docs: add documentation for v1.8 (#3669)

docs/content/development/cache/modules/in-memory.md

@@ -0,0 +1,70 @@
+---
+description: 'In this document, you’ll learn about the in-memory cache module and how you can install it in your Medusa backend.'
+---
+
+# In-Memory Cache Module
+
+In this document, you’ll learn about the in-memory cache module and how you can install it in your Medusa backend.
+
+## Overview
+
+Medusa’s modular architecture allows developers to extend or completely replace the logic used for caching. You can create a custom module, or you can use the modules Medusa provides.
+
+Medusa’s default starter project uses the in-memory cache module. The in-memory cache module uses a plain JavaScript Map object to store the cache data.
+
+This module is helpful for development or when you’re testing out Medusa, but it’s not recommended to be used in production. For production, it’s recommended to use modules like [Redis Cache Module](./redis.md).
+
+This document will guide you through installing the in-memory cache module.
+
+---
+
+## Prerequisites
+
+### Medusa Backend
+
+It’s assumed you already have a Medusa backend installed. If not, you can learn how to install it by following [this guide](../../backend/install.mdx).
+
+---
+
+## Step 1: Install the Module
+
+In the root directory of your Medusa backend, install the in-memory cache module with the following command:
+
+```bash npm2yarn
+npm install @medusajs/cache-inmemory
+```
+
+---
+
+## Step 2: Add Configuration
+
+In `medusa-config.js`, add the following to the exported object:
+
+```js title=medusa-config.js
+module.exports = {
+  // ...
+  modules: {
+    // ...
+    cacheService: {
+      resolve: "@medusajs/cache-inmemory",
+      options: {
+        ttl: 30,
+      },
+    },
+  },
+}
+```
+
+This registers the in-memory cache module as the main cache service to use. You pass it the option `ttl`. This means time-to-live, and it indicates the number of seconds an item can live in the cache before it’s removed.
+
+---
+
+## Step 3: Test Module
+
+To test the module, run the following command to start the Medusa backend:
+
+```bash npm2yarn
+npm run start
+```
+
+The backend should then start with no errors, indicating that the module was installed successfully.