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
event is emitted with the order data.Copy to clipboardorder.placed
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
event and send the email when the event is emitted.Copy to clipboardorder.placed
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
directory. Files here should export classes, which will be treated as subscribers by Medusa. By convention, the class name should end withCopy to clipboardsrc/subscribers
and the file name should be the camel-case version of the class name withoutCopy to clipboardSubscriber
. For example, theCopy to clipboardSubscriber
class is in the fileCopy to clipboardWelcomeSubscriber
.Copy to clipboardsrc/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
event will be emitted and all the handlers will receive the order id in the parameter object.Copy to clipboardorder.placed
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
. If you use a different one, set the following environment variable inCopy to clipboardredis://localhost:6379
:Copy to clipboard.env
1REDIS_URL=<YOUR_REDIS_URL>
After that, in
, you’ll need to comment out the following line:Copy to clipboardmedusa-config.js
123456module.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
, in the constructor of your subscriber, you should listen to events usingCopy to clipboardsrc/subscribers
, whereCopy to clipboardeventBusService.subscribe
is a service injected into your subscriber’s constructor.Copy to clipboardeventBusService
The
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.Copy to clipboardeventBusService.subscribe
For example, here is the
class which is created inCopy to clipboardOrderNotifierSubscriber
:Copy to clipboardsrc/subscribers/orderNotifier.js
1234567891011class 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
as one of the handlers of theCopy to clipboardhandleOrder
event. The methodCopy to clipboardorder.placed
will be executed every time an order is placed, and it will receive the order ID in theCopy to clipboardhandleOrder
parameter. You can then use the order’s details to perform any kind of task you need.Copy to clipboarddata
The
object will not contain other order data. Only the ID of the order. You can retrieve the order information using theCopy to clipboarddata
.Copy to clipboardorderService
Using Services in Subscribers
You can access any service through the dependencies injected to your subscriber’s constructor.
For example:
12345constructor({ productService, eventBusService }) {this.productService = productService;eventBusService.subscribe("order.placed", this.handleOrder);}
You can then use
anywhere in your subscriber’s methods.Copy to clipboardthis.productService
Conclusion
Subscribers are just one part of Medusa’s architecture. Here are some documentation that can guide you into doing more with Medusa:
- How to Add an Endpoint for Storefront and Admin
- What are Services and how to create a custom service
- 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.
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