docs: added documentation for logging (#5469)

www/apps/docs/content/development/logging/index.md

@@ -0,0 +1,172 @@
+# Logging
+
+In this document, you’ll learn about how you can log messages in the Medusa backend and available logging configurations.
+
+## Overview
+
+In the Medusa backend, the `Logger` class is registered in the [dependency container](../fundamentals/dependency-injection.md). This class provides helper methods to log messages in the console while the Medusa backend is running.
+
+You can resolve this class in your resources, such as services or loaders, to show a message in the console. You can also configure how and where logs are shown in your backend using environment variables.
+
+---
+
+## How to Log a Message
+
+To log a message, resolve the `logger` registration name using dependency injection in your resource.
+
+For example, to log a message in a [loader](../loaders/overview.mdx):
+
+```ts title=src/loaders/my-loader.ts
+import { 
+  ProductService, 
+  ConfigModule, 
+  Logger,
+} from "@medusajs/medusa"
+import { AwilixContainer } from "awilix"
+
+export default async (
+  container: AwilixContainer,
+  config: ConfigModule
+): Promise<void> => {
+  const logger = container.resolve<Logger>("logger")
+
+  logger.info("Starting loader...")
+
+  const productService = container.resolve<ProductService>(
+    "productService"
+  )
+
+  logger.info(`Products count: ${
+    await productService.count()
+  }`)
+  
+  logger.info("Ending loader")
+}
+```
+
+In the example above, you resolve the `logger` from the dependency container and then use its `info` method to show an info message in the console.
+
+If you start your backend, you should see the messages in the loader logged in the console.
+
+:::note
+
+Learn how to resolve dependencies within other types of resources in the [dependency container guide](../fundamentals/dependency-injection.md#resolve-resources).
+
+:::
+
+---
+
+## Log Levels
+
+The following `Logger` methods accept a string parameter that should be logged in the console with a specific level:
+
+- `info`: The message is logged with level `info`.
+- `warn`: The message is logged with level `warn`.
+- `error`: The message is logged with level `error`.
+- `debug`: The message is logged with level `debug`.
+
+---
+
+## Show Log with Progress
+
+The `activity` method is used to log a message of level `info`. If the Medusa backend is running in a development environment, a spinner starts that can be used to show progress and succeed or fail the progress.
+
+The `activity` method returns the ID of the started activity. This ID can then be passed to one of the following methods:
+
+- `progress`: Log a message of level `info` that indicates progress within that same activity.
+- `failure`: Log a message of level `error` that indicates that the activity has failed. This also ends the associated activity.
+- `success`: Log a message of level `info` that indicates that activity has succeeded. This also ends the associated activity.
+
+:::note
+
+If you configured the `LOG_LEVEL` environment variable to a level higher than those associated with the above methods, their messages won’t be logged.
+
+:::
+
+For example:
+
+```ts title=src/loaders/my-loader.ts
+import { 
+  ProductService, 
+  ConfigModule, 
+  Logger,
+} from "@medusajs/medusa"
+import { AwilixContainer } from "awilix"
+
+export default async (
+  container: AwilixContainer,
+  config: ConfigModule
+): Promise<void> => {
+  const logger = container.resolve<Logger>("logger")
+
+  const activityId = logger.activity("Starting loader...")
+
+  const productService = container.resolve<ProductService>(
+    "productService"
+  )
+
+  try {
+    logger.progress(activityId, `Products count: ${
+      await productService.count()
+    }`)
+  } catch (e) {
+    logger.failure(activityId, `An error occurrect: ${e}`)
+  }
+  
+  logger.success(activityId, "Ending loader")
+}
+```
+
+---
+
+## Logging Configuration
+
+### Log Level
+
+By default, the minimum logged level is `silly`, meaning that messages of all levels are logged.
+
+You can change that by setting the `LOG_LEVEL` environment variable to the minimum level you want to be logged.
+
+The available log levels, from lowest to highest levels, are:
+
+1. `silly`: this is used to indicate that messages of all levels should be logged.
+2. `debug`
+3. `info`
+4. `warn`
+5. `error`
+
+For example, to log only `error` messages set the `LOG_LEVEL` environment variable to `error`:
+
+```bash
+LOG_LEVEL=error
+```
+
+:::note
+
+The environment variable must be set as a system environment variable and not in `.env`.
+
+:::
+
+### Save Logs in File
+
+Aside from showing the logs in the Medusa backend’s console, you can save the backend’s logs in a file by setting the `LOG_FILE` environment variable to the path of the file relative to the Medusa backend’s root directory.
+
+:::note
+
+This doesn’t save logs of requests sent to the Medusa backend.
+
+:::
+
+For example:
+
+```bash
+LOG_FILE=all.log
+```
+
+Your logs are now saved in the `all.log` file at the root of your Medusa backend.
+
+:::note
+
+The environment variable must be set as a system environment variable and not in `.env`.
+
+:::