How to Create a Subscriber

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

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.

For the order.placed event, the data object won't contain other order data. Only the ID of the order. You can retrieve the order information using the orderService.

Subscriber ID

The subscribe method of the eventBusService accepts a third optional parameter which is a context object. This object has a property subscriberId with its value being a string. This ID is useful when there is more than one handler method attached to a single event or if you have multiple Medusa backends running. This allows the events bus service to differentiate between handler methods when retrying a failed one. If a subscriber ID is not passed on subscription, all handler methods are run again. This can lead to data inconsistencies or general unwanted behavior in your system. On the other hand, if you want all handler methods to run again when one of them fails, you can omit passing a subscriber ID.

An example of using the subscribe method with the third parameter:

eventBusService.subscribe("order.placed", this.handleOrder, {
  subscriberId: "my-unique-subscriber",
})

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Using Services in Subscribers

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

For example:

src/subscribers/orderNotifier.ts

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:

src/subscribers/orderNotifier.ts

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.

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See Also

• Create a Plugin