Deprecate keep_warm in favor of AWS-native cold start solutions

README.md

@@ -57,7 +57,7 @@
 - [Advanced Settings](#advanced-settings)
     - [YAML Settings](#yaml-settings)
 - [Advanced Usage](#advanced-usage)
-  - [Keeping The Server Warm](#keeping-the-server-warm)
+  - [Keeping The Server Warm (Deprecated)](#keeping-the-server-warm-deprecated)
     - [Serving Static Files / Binary Uploads](#serving-static-files--binary-uploads)
   - [Enabling CORS](#enabling-cors)
   - [Large Projects](#large-projects)
@@ -1239,8 +1239,8 @@ to change Zappa's behavior. Use these at your own risk!
             "token_header": "Authorization", // Optional. Default 'Authorization'. The name of a custom authorization header containing the token that clients submit as part of their requests.
             "validation_expression": "^Bearer \\w+$", // Optional. A validation expression for the incoming token, specify a regular expression.
         },
-        "keep_warm": true, // Create CloudWatch events to keep the server warm. Default true. To remove, set to false and then `unschedule`.
-        "keep_warm_expression": "rate(4 minutes)", // How often to execute the keep-warm, in cron and rate format. Default 4 minutes.
+        "keep_warm": false, // Deprecated. Create CloudWatch events to keep the server warm. Default false. Does not reduce cold start duration. Consider snap_start or provisioned_concurrency instead. See #1451.
+        "keep_warm_expression": "rate(4 minutes)", // How often to execute the keep-warm, in cron and rate format. Default 4 minutes. Only used when keep_warm is true.
         "lambda_description": "Your Description", // However you want to describe your project for the AWS console. Default "Zappa Deployment".
         "lambda_handler": "your_custom_handler", // The name of Lambda handler. Default: handler.lambda_handler
         "layers": ["arn:aws:lambda:<region>:<account_id>:layer:<layer_name>:<layer_version>"], // optional lambda layers
@@ -1317,9 +1317,11 @@ Similarly, you can supply a `zappa_settings.toml` file:
 
 ## Advanced Usage
 
-### Keeping The Server Warm
+### Keeping The Server Warm (Deprecated)
 
-Zappa will automatically set up a regularly occurring execution of your application in order to keep the Lambda function warm. This can be disabled via the `keep_warm` setting.
+> **Deprecated**: `keep_warm` is disabled by default and will be removed in a future version. Investigation ([#1445](https://github.com/zappa/Zappa/issues/1445)) found that `keep_warm` does not reduce cold start duration — only frequency for a single execution environment. Use `snap_start` (45-72% faster cold starts, ~$1.95/mo) or `provisioned_concurrency` (eliminates cold starts entirely, ~$5.40/mo) instead. See [#1451](https://github.com/zappa/Zappa/issues/1451).
+
+Zappa can set up a regularly occurring execution of your application in order to keep the Lambda function warm. This can be enabled via the `keep_warm` setting.
 
 #### Serving Static Files / Binary Uploads
 

tests/test_core.py

@@ -4557,5 +4557,78 @@ def test_upload_to_s3_reraises_unexpected_errors(self):
         z.s3_client.create_bucket.assert_not_called()
 
