A REST MVC application typically has some supporting infrastructure. That is to say, MVC is not a complete blueprint on how to structure the entirety of your program's code - only a general rule of thumb of how to assign certain responsibilities.

The library you're using needs initialization, and apparently this code should execute only once. There are several ways to go about it:

1. Initialize the client once before starting up the express server, and then pass in the ready-to-use client for use by route handlers. This may be the easiest to use, but must necessarily delay the .listen() call - so the time until your application starts responding to HTTP may be longer.
2. Use a pattern known as the Singleton to allow route handlers to initialize the client, but only execute the initialization once under the hood. Depending on how exactly the library is initialized (does it return a Promise? does it use a callback?), this may require some careful design - for example, you may need to store and return a Promise instance, so multiple consumers will be calling .then() on the same Promise.

I implemented the Singleton pattern as suggested:

import oracledb from 'oracledb';

class PrivateOraInitSingleton {
  constructor() {
    try {
      oracledb.initOracleClient({libDir: '/usr/local/lib/instantclient_19_8'});
    } catch (err) {
      console.error(err);
      process.exit(1);
    }
  }
}

class OraInitSingleton {
    constructor() {
        throw new Error('Use OraInitSingleton.getInstance()');
    }
    static getInstance() {
        if (!OraInitSingleton.instance) {
            OraInitSingleton.instance = new PrivateOraInitSingleton();
        }
        return OraInitSingleton.instance;
    }
}

export default OraInitSingleton;

Usage:

const object = OraInitSingleton.getInstance();

try {
  connectionPromise = oracledb.getConnection({
    user : process.env.DB_USER,
    password : process.env.DB_PASSWORD,
    connectString : process.env.CONNECT_STRING
  });
} catch (err) {
  console.error(err);
  process.exit(1);
}