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Basic Concepts

Last updated: 2022-04-28 16:31:41

    Serverless Cloud Function (SCF) provides two deployment methods of code deployment and image deployment and supports two function types of event-triggered function and HTTP-triggered function. Different deployment methods and function types require different specifications during code development. This document describes the writing specifications and related concepts of event-triggered function in code deployment. For more information on image deployment and HTTP-triggered function, please see the corresponding documents.

    An SCF event-triggered function involves three basic concepts: execution method, function input parameter, and function return.


    The above concepts correspond respectively to the following in general project development:
    Execution method: corresponds to the main function of the project and is the starting point of program execution.
    Function input parameter: refers to function input parameters in a normal sense. However, in the SCF environment, the input parameters of an entry function are fixed values. For more information, please see Function Input Parameters.
    Function return: corresponds to the returned value of the main function in the project. After the function returns, the code execution ends.

    Execution Method

    When the SCF platform invokes a function, it will first find an execution method as the entry point to execute your code. At this time, you need to set in the format of filename.execution method name.
    For example, if the user-configured execution method is index.handler, the SCF platform will first look for the index file in the code package and find the handler method in the file to start execution.
    In the execution method, you can process the input parameters of the entry function and call other methods in the code arbitrarily. In SCF, the completion of the execution of the entry function or the exception of the execution of the function marks the end of execution.

    Function Input Parameters

    Function input parameters refer to the content that is passed to the function when the function is triggered. Usually, there are two input parameters: event and context. However, the number of input parameters may vary by programming language and environment. For more information, please see Serverless Cloud Function.


    The event parameter is of dict type and contains the basic information that triggers the function. It can be in a platform-defined or custom format. After the function is triggered, the event can be processed inside the code.


    There are two ways to trigger an SCF function:

    1. Trigger by calling TencentCloud API.
    2. Trigger by binding a trigger.
      These two SCF trigger methods correspond to two event formats:
    • TencentCloud API:
      You can freely define a parameter of dict type between the invoker and the function code, where the invoker passes in the data in the format agreed upon, and the function code gets the data in the format.

    You can define a data structure {"key":"XXX"} of dict type, and when the invoker passes in the data {"key":"abctest"}, the function code can get the value abctest through event[key].

    • Trigger:
      SCF can be connected with various Tencent Cloud services such as API Gateway, COS, and CKafka, so you can bind a corresponding Tencent Cloud service trigger to a function. When the function is triggered, the service will pass the event to SCF as the event parameter in a platform-predefined unchangeable format. You can write code based on this format and get information from the event parameter.

    When COS triggers a function, the specific information of the bucket and the file will be passed to the event parameter in JSON format. The processing of the triggering event can be completed by parsing the event information in the function code.

    After understanding the basic usage of event and context input parameters, you should pay attention to the following points when writing function code:

    • To ensure uniformity for each programming language and environment, `event` and `context` should be uniformly encapsulated in the `JSON` data format.
    • Different triggers pass different data structures when triggering functions. For more information, please see Trigger Overview.
    • If the function does not need any input, you can ignore the `event` and `context` parameters in your code.

    Function Return

    The SCF platform will get the returned value after the function is executed and handle according to different trigger type as listed below.

    Trigger Type Handling Method
    Sync triggering
    • If triggered by API Gateway or the TencentCloud API for sync invocation, the function will be triggered synchronously.
    • For a function triggered synchronously, the SCF platform will not return the trigger result during function execution.
    • After the function is executed, the SCF platform will encapsulate the returned value into JSON format and return it to the invoker.
    Async triggering
    • For a function that is triggered asynchronously, the SCF will return the triggering request ID after receiving the triggering event.
    • After the function is executed, the returned value will be encapsulated into JSON format and stored in the log.
    • After the function execution is completed, you can query the log by the request ID in the return to get the returned value of the asynchronously triggered function.

    When the code in a function returns a specific value, it usually returns a specific data structure; for example:

    Runtime Environment Returned Structure Type
    Python Simple or dict data structure
    Node.js JSON Object
    PHP Array structure
    Go Simple data structure or struct with JSON description

    To ensure uniformity for different programming languages and environments, the function return will be uniformly encapsulated in the JSON data format. For example, after SCF gets the returned value of the function in the above runtime environment, it will convert the returned data structure to JSON and return it to the invoker.


    • You should ensure that the returned value of the function can be converted to JSON format. If the object is returned directly and there is no JSON conversion method, SCF will fail when executing JSON conversion and prompt an error.
    • For example, the returned value in the above runtime environment does not need to be converted to JSON format before it is returned; otherwise, the output string will be converted again.

    Exception Handling

    If an exception occurs during testing and executing a function, the SCF platform will handle the exception as much as possible and write the exception information into the log. Exceptions generated by function execution include caught exceptions (handled errors) and uncaught exceptions (unhandled errors).

    Handling method

    You can log in to the SCF console and follow the steps below to test exception handling:

    1. Create a function and copy the following function code without adding any triggers.
    2. Click Test in the console and select the "Hello World" test sample for testing.

    This document provides the following three ways to throw exceptions, and you can choose how to handle exceptions in the code based on your actual needs.


    def always_failed_handler(event,context):
      raise Exception('I failed!')

    This function will throw an exception during execution and return the following error message. The SCF platform will record this error message in the function log.
    File "/var/user/index.py", line 2, in always_failed_handler
    raise Exception('I failed!')
    Exception: I failed!

    Returned error message

    If exception handling and error capture are not performed in your code logic, the SCF platform will capture errors as much as possible such as when your function suddenly crashes and exits during execution. The platform will return a general error message if it cannot capture an error that occurs.
    The table below lists some common errors in code execution:

    Error Scenario Error Message
    raise is used to throw an exception {File "/var/user/index.py", line 2, in always_failed_handler raise Exception('xxx') Exception: xxx}
    The handler does not exist {'module' object has no attribute 'xxx'}
    The dependent module does not exist {global name 'xxx' is not defined}
    Timed out {"time out"}


    The SCF platform stores all the records of function invocations and the outputs of the function code in logs. You can use the printout or log statement in the programming language to generate the output logs for debugging and troubleshooting. For more information, please see Log Search Guide.


    Because of the nature of SCF, you must write your function code in a stateless style. State characteristics in the lifecycle of a function such as local file storage will be destroyed after the function invocation ends.
    Therefore, you are recommended to store persistent states in TDSQL, COS, TencentDB for Memcached, or other cloud storage services.

    Development Process

    For more information on the function development process, please see Getting Started.

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