docs: added loaders documentation (#4019)

* docs: added loaders documentation * added a link to the loaders documentation
2023-05-04 18:55:50 +03:00
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+---
+description: "earn what loaders are in Medusa. A loader is a script that runs when the Medusa backend starts."
+---
+
+import DocCard from '@theme/DocCard';
+import Icons from '@theme/Icon';
+
+# Loaders
+
+In this document, you’ll learn what loaders are in Medusa.
+
+## Overview
+
+A loader is a script that runs when the Medusa backend starts. The Medusa backend uses loaders to initialize the database connection, load plugins, register resources in the dependency container, and more.
+
+Loaders can be created within the Medusa backend codebase, in a plugin, or in a module to perform custom actions when the backend starts. The loader is created in a TypeScript or JavaScript file located in the `src/loaders` directory, then transpiled using the `build` command into the `dist/loaders` directory.
+
+### Loader Examples
+
+For example, the Redis Event Bus module uses a loader to establish a connection with Redis and log a message in the console.
+
+Another example is the Algolia plugin, which uses a loader to update the index settings when the Medusa backend starts based on the plugin’s options.
+
+Loaders can be used to access the dependency container and register custom resources in it.
+
+Loaders can also be used to create [scheduled jobs](../scheduled-jobs/overview.mdx) that run at a specified interval of time. For example:
+
+```ts
+const publishJob = async (container, options) => {
+  const jobSchedulerService = 
+    container.resolve("jobSchedulerService")
+  jobSchedulerService.create(
+    "publish-products", 
+    {}, 
+    "0 0 * * *", 
+    async () => {
+      // job to execute
+      const productService = container.resolve("productService")
+      const draftProducts = await productService.list({
+        status: "draft",
+      })
+
+      for (const product of draftProducts) {
+        await productService.update(product.id, {
+          status: "published",
+        })
+      }
+    }
+  )
+}
+
+export default publishJob
+```
+
+---
+
+## Custom Development
+
+Developers can create a loader with the desired functionality directly within the Medusa Core, in a plugin, or in a module.
+
+<DocCard item={{
+  type: 'link',
+  href: '/development/loaders/create',
+  label: 'Create a Loader',
+  customProps: {
+    icon: Icons['academic-cap-solid'],
+    description: 'Learn how to create a loader.'
+  }
+}} />