NodeJS class with Promise in method - node.js

Question

I am starting with NodeJS, been writing some code for a couple days. Right now I am creating a new Winston Transport, in order to use MSSQL or OracleDB as the log destination.
I decided to create a DB class to manage connections, inserts, etc to the DB.
My idea is to declare a connection (null) in the constructor, and then have methods to create the connection, insert, etc.
However, as connecting is async, and Winston transport calls the log method, I need to create the connection, make the insert and close (or leave open the connection).
So I went with this:
class db {
constructor(){
this.connection = null;
}
connect(){
return new Promise(function(resolve, reject){
if (this.connection == null){
if (config.Logging.DB.type == 'mssql'){
const dbOptions = {
user: config.Logging.DB.user,
password: config.Logging.DB.password,
server: config.Logging.DB.mssql.server,
database: config.Logging.DB.mssql.database,
options: {
encrypt: config.Logging.DB.encrypt
}
};
this.connection = new mssql.ConnectionPool(dbOptions, err => {
if (err) reject('Can\'t establish a DB connection.');
this.connection.connect(err => {
if (err) reject(err);
resolve();
});
});
}
}else{
resolve();
}
});
}
insert(query){
if (config.Logging.DB.type == 'mssql'){
this.connection.request().query(query, err => {
console.log(err);
});
}
}
}
My idea is to create a db class instance upon starting the app, and then use that open connection to make any inserts (each time winston calls the log method), like this:
const db = require('./functions/db');
const db_instance = new db();
//Custom DB transport
class DBTransport extends Transport {
constructor(opts){
super(opts);
//db.getConnection();
}
log(info, callback) {
//write
db_instance.connect().then(()=>{
db_instance.insert(`insert into logs (message) values ("${info.message}")`);
}).catch((err)=>{
console.log(err);
});
callback();
}
}
But this isn't working at all. First of all I get an error as in the db class connect method I am trying to access 'this.connection' inside a the promise and I am not being able to.
And I also want to keep the connection open, as winston may call the log method lots of times (I have a process that will call the log method over 5K times).
I did try using some of the available winston-mssql transports out there... but they are all updated, and not working with the current version of winston.
Can anyone help?

Answer 1

You'll need to save the promise outside of your connect function and return that memoized promise each time like this:
class DBTransport {
connect() {
if (!this._pool) {
// Memoize (cache) the connection so you don't have to remake it every time
this._pool = new Promise(function(resolve, reject) {
const pool = new mssql.ConnectionPool(dbOptions);
return pool.connect().then(function() {
return pool;
});
});
}
return this._pool;
}
insert(query) {
// Always refetch your connection
return this.connect().then(function(pool) {
return pool.request().query(query);
})
}
}