc.
+import json
 import logging
 import re
+import sys
 
 logger = logging.getLogger(__name__)
 
+# When changing the schema of the durable invocation log, bump this version so
+# the extension can handle compatibility between old and new schemas.
+DURABLE_INVOCATION_LOG_SCHEMA_VERSION = "1.0.0"
+
 
 def _parse_durable_execution_arn(arn):
     """
@@ -47,3 +53,18 @@ def extract_durable_function_tags(event):
         "durable_function_execution_name": execution_name,
         "durable_function_execution_id": execution_id,
     }
+
+
+def emit_durable_execution_log(request_id, execution_name, execution_id):
+    """
+    Emits a structured JSON log to stdout mapping the Lambda request_id to the
+    durable execution name and ID. This is consumed by the Lambda extension layer
+    to correlate request IDs with durable executions.
+    """
+    log = {
+        "request_id": request_id,
+        "durable_execution_name": execution_name,
+        "durable_execution_id": execution_id,
+        "schema_version": DURABLE_INVOCATION_LOG_SCHEMA_VERSION,
+    }
+    print(json.dumps(log), file=sys.stdout, flush=True)

datadog_lambda/wrapper.py

@@ -42,7 +42,7 @@
     tracer,
     propagator,
 )
-from datadog_lambda.durable import extract_durable_function_tags
+from datadog_lambda.durable import extract_durable_function_tags, emit_durable_execution_log
 from datadog_lambda.trigger import (
     extract_trigger_tags,
     extract_http_status_code_tag,
@@ -245,6 +245,12 @@ def _before(self, event, context):
 
             self.trigger_tags = extract_trigger_tags(event, context)
             self.durable_function_tags = extract_durable_function_tags(event)
+            if self.durable_function_tags:
+                emit_durable_execution_log(
+                    context.aws_request_id,
+                    self.durable_function_tags["durable_function_execution_name"],
+                    self.durable_function_tags["durable_function_execution_id"],
+                )
             # Extract Datadog trace context and source from incoming requests
             dd_context, trace_context_source, event_source = extract_dd_trace_context(
                 event,

tests/test_durable.py

@@ -2,11 +2,16 @@
 # under the Apache License Version 2.0.
 # This product includes software developed at Datadog (https://www.datadoghq.com/).
 # Copyright 2019 Datadog, Inc.
+import io
+import json
+import sys
 import unittest
 
 from datadog_lambda.durable import (
     _parse_durable_execution_arn,
     extract_durable_function_tags,
+    emit_durable_execution_log,
+    DURABLE_INVOCATION_LOG_SCHEMA_VERSION,
 )
 
 
@@ -89,3 +94,37 @@ def test_returns_empty_dict_when_durable_execution_arn_cannot_be_parsed(self):
     def test_returns_empty_dict_when_event_is_empty(self):
         result = extract_durable_function_tags({})
         self.assertEqual(result, {})
+
+
+class TestEmitDurableExecutionLog(unittest.TestCase):
+    def _capture_stdout(self, fn):
+        captured = io.StringIO()
+        original = sys.stdout
+        sys.stdout = captured
+        try:
+            fn()
+        finally:
+            sys.stdout = original
+        return captured.getvalue()
+
+    def test_emits_json_with_all_fields(self):
+        output = self._capture_stdout(
+            lambda: emit_durable_execution_log(
+                "req-123", "my-execution", "exec-id-456"
+            )
+        )
+        data = json.loads(output.strip())
+        self.assertEqual(data["request_id"], "req-123")
+        self.assertEqual(data["durable_execution_name"], "my-execution")
+        self.assertEqual(data["durable_execution_id"], "exec-id-456")
+        self.assertEqual(data["schema_version"], DURABLE_INVOCATION_LOG_SCHEMA_VERSION)
+
+    def test_emits_single_json_line(self):
+        output = self._capture_stdout(
+            lambda: emit_durable_execution_log("req-1", "name", "id")
+        )
+        lines = [l for l in output.splitlines() if l.strip()]
+        self.assertEqual(len(lines), 1)
+
+    def test_schema_version_is_correct(self):
+        self.assertEqual(DURABLE_INVOCATION_LOG_SCHEMA_VERSION, "1.0.0")