Waiting for database connection before accepting requests - node.js

Question

The problem that I have is that my express server starts before the database connection is established. People can send requests to the application while the connection is not yet there for some time:
const app = express();
dbClient.connect()
.subscribe(() => console.log('connection established!'));
module.exports = app.listen(8080, () => {
console.log('the server is running');
});
The outcome of this is:
the server is running // some seconds break
connection established! // now everything works properly
How can I start listening to events only after the subscriber has been run?

Answer 1

The only way to ensure you are connected before to listen is to chain it. If you experience trouble about exporting the result is because you are mistaking import/export. Import/export should not have any impact on your system. Every actions should be triggered and not implied.
You should consider putting all your Express handling into a class and then use it in your controller. This way you could handle errors ... As example :
// File a.js
let instance = null;
export default class ServerApi {
constructor() {
if (instance) return instance;
instance = this;
return instance;
}
static getInstance() {
return instance || new ServerApi();
}
startServer(callback) {
const app = express();
dbClient.connect()
.subscribe(() => {
console.log('connection established!');
app.listen(8080, () => {
console.log('the server is running');
callback();
});
});
}
stopServer() { ... }
getServerStatus() { ... }
};
// File controller.js
import ServerApi from 'a.js';
ServerApi.getInstance().startServer(() => {
// handle error
// handle success
});

Answer 2

There are few ways to achieve this.
one a ways to wrap app.listen into observable
const app = express();
const connectApp = (port = 8080) => new Observable(observer => app.listen(port, () => {
observer.next(app);
observer.complete();
});
dbClient.connect()
.pipe(
tap(() => console.log('connection established!')),
mergeMap(() => connectApp()),
tap(() => console.log('the server is running')),
)
.subscribe(() => console.log('Enjoy'));