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88 lines
3.0 KiB
Markdown
88 lines
3.0 KiB
Markdown
# Create a Subscriber
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In this document, you’ll learn how to create a [Subscriber](overview.md) in your Medusa server that listens to events to perform an action.
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## Prerequisites
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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.
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You can learn how to [install Redis](../../../tutorial/0-set-up-your-development-environment.mdx#redis) and [configure it with Medusa](../../../usage/configurations.md#redis) before you get started.
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---
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## Implementation
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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`.
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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.
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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.
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For example, here is the `OrderNotifierSubscriber` class created in `src/subscribers/orderNotifier.ts`:
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```ts title=src/subscribers/orderNotifier.ts
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class OrderNotifierSubscriber {
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constructor({ eventBusService }) {
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eventBusService.subscribe("order.placed", this.handleOrder)
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}
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handleOrder = async (data) => {
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console.log("New Order: " + data.id)
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}
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}
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export default OrderNotifierSubscriber
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```
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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.
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:::note
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The `data` object won't contain other order data. Only the ID of the order. You can retrieve the order information using the `orderService`.
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:::
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---
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## Using Services in Subscribers
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You can access any service through the dependencies injected to your subscriber’s constructor.
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For example:
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```ts
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class OrderNotifierSubscriber {
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constructor({ productService, eventBusService }) {
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this.productService = productService
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eventBusService.subscribe("order.placed", this.handleOrder)
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}
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// ...
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}
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```
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You can then use `this.productService` anywhere in your subscriber’s methods. For example:
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```ts
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class OrderNotifierSubscriber {
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// ...
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handleOrder = async (data) => {
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// ...
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const product = this.productService.list()
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}
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}
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```
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:::note
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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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:::
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---
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## See Also
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- [Events reference](events-list.md)
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- [Create a Service](../services/create-service)
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