laf/docs/en/guide/function/trigger.md

title
Cloud Function Triggers

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A trigger is a way to initiate the execution of a function. We can bind triggers to cloud functions to control when they are invoked.

::: info Currently, triggers only support the scheduled trigger type. :::

Creating a Trigger

Step 1: Click on the trigger button on the right side of the function list.

Step 2: Create a new trigger.

Step 3: Enter a name for the trigger for easy search and management in the future.

Step 4: Select the type of trigger. Currently, only scheduled triggers are supported.

Step 5: Choose the execution strategy for the scheduled task. Here, you can use cron expressions.

If you are not familiar with cron expressions, you can click on the link below to choose from three preset options.

Alternatively, you can click on the More Examples button to find the scheduled strategy you need or generate your own expression using the Online Cron Expression website.

A specific example

In the development process of WeChat Official Accounts, it is usually necessary to use a central control server to obtain and refresh the access_token. The access_token used by other business logic servers should all come from this central control server, and should not be refreshed individually. Otherwise, conflicts may occur, resulting in the access_token being overwritten and affecting the business. Generally, the validity period of the access_token is 2 hours, and this problem can be solved well using a scheduled trigger.

1. First, let's create a cloud function named get-access-token.

import cloud from '@lafjs/cloud'
import axios from 'axios'

const AT_BASEURL = 'https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential'
const APP_ID = 'your appid'
const APP_SECRET = 'your app secret'

export async function main(ctx: FunctionContext) {

  try {
    const request_url = `${AT_BASEURL}&appid=${APP_ID}&secret=${APP_SECRET}`
    const { data: { access_token, expires_in, errcode, errmsg } } = await axios.get(request_url)

    if (errcode) return { errcode, errmsg }
    
    // write into cloud.shared
    cloud.shared.set('_access_token', access_token)
    cloud.shared.set('_expired_time', expires_in)

  } catch (error) {
    return { 
      errcode: 500,
      err_msg: 'Get access token error: ' + error.toString() }
  }
  
}

In the above code, we follow the Interface Call Request Instructions in the WeChat official documentation to request the access_token and store it in cloud.shared. Other business functions can read it at any time.

2. Use a trigger

Do you remember the process of creating a trigger just now? We only need to select the associated function as get-access-token and set the Cron expression to 2 hours to request the WeChat API every two hours to refresh the access_token to ensure validity.