Why Nestjs + Firebase Cloud Functions will "Starting Nest application" multiple times - google-cloud-functions

Question

Main problem
Why nestjs on firebase cloud function has to initialize once at firebase serve --only functions, and re-initialize again on the first API request.
Details
This is my main.ts:
import { NestFactory } from '#nestjs/core';
import {
ExpressAdapter,
NestExpressApplication,
} from '#nestjs/platform-express';
import * as express from 'express';
import { HttpsFunction, https, pubsub } from 'firebase-functions';
import { AppModule } from './app/app.module';
const server: express.Express = express();
export const createNestServer = async (expressInstance: express.Express) => {
const adapter = new ExpressAdapter(expressInstance);
const app = await NestFactory.create<NestExpressApplication>(
AppModule,
adapter,
{},
);
app.enableCors();
return app.init();
};
createNestServer(server)
.then((v) => console.log('Nest Ready'))
.catch((err) => console.error('Nest broken', err));
export const api: HttpsFunction = https.onRequest(server);
And app/test/test.service.ts:
import { Injectable } from '#nestjs/common';
#Injectable()
export class TestService {
constructor(
) {
console.log(11, 'constructor');
}
async testApi() {
console.log(22, 'gotten api')
return `Hello World!!!`;
}
}
When I firebase serve --only functions here are the logs:
> firebase serve --only functions
i functions: Watching "/Users/jingles/GitHub/pixelhead-backend" for Cloud Functions...
✔ functions[us-central1-api]: http function initialized (http://localhost:5000/test-application/us-central1/api).
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [NestFactory] Starting Nest application...
> 11 constructor
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [InstanceLoader] AppModule dependencies initialized +41ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [InstanceLoader] DiscoveryModule dependencies initialized +0ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [InstanceLoader] TestModule dependencies initialized +0ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [RoutesResolver] TestController {/test}: +1ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [RouterExplorer] Mapped {/test, GET} route +0ms
> [Nest] 26036 - 27/04/2022, 04:06:31 LOG [NestApplication] Nest application successfully started +1033ms
> Nest Ready
As you can see here Starting Nest application, 11 constructor and Nest Ready. All good.
But when I make a request /test/ this is the log I get:
> [Nest] 26052 - 27/04/2022, 04:06:53 LOG [NestFactory] Starting Nest application...
i functions: Beginning execution of "us-central1-api"
> 11 constructor
> [Nest] 26052 - 27/04/2022, 04:06:53 LOG [InstanceLoader] AppModule dependencies initialized +44ms
> [Nest] 26052 - 27/04/2022, 04:06:53 LOG [InstanceLoader] DiscoveryModule dependencies initialized +1ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [InstanceLoader] TestModule dependencies initialized +0ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [RoutesResolver] TestController {/test}: +1ms
> [Nest] 26036 - 27/04/2022, 04:06:30 LOG [RouterExplorer] Mapped {/test, GET} route +0ms
> 22 gotten api
i functions: Finished "us-central1-api" in ~1s
> [Nest] 26052 - 27/04/2022, 04:06:54 LOG [NestApplication] Nest application successfully started +985ms
> Nest Ready
The problem here you can see, it Starting Nest application again. Also Nest Ready comes after 22 gotten api.
What is the issue?

Answer 1

I believe you have two separates issues here:
Having "Starting Nest application" 2 times: Firebase functions are a serverless environment, that said you have no guarantees on which instance in the cloud will serve your request: an already initialized or not.
The emulator is simply emulating a "cold start" in this case, on the "/test/" request arrival, which means the index code is executed (env boostrapped) and then the request is served.
"Nest Ready comes after 22 gotten api"
This is basically due to these two lines of code:
createNestServer(server)
.then((v) => console.log('Nest Ready'))
.catch((err) => console.error('Nest broken', err));
export const api: HttpsFunction = https.onRequest(server);
the first initializes the Nest app on the Express instance, but does not stop the second line from serving the requests arriving at "/test/" endpoint. The app is probably "half" initialized when the request reaches the code so you don't get any error and the request is served anyways, but in general you should organize your code in a way that firstly bootstraps the Nest app AND THEN serve the requests.
Important note: As we are talking about a serverless environment, each time a request comes in you could be either in a situation in which the Nest app is already initialized or not. You should then organize your code accordingly.
Check here for reference:
https://docs.nestjs.com/faq/serverless