Global variables values Node.js are missing on App Engine - node.js

Question

I have a Node.js service deployed on App Engine which uses the Dialogflow fulfillment library. The scenario is this: I have an async function which retrieves the credentials using Secret manager and, with that info, calls an API that brings a url instance and a token back. This is a server-to-server authentication (OAuth), so it is the same for all users that access it. I set those values in global variables, like this:
let globalUser = "";
let globalPass = "";
...
async function credentials() {
const credentials = await secretsInstance.getCredentials();
const parsedCredentials = JSON.parse(credentials);
const user = parsedCredentials.user;
const pass = parsedCredentials.pass;
//setting the values to the global variables
globalUser = user;
globalPass = pass;
//call the authentication API - in the callback I set other global variables
await authApiInstance.authenticate(user, pass, callback);
}
After the callback function is called, I set the instance url and token to the global variables.
The token gets expired each 20 minutes, so I need to keep it updated. For that I call a setInterval function in which I call the authApiInstance.authenticate(...)
The problem here is that, when receiving a POST request coming from Dialogflow, I need to call another API that needs that url, which in this stage is empty for the first time, so it throws ECONNREFUSED. Then if I call the server other times, the variable is set.
The logs in GCP are like this:
2020-08-14 23:29:49.078 BRT
"Calling the loadQuestions API
2020-08-14 23:29:49.078 BRT
"The url is: /services/…
2020-08-14 23:29:49.091 BRT
"CATCH: Error: connect ECONNREFUSED 127.0.0.1:80"
2020-08-14 23:29:49.268 BRT
dialogflowGatewayProdxjmztxaet4d8Function execution took 764 ms, finished with status code:
200
2020-08-14 23:29:49.278 BRT
{ message_id: '39045207393', status: 200 }
2020-08-14 23:29:49.289 BRT
"Credentials ok"
2020-08-14 23:29:49.976 BRT
"Url set"
As it can be seen, the credentials and url were set after the API got called, so it didn't have a url to proceed successfully with the call.
I could call the function inside the POST, each time there is a request to guarantee that it will always exist, but the performance would be lost, especially dealing with Chatbots that must be quick.
I also tried the warmup approach, in which theoretically it would be called when deploying and changing the instance (but it could not be called, as by docs):
app.get('/_ah/warmup', (req, res) => {
credentials();
});
How could I approach this? I'm pretty new to Node.js and the server world.
Thanks

Answer 1

credentials(); by itself. no need to do it in express. The issue i would be race condition on the the shared credential.
crude example assuming the event loop has only these script in queue :
let say, you have 2 concurrent users A and B. A request and found the credential expire which in turn request new credential. B request before the credential return from A request, which in turn request another credential. Based on node eventloop, A then get credential_A , B will get credential B. If your third party only allow single credential then A will get an error from api call.
So the approach would be to forward the credential related task to one module, which manages the credential. background task or on request ( get token it expires on request) will face the same race problem. since node doesn't have context of thread, it is simple.
let credential = {}
let isUpdating = false;
const _updateCrediental = (newCrediential){
//map here
}
const _getCredential = async()=> {
try{
if(!updating){
updating = true;
const newCrediential = await apiCall();
updateCrediential(newCrediential);
updating = false;
return credential;
}else{
return false;
}
}catch(err){
throw err;
}
}
export.getCredential = ()=>{
if(credentialIsValid()){
return credential;
}
return __getCredential();
}
/// check the return if it promise type then waaait for it if its false then wait for certain time and check again.
An improvement to this would be using event to instead of using timeout.
I myself would prefer work with database as well as you might want to log credential generation as well. Most database promise certain kind of transaction or locking. (feel safer)