 
+class TestKeepWarmDeprecation(unittest.TestCase):
+    def setUp(self):
+        self.users_current_region_name = os.environ.get("AWS_DEFAULT_REGION", None)
+        os.environ["AWS_DEFAULT_REGION"] = "us-east-1"
+
+    def tearDown(self):
+        del os.environ["AWS_DEFAULT_REGION"]
+        if self.users_current_region_name is not None:
+            os.environ["AWS_DEFAULT_REGION"] = self.users_current_region_name
+
+    def test_keep_warm_default_false(self):
+        """keep_warm should default to false after deprecation."""
+        zappa_cli = ZappaCLI()
+        zappa_cli.api_stage = "ttt888"
+        zappa_cli.load_settings("test_settings.json")
+        # Default should be False when not explicitly set
+        self.assertFalse(zappa_cli.stage_config.get("keep_warm", False))
+
+    def test_keep_warm_deprecation_warning(self):
+        """schedule() should emit a deprecation warning when keep_warm is enabled."""
+        zappa_cli = ZappaCLI()
+        zappa_cli.api_stage = "ttt888"
+        zappa_cli.load_settings("test_settings.json")
+        zappa_cli.override_stage_config_setting("keep_warm", True)
+        zappa_cli.override_stage_config_setting("events", [])
+        zappa_cli.lambda_name = "test-lambda"
+
+        zappa_cli.zappa = mock.MagicMock()
+        zappa_cli.zappa.lambda_client.get_function.return_value = {
+            "Configuration": {"FunctionArn": "arn:aws:lambda:us-east-1:123:function:test"}
+        }
+
+        with mock.patch("click.echo") as mock_echo:
+            zappa_cli.schedule()
+
+        echo_calls = " ".join(str(c) for c in mock_echo.call_args_list)
+        self.assertIn("deprecated", echo_calls)
+        self.assertIn("snap_start", echo_calls)
+        self.assertIn("provisioned_concurrency", echo_calls)
+
+    def test_keep_warm_no_warning_when_disabled(self):
+        """schedule() should not emit a deprecation warning when keep_warm is false."""
+        zappa_cli = ZappaCLI()
+        zappa_cli.api_stage = "ttt888"
+        zappa_cli.load_settings("test_settings.json")
+        zappa_cli.override_stage_config_setting("keep_warm", False)
+        zappa_cli.override_stage_config_setting("events", [])
+        zappa_cli.lambda_name = "test-lambda"
+
+        with mock.patch("click.echo") as mock_echo:
+            zappa_cli.schedule()
+
+        echo_calls = " ".join(str(c) for c in mock_echo.call_args_list)
+        self.assertNotIn("deprecated", echo_calls)
+
+    def test_keep_warm_callback_deprecation_warning(self):
+        """keep_warm_callback should emit a DeprecationWarning."""
+        import warnings
+
+        from zappa.handler import keep_warm_callback
+
+        mock_context = mock.MagicMock()
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            with mock.patch("zappa.handler.lambda_handler"):
+                keep_warm_callback(event={}, context=mock_context)
+
+            self.assertEqual(len(w), 1)
+            self.assertTrue(issubclass(w[0].category, DeprecationWarning))
+            self.assertIn("deprecated", str(w[0].message))
+
+
 if __name__ == "__main__":
     unittest.main()

zappa/cli.py

@@ -1452,7 +1452,21 @@ def schedule(self):
         for event in events:
             self.collision_warning(event.get("function"))
 
-        if self.stage_config.get("keep_warm", True):
+        keep_warm = self.stage_config.get("keep_warm", False)
+        if keep_warm:
+            click.echo(
+                click.style("Warning!", fg="yellow", bold=True)
+                + " keep_warm is "
+                + click.style("deprecated", bold=True)
+                + " and disabled by default."
+                + " It does not reduce cold start duration — only frequency for a single execution environment."
+                + " Consider using "
+                + click.style("snap_start", bold=True)
+                + " or "
+                + click.style("provisioned_concurrency", bold=True)
+                + " instead."
+                + " See https://github.com/zappa/Zappa/issues/1451"
+            )
             if not events:
                 events = []
 

zappa/handler.py

@@ -993,7 +993,21 @@ def lambda_handler(event, context):  # pragma: no cover
 
 
 def keep_warm_callback(event, context):
-    """Method is triggered by the CloudWatch event scheduled when keep_warm setting is set to true."""
+    """Deprecated: Method is triggered by the CloudWatch event scheduled when keep_warm setting is set to true.
+
+    keep_warm does not reduce cold start duration — only frequency for a single execution environment.
+    Consider using snap_start or provisioned_concurrency instead.
+    See https://github.com/zappa/Zappa/issues/1451
+    """
+    import warnings
+
+    warnings.warn(
+        "keep_warm is deprecated and will be removed in a future version. "
+        "Use snap_start or provisioned_concurrency instead. "
+        "See https://github.com/zappa/Zappa/issues/1451",
+        DeprecationWarning,
+        stacklevel=1,
+    )
     lambda_handler(event={}, context=context)  # overriding event with an empty one so that web app initialization will
     # be triggered.