docs: added idempotency key documentation (#4039)

docs/content/development/idempotency-key/overview.mdx

@@ -0,0 +1,72 @@
+---
+description: "Learn what an idempotency key is in Medusa. An Idempotency Key is a unique key associated with an operation that allows you to safely retry requests."
+---
+
+import DocCard from '@theme/DocCard';
+import Icons from '@theme/Icon';
+
+# Idempotency Key
+
+In this document, you'll learn what an idempotency key is in Medusa.
+
+## Overview
+
+An Idempotency Key is a unique, randomly generated key associated with an operation, such as the cart completion process. The idempotency key can be passed in the header of a request to an endpoint. This allows you to safely retry requests without accidentally performing the same operation twice.
+
+For example, if a connection error occurs while the customer is completing their cart and placing an order, you can retry from the last recovery point before the error occurred.
+
+When an operation first starts, the idempotency key is generated using the `uuid` package's `v4` method. Then, the backend sets the following headers in the response:
+
+```bash
+Access-Control-Expose-Headers: Idempotency-Key
+Idempotency-Key: <IDEM_VAL>
+```
+
+Where `<IDEM_VAL>` is the idempotency key generated.
+
+These headers can then be passed again for subsequent retrying requests and will be available on the response of these requests as well. The value of the idempotency key remains the same across requests and responses, even if an error occurs and you retry the request.
+
+For example, when the cart completion process starts, the Medusa backend generates the idempotency key and sets the necessary headers on the response. If an error occurs in the request, you can later retry the request by passing these same headers in your request.
+
+---
+
+## IdempotencyKey Entity Overview
+
+The idempotency key's data is stored within the `IdempotencyKey` entity. Some of its attributes include:
+
+- `idempotency_key`: a unique string indicating the value of the idempotency key.
+- `request_method`: a string indicating the method of the latest request related to the idempotency key's operation. For example, `POST`.
+- `request_params`: a JSONB object indicating the parameters of the latest request related to the idempotency key's operation.
+- `request_path`: a string indicating the path of the latest request related to the idempotency key's operation.
+- `response_code`: a number indicating the response code of the latest request related to the idempotency key's operation.
+- `response_body`: a JSONB object indicating the response body of the latest request related to the idempotency key's operation.
+- `recovery_point`: a string indicating the point to continue from when retrying the request. The default value is `started`.
+
+---
+
+## Idempotency Key Stages
+
+Idempotency key stages are the different recovery points that are available for an operation. Every operation has at least the `started` and `finished` stages.
+
+For example, the cart completion operation has the following stages or recovery points:
+
+- `started`
+- `tax_lines_created`
+- `payment_authorized`
+- `finished`
+
+---
+
+## Custom Development
+
+You can use the `IdempotencyKeyService` in your custom development to ensure requests can be safely retried or continued.
+
+<DocCard item={{
+  type: 'link',
+  href: '/development/idempotency-key/use-service',
+  label: 'Use IdempotencyKeyService',
+  customProps: {
+    icon: Icons['academic-cap-solid'],
+    description: 'Learn how to use the IdempotencyKeyService in your code.'
+  }
+}} />

