Should I keep Sequelize instance throughout server running time?

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I have a Sequelize instance and it is exported in a file to be accessed when doing DB operations.

const sequelize = new Sequelize('database', 'username', null, {
  dialect: 'mysql'
});
module.exports = sequelize;

So the instance is created when the expressjs server starts and never destroys. I wonder if this is the correct way to do, or should I call new Sequelize every time I use the DB operation?

I think it should be kept alive because that's how DB pooling could take effect. Right?

sequelize.js node.js
1 Answers

The bottom line is - yes, it should stay alive. There is no performance hit whatsoever if you keep the instance alive. Because it will be the Sequelize instance (and by extension the ORM) which will handle the future connections. This also includes (as you noted) pooling.

The connections

When it comes to the pooling configuration itself, it get's a little tricky though. Depending on your configuration, the pool has some amount of "space" to work with - the limit to create connections, the idle duration after which connections are removed etc. I can certainly imagine a situation when keeping a connection alive is simply not needed - for example an internal system for a company which is not used overnight.

The Sequelize ORM gives you a good set of options to choose from when configuring your connection pool. In general, you do want to reuse connections as establishing new ones is quite expensive - not just because of the network (e.g. authorization, maybe proxy etc.) but also because of memory allocation which happens when you create a database connection (which is why reconnecting on every request is not a good idea..).

However, it all comes down to which database engine you use (and how busy your system is); MySQL can for example cache connections. When a connection is closed, it is returned to the thread cache rather than discarded (for some period of time). When a new connection opens, MySQL will look into the thread cache rather than try and establish a new connection.

You might want to go through these:

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