Best practices for extending CartService in Medusa.js

[rob0pup]

Hi folks 👋

I’m currently working on a Medusa.js project where I need to extend the CartService to add some custom business logic whenever a cart is created or updated.

For example:

• Adding a custom field to track cart source (e.g., web, mobile, wholesale)
• Applying specific discounts automatically based on metadata
• Running validation before checkout (like checking stock from an external system)

I’ve read the docs about [extending services](https://docs.medusajs.com/development/fundamentals/services#extending-services), but I’d like to understand the best practices for doing this in a way that:

1. Doesn’t break when upgrading Medusa.js versions
2. Keeps my logic clean and maintainable
3. Works well with other plugins I might install later

So my questions are:

• Is extending CartService directly the best approach, or should I use a custom subscriber to listen to events (cart.created, cart.updated)?
• If I extend the service, should I override methods like create/update or is there a better hook point?
• Are there any examples of real-world implementations (beyond the docs) that show how to handle these kinds of customizations?

Any advice, code snippets, or pointers to examples would be greatly appreciated 🙏

Thanks in advance!

[wasaflash]

Hi 👋 great question!

There are two common approaches to customizing cart logic in Medusa:

Extending CartService

• You can create a custom service that extends CartService and override methods like create or update.
• This is useful if you need to change how carts behave at the core level (e.g., automatically attaching metadata or enforcing custom rules).

Using Event Subscribers

• If you don’t want to modify the service directly, you can listen to events like cart.created or cart.updated.
• This is safer for upgrades since you’re not overriding Medusa’s core methods.