docs/content/development/idempotency-key/use-service.md

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+---
+description: 'Learn how to create a loader in Medusa. A loader can be created in the Medusa backend codebase, in a plugin, or in a module.'
+addHowToData: true
+---
+
+# How to Use IdempotencyKeyService
+
+In this document, you'll learn how to use the `IdempotencyKeyService`.
+
+## Overview
+
+You can use the `IdempotencyKeyService` within your custom development to ensure that your custom endpoints and operations can be safely retried or continued if an error occurs. This guide is also useful if you're overriding an existing feature in Medusa that uses the `IdempotencyKeyService` and you want to maintain its usage, such as if you're overriding the cart completion strategy.
+
+The `IdempotencyKeyService` includes methods that can be used to create and update idempotency keys, among other functionalities.
+
+---
+
+## Create Idempotency Key
+
+You can create an idempotency key within an endpoint using the `create` method of the `IdempotencyKeyService`:
+
+```ts
+router.post("/custom-route", async (req, res) => {
+  // ...
+  const idempotencyKey = await idempotencyKeyService.create({
+    request_method: req.method,
+    request_params: req.params,
+    request_path: req.path,
+  })
+  // ...
+})
+```
+
+The method requires as a parameter an object having the following properties:
+
+- `request_method`: a string indicating the request method to be associated with the idempotency key.
+- `request_params`: an object indicating the request parameters to be associated with the idempotency key.
+- `request_path`: a string indicating the request path to be associated with the idempotency key.
+
+The method handles generating the idempotency key value and saving the idempotency key with its details in the database. It returns the full idempotency key object.
+
+Alternativaly, you can use the `initializeRequest` method that allows you to retrieve an idempotency key based on the value passed in the `Idempotency-Key` header of the request if it exists, or create a new key otherwise. For example:
+
+```ts
+router.post("/custom-route", async (req, res) => {
+  // ...
+  const headerKey = req.get("Idempotency-Key") || ""
+
+  const idempotencyKey = await idempotencyKeyService
+    .initializeRequest(
+      headerKey,
+      req.method,
+      req.params,
+      req.path
+    )
+  // ...
+})
+```
+
+The method requires the following parameters:
+
+1. The first parameter is the key existing in the header of the request, if there's any.
+2. The second parameter is the request's method.
+3. The third parameter is the request's parameters.
+4. The fourth method is the request's path.
+
+The method returns the full idempotency key object.
+
+---
+
+## Perform Actions Within Idempotency Key Stages
+
+Each [idempotency key stage](./overview.mdx#idempotency-key-stages) typically has transactions performed within it. Using the `IdempotencyKeyService`'s `workStage` method allows you to perform related functionalities in transactional isolation within each stage. You can access the stage or recovery point of an idempotency key using the `recovery_point` attribute.
+
+The following example is taken from the `CartCompletionStrategy` implemented in the Medusa backend:
+
+<!-- eslint-disable no-fallthrough -->
+
+```ts
+class CartCompletionStrategy
+  extends AbstractCartCompletionStrategy {
+  // ...
+
+  async complete(
+    id: string,
+    ikey: IdempotencyKey,
+    context: RequestContext
+  ): Promise<CartCompletionResponse> {
+    // ...
+    let inProgress = true
+    let err: unknown = false
+
+    while (inProgress) {
+      switch (idempotencyKey.recovery_point) {
+        case "started": {
+          await this.activeManager_
+            .transaction(
+              "SERIALIZABLE", 
+              async (transactionManager) => {
+                idempotencyKey = 
+                  await this.idempotencyKeyService_
+                    .withTransaction(transactionManager)
+                    .workStage(
+                      idempotencyKey.idempotency_key,
+                      async (manager) =>
+                        await this.handleCreateTaxLines(
+                          id, 
+                          {
+                            manager,
+                          }
+                        )
+                    )
+              }
+            )
+            .catch((e) => {
+              inProgress = false
+              err = e
+            })
+          break
+        }
+
+        case "tax_lines_created": {
+          // ...
+        }
+
+        case "payment_authorized": {
+          // ...
+        }
+
+        case "finished": {
+          // ...
+        }
+
+        default: {
+          // ...
+        }
+      }
+    }
+
+    // ...
+  }
+}
+```
+
+The method requires the following parameters:
+
+1. The first parameter is the idempotency key value.
+2. The second parameter is a callback function to be executed. The function should return an object that is used to update the idempotency key's details. The object can include the following parameters:
+   1. `recovery_point`: a string indicating the new recovery point associated with the idempotency key's operation. If no `recovery_point` is returned in the object, the `finished` recovery point is assigned by default.
+   2. `response_code`: a number indicating the latest response code of the idempotency key's operation.
+   3. `response_body`: an object indicating the latest response body of the idempotency key's operation.
+
+The method returns an updated idempotency key object.

docs/content/development/loaders/overview.mdx

@@ -1,5 +1,5 @@
 ---
-description: "earn what loaders are in Medusa. A loader is a script that runs when the Medusa backend starts."
+description: "Learn what loaders are in Medusa. A loader is a script that runs when the Medusa backend starts."
 ---
 
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