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How to Create a Subscriber

In this document, you’ll learn how to create a Subscriber in your Medusa backend that listens to events to perform an action.

Prerequisites​

Medusa's event system works by pushing data to a Queue that each handler then gets notified of. The queuing system is based on Redis, so it's required for subscribers to work.

You can learn how to install Redis and configure it with Medusa before you get started.

Implementation​

A subscriber is a TypeScript or JavaScript file that is created under src/subscribers. Its file name, by convension, should be the class name of the subscriber without the word Subscriber. For example, if the subscriber is HelloSubscriber, the file name should be hello.ts.

After creating the file under src/subscribers, in the constructor of your subscriber, listen to events using eventBusService.subscribe , where eventBusService is a service injected into your subscriber’s constructor.

The eventBusService.subscribe method receives the name of the event as a first parameter and as a second parameter a method in your subscriber that will handle this event.

For example, here is the OrderNotifierSubscriber class created in src/subscribers/orderNotifier.ts:

src/subscribers/orderNotifier.ts
class OrderNotifierSubscriber {
  constructor({ eventBusService }) {
    eventBusService.subscribe("order.placed", this.handleOrder)
  }

  handleOrder = async (data) => {
    console.log("New Order: " + data.id)
  }
}

export default OrderNotifierSubscriber

This subscriber registers the method handleOrder as one of the handlers of the order.placed event. The method handleOrder will be executed every time an order is placed. It receives the order ID in the data parameter. You can then use the order’s details to perform any kind of task you need.

The data object won't contain other order data. Only the ID of the order. You can retrieve the order information using the orderService.

Using Services in Subscribers​

You can access any service through the dependencies injected to your subscriber’s constructor.

For example:

class OrderNotifierSubscriber {
  constructor({ productService, eventBusService }) {
      this.productService = productService

      eventBusService.subscribe("order.placed", this.handleOrder)
  }
  // ...
}

You can then use this.productService anywhere in your subscriber’s methods. For example:

class OrderNotifierSubscriber {
  // ...
  handleOrder = async (data) => {
    // ...
    const product = this.productService.list()
  }
}

When using attributes defined in the subscriber, such as the productService in the example above, you must use an arrow function to declare the method. Otherwise, the attribute will be undefined when used.

See Also​