How to Extend a Service

In this document, you’ll learn how to extend a core service in Medusa.

Overview

Medusa’s core services cover a wide range of functionalities related to each domain or entity. You can extend these services to add custom methods or override existing methods.

Word of Caution about Overriding

Extending services to add new methods shouldn't cause any issues within your commerce application. However, if you extend them to override their existing methods, you should be aware that this could have negative implications, such as unanticipated bugs, especially when you try to upgrade the core Medusa package to a newer version.

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Step 1: Create the Service File

In your Medusa backend, create the file src/services/product.ts. This file will hold your extended service.

Note that the name of the file must be the same as the name of the original service in the core package. So, if you’re extending the ProductService, the file’s name should be product.ts. On the other hand, if you’re extending the CustomerService, the file’s name should be customer.ts.

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Step 2: Implementing the Service

In the file, you can import the original service from the Medusa core, then create your service that extends the core service.

For example, to extend the Product service:

src/services/product.ts

import { 
  ProductService as MedusaProductService,
} from "@medusajs/medusa"

class ProductService extends MedusaProductService {
  // TODO add customizations
}

export default ProductService

Notice that you alias the ProductService of the core to avoid naming conflicts.

Within the service, you can add new methods or extend existing ones.

You can also change the lifetime of the service:

src/services/product.ts

import { Lifetime } from "awilix"
import { 
  ProductService as MedusaProductService,
} from "@medusajs/medusa"

class ProductService extends MedusaProductService {
  // The default life time for a core service is SINGLETON
  static LIFE_TIME = Lifetime.SCOPED
    
  // ...
}

export default ProductService

You can learn more details about the service lifetime and other considerations when creating a service in the Create Service documentation.

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Step 3: Test it Out

To test out your customization, start by transpiling your files by running the following command in the root directory of the Medusa backend:

• npm
• Yarn
• pnpm

npm run build

yarn build

pnpm run build

Then, start the backend:

• npm
• Yarn
• pnpm

npx @medusajs/medusa-cli develop

npx @medusajs/medusa-cli develop

npx @medusajs/medusa-cli develop

You should see the customizations you made in effect.

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Troubleshooting

AwilixResolutionError: Could Not Resolve X (Service Lifetime)

If you're registering a custom resource within a middleware, for example a logged-in user, then make sure that all services that are using it have their LIFE_TIME static property either set to Lifetime.SCOPED or Lifetime.TRANSIENT. This mainly applies for services in the core Medusa package, as, by default, their lifetime is Lifetime.SINGLETON.

For example:

import { Lifetime } from "awilix"
import { 
  ProductService as MedusaProductService,
} from "@medusajs/medusa"

// extending ProductService from the core
class ProductService extends MedusaProductService {
  // The default life time for a core service is SINGLETON
  static LIFE_TIME = Lifetime.SCOPED

  // ...
}

export default ProductService

This may require you to extend a service as explained in this documentation if necessary.

If you're unsure which service you need to change its LIFE_TIME property, it should be mentioned along with the AwilixResolutionError message. For example:

AwilixResolutionError: Could not resolve 'loggedInUser'.

Resolution path: cartService -> productService -> loggedInUser

As shown in the resolution path, you must change the LIFE_TIME property of both cartService and productService to Lifetime.SCOPED or Lifetime.TRANSIENT.

You can learn about the service lifetime in the Create a Service documentation.