在app.yaml上指定但未在main.go上获取的环境变量

Question

I've specified my environment variables in app.yaml and it's being fetched when I'm running it on my local machine but once I have it deployed - it's not fetching it.

Here's how I've set it up:

application: some-application
version: 1
runtime: go
api_version: go1
threadsafe: true

handlers: 
- url: /.*
  script: main.go
  secure: always

env_variables: 
  ENVIRONMENT_VAR1: 'some key'
  ENVIRONMENT_VAR2: 'some key'
  ENVIRONMENT_VAR3: 'some key'

And I'm using os.Getenv("ENVIRONMENT_VAR1") to retrieve the key and it works when I run it on my local but fails to work when deployed on google app engine.

Answer 1

It is undocumented in the official doc: Defining environment variables, but environment variables defined in app.yaml in production are not set before init() functions are called. They are only set before serving the first request.

This issue was reported here. Quoting an AppEngine engineer's answer:

Right. Due to the nature of the implementation, environment variables are not available in your init functions, unfortunately. Although they are not tied to requests, they are not set until after all the init functions have already run, but are set before the first request is handled.

As a result, you could use a sync.DoOnce in your main handler to perform any actions required based on the value of an environment variable since it will be properly set by that time.

Example of achieving this with Once.Do():

var once = sync.Once{}

func MainHandler(w http.ResponseWriter, r *http.Request) {
    once.Do(mysetup)
    // do your regular stuff here
}

func mysetup() {
    // This function is executed only once. Read / use env vars here.
    var1 := os.Getenv("ENVIRONMENT_VAR1")
    _ = var1 // use var1
}