feat: add docs to main (#1559)

docs/developer/develop/advanced/database.md

@@ -0,0 +1,65 @@
+# Database
+
+The Olares system provides three most popular data storage cluster for all APPs, covering `RDS`, `NoSQL`, and `Cache` data storage use cases.
+
+## RDS
+
+The system has deployed **PostgreSQL** and provides two types of databases.
+
+- **Standalone PostgreSQL**, providing the most commonly used `RDS` database layer functions.
+- **Distributed PostgreSQL** extension, powered by **Citus**. Provides the ability to horizontally scale the database.
+
+When setting up a **PostgreSQL** database, you can specify the type of database to be used in [OlaresManifest.yaml](../package/manifest.md#middleware).
+
+```yaml
+middleware:
+postgres:
+  username: postgres
+  databases:
+    - name: db
+      distributed: true # Whether the database is distributed in the cluster.
+```
+
+If you use **Citus**, **Olares** will automatically shard the database tables and perform rebalancing during the horizontal scaling of **PostgreSQL** replicas.
+
+## NoSQL
+
+The NoSQL cluster is not deployed by default in Olares, but it can be easily installed from the Market. To set up a NoSQL cluster, the administrator needs to install the [**MongoDB**](https://market.olares.com/middleware/mongodb) middleware. Once installed, the [Percona Operator for MongoDB](https://github.com/percona/percona-server-mongodb-operator) automatically manages the **MongoDB** cluster. Users can then horizontally scale **MongoDB** cluster replicas, as well as perform backup and restore operations on databases.
+
+You can specify detailed configuration for MongoDB in [OlaresManifest.yaml](../package/manifest.md#middleware) as follows:
+
+```yaml
+middleware:
+  mongodb:
+    username: mongodb
+    databases:
+      - name: db0
+      - name: db1
+options:
+  dependencies:
+    - name: olares
+      type: system
+      version: ">=1.6.0-0"
+    - name: mongodb
+      version: ">=6.0.0-0"
+      type: middleware
+```
+
+## Cache
+
+In terms of the Cache cluster, Olares uses Redis Cluster. The cluster is managed by a customized [Redis Cluster Operator](https://github.com/beclab/redis-cluster-operator) to achieve cloud nativeness. It enables us to scale replicas horizontally in a convenient and effective manner.
+
+To ensure **data isolation** between users and apps in the **Redis cluster**, the **Olares** system has added a **Redis cluster proxy**. It isolates data based on the `namespace`. This operation is transparent, meaning app developers typically do not need to be aware of it.
+
+Additionally, this proxy simplifies the process of connecting to clusters. It eliminates the need to switch from a **standalone Redis Client** to a **Redis Cluster client** in the app, thus simplifying app code modifications.
+
+```
+middleware:
+  redis:
+    password: password
+    namespace: db0
+```
+
+:::info NOTE
+Since Olares uses the Redis Cluster version, developers need to understand the usage restrictions of Redis Cluster in detail when using it.
+:::