[Serving] Support default env var override behavior for connection (#713)

# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [ ] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [ ] Title of the pull request is clear and informative.
- [ ] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

---------

Co-authored-by: Ying Chen <2601502859@qq.com>

Author: YingChen1996

docs/how-to-guides/deploy-a-flow/deploy-using-dev-server.md

@@ -14,6 +14,13 @@ an example to show how to deploy a flow.
 Please ensure you have [create the connection](../manage-connections.md#create-a-connection) required by flow, if not, you could
 refer to [Setup connection for web-classification](https://github.com/microsoft/promptflow/tree/main/examples/flows/standard/web-classification).
 
+Note: We will use relevant environment variable ({connection_name}_{key_name}) to override connection configurations in 
+serving mode, white space in connection name will be removed directly from environment variable name. For instance, 
+if there is a custom connection named 'custom_connection' with a configuration key called 'chat_deployment_name,' the 
+function will attempt to retrieve 'chat_deployment_name' from the environment variable 
+'CUSTOM_CONNECTION_CHAT_DEPLOYMENT_NAME' by default. If the environment variable is not set, it will use the original 
+value as a fallback.
+
 
 The following CLI commands allows you serve a flow folder as an endpoint. By running this command, a [flask](https://flask.palletsprojects.com/en/) app will start in the environment where command is executed, please ensure all prerequisites required by flow have been installed.
 ```bash

src/promptflow/promptflow/_sdk/_serving/flow_invoker.py

@@ -12,6 +12,7 @@
     get_local_connections_from_executable,
     resolve_connections_environment_variable_reference,
     update_environment_variables_with_connections,
+    override_connection_config_with_environment_variable,
 )
 from promptflow._sdk.entities._connection import _Connection
 from promptflow.executor import FlowExecutor
@@ -61,6 +62,7 @@ def _init_connections(self, connection_provider):
         else:
             raise UnsupportedConnectionProvider(connection_provider)
 
+        override_connection_config_with_environment_variable(self.connections)
         resolve_connections_environment_variable_reference(self.connections)
         update_environment_variables_with_connections(self.connections)
         logger.info(f"Promptflow get connections successfully. keys: {self.connections.keys()}")

src/promptflow/promptflow/_sdk/_utils.py

@@ -317,6 +317,29 @@ def _match_env_reference(val: str):
     return name
 
 
+def override_connection_config_with_environment_variable(connections: Dict[str, dict]):
+    """
+    The function will use relevant environment variable to override connection configurations. For instance, if there
+    is a custom connection named 'custom_connection' with a configuration key called 'chat_deployment_name,' the
+    function will attempt to retrieve 'chat_deployment_name' from the environment variable
+    'CUSTOM_CONNECTION_CHAT_DEPLOYMENT_NAME' by default. If the environment variable is not set, it will use the
+    original value as a fallback.
+    """
+    logger = logging.getLogger(LOGGER_NAME)
+    for connection_name, connection in connections.items():
+        values = connection.get("value", {})
+        for key, val in values.items():
+            connection_name = connection_name.replace(" ", "_")
+            env_name = f"{connection_name}_{key}".upper()
+            if env_name not in os.environ:
+                continue
+            values[key] = os.environ[env_name]
+            logger.info(
+                f"Connection {connection_name}'s {key} is overridden with environment variable {env_name}"
+            )
+    return connections
+
+
 def resolve_connections_environment_variable_reference(connections: Dict[str, dict]):
     """The function will resolve connection secrets env var reference like api_key: ${env:KEY}"""
     for connection in connections.values():

src/promptflow/tests/sdk_cli_test/unittests/test_utils.py

@@ -30,6 +30,7 @@
     generate_flow_tools_json,
     refresh_connections_dir,
     resolve_connections_environment_variable_reference,
+    override_connection_config_with_environment_variable,
     snake_to_camel,
 )
 
@@ -87,6 +88,29 @@ def test_resolve_connections_environment_variable_reference(self):
         assert connections["test_connection"]["value"]["api_base"] == "BASE"
         assert connections["test_custom_connection"]["value"]["key"] == "CUSTOM_VALUE"
 
+    def test_override_connection_config_with_environment_variable(self):
+        connections = {
+            "test_connection": {
+                "type": "AzureOpenAIConnection",
+                "value": {
+                    "api_key": "KEY",
+                    "api_base": "https://gpt-test-eus.openai.azure.com/",
+                },
+            },
+            "test_custom_connection": {
+                "type": "CustomConnection",
+                "value": {"key": "value1", "key2": "value2"},
+            },
+        }
+        with mock.patch.dict(
+            os.environ, {"TEST_CONNECTION_API_BASE": "BASE", "TEST_CUSTOM_CONNECTION_KEY": "CUSTOM_VALUE"}
+        ):
+            override_connection_config_with_environment_variable(connections)
+        assert connections["test_connection"]["value"]["api_key"] == "KEY"
+        assert connections["test_connection"]["value"]["api_base"] == "BASE"
+        assert connections["test_custom_connection"]["value"]["key"] == "CUSTOM_VALUE"
+        assert connections["test_custom_connection"]["value"]["key2"] == "value2"
+
     def test_generate_flow_tools_json(self) -> None:
         # call twice to ensure system path won't be affected during generation
         for _ in range(2):