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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/subscribersCopy to Clipboard. Its file name, by convension, should be the class name of the subscriber without the word SubscriberCopy to Clipboard. For example, if the subscriber is HelloSubscriberCopy to Clipboard, the file name should be hello.tsCopy to Clipboard.

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

The eventBusService.subscribeCopy to Clipboard 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 OrderNotifierSubscriberCopy to Clipboard class created in src/subscribers/orderNotifier.tsCopy to Clipboard:

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

The dataCopy to Clipboard object won't contain other order data. Only the ID of the order. You can retrieve the order information using the orderServiceCopy to Clipboard.

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)
  }
  // ...
}
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You can then use this.productServiceCopy to Clipboard anywhere in your subscriber’s methods. For example:

class OrderNotifierSubscriber {
  // ...
  handleOrder = async (data) => {
    // ...
    const product = this.productService.list()
  }
}
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When using attributes defined in the subscriber, such as the productServiceCopy to Clipboard in the example above, you must use an arrow function to declare the method. Otherwise, the attribute will be undefined when used.

See Also​