Medusa is an open source ecommerce platform with plenty of features that boost the ecommerce experience for merchants while providing a great developer experience.

As a headless commerce platform, Medusa’s server architecture is composed of a few components that have different purposes and usages.

This article in particular focuses on Subscribers, what’s their purpose, and how they can be used to handle events in Medusa.

Overview

In Medusa, there are events that are emitted when a certain action occurs. For example, if a customer places an order, the order.placed event is emitted with the order data.

These events are designed to allow other parts of the platform or third-party integrations to listen to them and take action accordingly.

Subscribers register handlers for an events and allows you to perform an action when that event occurs. For example, if you want to send your customer an email when they place an order, then you can listen to the order.placed event and send the email when the event is emitted.

Natively in Medusa there are subscribers to handle different events. However, you can also create your own custom subscribers.

Custom subscribers reside in your project's src/subscribers directory. Files here should export classes, which will be treated as subscribers by Medusa. By convention, the class name should end with Subscriber and the file name should be the camel-case version of the class name without Subscriber. For example, the WelcomeSubscriber class is in the file src/subscribers/welcome.js.

Whenever an event is emitted, the subscriber’s registered handler method is executed. The handler method receives as a parameter an object that holds data related to the event. For example, if an order is placed the order.placed event will be emitted and all the handlers will receive the order id in the parameter object.

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 and you will therefore need to make sure that Redis is installed and configured for your Medusa project.

Then, you need to set your Redis URL in your Medusa server. By default, the Redis URL is redis://localhost:6379. If you use a different one, set the following environment variable in .env:

REDIS_URL=<YOUR_REDIS_URL>

After that, in medusa-config.js, you’ll need to comment out the following line:

module.exports = {
  projectConfig: {
    redis_url: REDIS_URL, //this line is commented out
    ...
  }
}

After that, you are able to listen to events on your server.

How to Create a Custom Subscriber

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

The eventBusService.subscribe method takes an event name as its first parameter, and a callback function as its second parameter. This callback will be executed when the event is fired.

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

class OrderNotifierSubscriber {
  constructor({ eventBusService }) {
    eventBusService.subscribe("order.placed", this.handleOrder);
  }

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

export default OrderNotifierSubscriber;

This subscriber will register 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, and it will receive 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 will not 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:

constructor({ productService, eventBusService }) {
    this.productService = productService;

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

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

Conclusion

Subscribers are just one part of Medusa’s architecture. Here are some documentation that can guide you into doing more with Medusa:

1. How to Add an Endpoint for Storefront and Admin
2. What are Services and how to create a custom service
3. How to create a payment provider

Should you have any issues or questions related to Medusa, then feel free to reach out to the Medusa team via Discord.