Function Compute:Function instance lifecycle hooks

How it works

A function instance moves through three phases:

• Initialization: When Function Compute creates or unfreezes an instance, it calls the Initializer hook (if configured) before routing any traffic to the instance. Use this phase to load models, warm up connection pools, or run any setup that must complete before the first request.

• Invocation: The instance handles function invocations. The main handler routes requests to your business logic.

• Shutdown: Before Function Compute freezes or destroys an instance, it calls the PreStop hook (if configured). Use this phase to flush buffers, close connections, or clean up resources.

Lifecycle hooks are billed at the same rate as normal invocations. Their logs appear in Function Logs and Advanced Logs, but not in the Invocation Requests list.

For background on instance lifecycle configuration, see Configure instance lifecycle.

Hook interface reference

Your custom runtime HTTP server must expose the following endpoints:

Both hooks receive the common request headers, including x-fc-request-id.

Implement lifecycle hooks

The following Python 3.10 Flask example shows all three endpoints in a single HTTP server: the Initializer hook, the main invocation handler, and the PreStop hook.

import os
from flask import Flask, request

app = Flask(__name__)

# Initializer hook — runs once when the instance starts
@app.route('/initialize', methods=['POST'])
def init_invoke():
    rid = request.headers.get('x-fc-request-id')
    print("FC Initialize Start RequestId: " + rid)
    # Add setup logic here: load models, open connections, etc.
    print("FC Initialize End RequestId: " + rid)
    return "OK"

# Main invocation handler
@app.route('/', defaults={'path': ''})
@app.route('/<path:path>', methods=['GET', 'POST', 'PUT', 'DELETE'])
def hello_world(path):
    rid = request.headers.get('x-fc-request-id')
    print("FC invoke Start RequestId: " + rid)
    # Add business logic here
    print("FC invoke End RequestId: " + rid)
    return "Hello, World!", 200, [('Function-Name', os.getenv('FC_FUNCTION_NAME'))]

# PreStop hook — runs before the instance is frozen or destroyed
@app.route('/pre-stop', methods=['GET'])
def prestop_invoke():
    rid = request.headers.get('x-fc-request-id')
    print("FC PreStop Start RequestId: " + rid)
    # Add cleanup logic here: flush buffers, close connections, etc.
    print("FC PreStop End RequestId: " + rid)
    return "OK"


if __name__ == '__main__':
    app.run(host='0.0.0.0', port=9000)

The PreStop hook follows the same implementation pattern as the Initializer hook.

# Scenario 1: unhandled exception
@app.route('/initialize', methods=['POST'])
def init():
    raise Exception("initialization failed")
    return "OK", 200, []

# Scenario 2: explicit non-2xx response
@app.route('/initialize', methods=['POST'])
def init():
    return "OK", 404, []

Error handling

Function Compute responds differently to errors depending on which hook fails and what status code it returns.

Configure lifecycle hooks

1. Log on to the Function Compute console. In the left-side navigation pane, click Functions.

2. In the top navigation bar, select a region. On the Functions page, click the function you want to configure.

3. Click the Configuration tab. In the Instance Configuration section, click Modify.

4. In the Instance Configuration panel, set the hook type and timeout period, then click Deploy.

5. Implement the hook logic in your code. Click Deploy above the code editor, then click Test Function.

View lifecycle hook logs

1. Log on to the Function Compute console. In the left-side navigation pane, click Functions.

2. Select a region and click the function you want to inspect.

3. Click the Test tab, click Test Function, then choose Logs > Function Logs.

Function Logs displays both invocation logs and Initializer hook logs in chronological order. The following example shows an Initializer execution followed by a function invocation:

2024-06-26 10:59:23FC Initialize Start RequestId: 529eab23-9b3a-4ffc-88c8-9a686*******
2024-06-26 10:59:23FC Initialize End RequestId: 529eab23-9b3a-4ffc-88c8-9a686*******
2024-06-26 10:59:25FC Invoke Start RequestId: 1-667b840c-15c49df0-b7dc1*******
2024-06-26 10:59:25FC Invoke End RequestId: 1-667b840c-15c49df0-b7dc1*******

PreStop hook logs may not appear immediately because Function Compute caches the instance for a period before destroying it. To trigger the PreStop hook on demand, update the function's configuration or code, then check Function Logs:

2024-06-26 11:04:33FC PreStop Start RequestId: c4385899-f071-490e-a8b7-e33c5*******
2024-06-26 11:04:33FC PreStop End RequestId: c4385899-f071-490e-a8b7-e33c5*******