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Function Compute:Event handlers

Last Updated:Jun 26, 2023

This topic describes the structure of event handlers in a custom runtime. This topic also provides examples on how to use event handlers in a custom runtime and answers to commonly asked questions.

Background information

In a custom runtime, Function Compute forwards the common headers, the request body, the POST method, the /invoke path, and the /initialize path to your HTTP server. You can build an input parameter that is similar to the context parameter in an official runtime based on the information in common headers. You can also build an input parameter that is similar to the event parameter in an official runtime based on the request body of the invoked function. fccustomeruntimeruntime

Signatures for event handlers

If a function uses an event handler, the HTTP server needs to implement the logic that corresponds to the /invoke path and the POST method. Function Compute automatically routes requests to POST /invoke.

PathRequestExpected response
POST /invoke
Important Content-Type must be application/octet-stream.
Response body: the returned value of the function handler.

Sample code

In the following sample code, the Flask web framework that is written in Python 3.10 runtime is used.

from flask import Flask
from flask import request
import sys

REQUEST_ID_HEADER = 'x-fc-request-id'

app = Flask(__name__)

@app.route("/invoke", methods = ["POST"])
def hello_world():
        rid = request.headers.get(REQUEST_ID_HEADER)
        print("FC Invoke Start RequestId: " + rid)

        data =
        print("FC Invoke End RequestId: " + rid)
        return "Hello, World!", 200, [()]

if __name__ == '__main__':'',port=9000)
In the preceding sample code:
  • @app.route("/invoke", methods = ["POST"]): adds the hello_world function that uses the /invoke path and the POST method by using the Flask web framework,
  • rid = request.headers.get(REQUEST_ID_HEADER): obtains the request ID in the request header.
  • data = obtains the request body.
  • return "Hello, World!": sends the request body.
Important Event handlers cannot return status code or response headers. For example, if you use the Flask web framework, the status code and response headers returned by return "Hello, World!", 400, [('Author', 'Aliyun-FC')] do not take effect.

Examples of other programming languages

You can use Serverless Devs to migrate your applications to Function Compute with a few clicks. The following example shows how to use Serverless Devs to deploy and invoke a function in an efficient manner. You can modify the sample code based on your business requirements.