Managing MySQL Connections

Connection Strings

It is not a good idea to store your MySQL connection details in your source code:

1. It is often different between environments. Storing it inline as part of the code will make it harder to deploy & manage your application.

2. It often contains secret values such passwords. Even if your code is never intentionally made public, there is a risk that someone gains access.

Instead, you should store your connection details in an environment variable. If you put your connection string in an environment variable called DATABASE_URL, it will be automatically detected by @databases/mysql, but you can also manually pass in your connection string from an environment variable to use a custom name:

import createConnectionPool from '@databases/mysql';

createConnectionPool(process.env.MY_CUSTOM_DATABASE_ENV_VAR);

There are many other ways you can configure your connection. You can find the full list of options is in Connection Options.

Connection Pools

createConnectionPool creates a "pool" of connections to the database. Creating a connection to a MySQL database takes time, so to ensure your application remains fast, we keep a "pool" of connections (10 connections by default), and when you run a query or transaction, we allocate one of these connections to run your query.

This means that it is very important to only create a single connection pool for your database for the entire lifetime of your application. The best way to do this is to create a single file for your connection:

// database.ts

import createConnectionPool, {sql} from '@databases/mysql';

export {sql};

const db = createConnectionPool();
export default db;

Then instead of referencing '@database/mysql' in other files you can use:

import db, {sql} from './database';

Disposing of connections

In a typical server application that is intended to run continuously, you can normally leave this connection pool running. If you are building a CLI or short lived process, you should make sure you disconnect. If you don't do this you will have two problems:

1. The node process will not exit automatically if there is an active connection pool.

2. If you forcibly kill the process, it will still use up a connection on your MySQL database until the connection times out.

To disconnect, you can add something like:

process.once('SIGTERM', () => {
  db.dispose().catch((ex) => {
    console.error(ex);
  });
});