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docs/backend.md

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+# Backend Documentation
+
+The backend is built with **FastAPI**, providing a high-performance API to serve system metrics.
+
+## Core Files
+
+### `main.py`
+- Entry point of the application.
+- Defines the FastAPI app and CORS middleware.
+- **Endpoints**:
+    - `GET /`: Serves the `monitor.html` file.
+    - `GET /api/status`: Orchestrates the monitoring tasks and returns a JSON response.
+- **Parallel Execution**: Uses `asyncio.gather` and `asyncio.to_thread` to run blocking SSH/HTTP calls concurrently.
+
+### `monitor_logic.py`
+- Contains the low-level monitoring functions.
+- **`get_ssh_data()`**:
+    - Connects via SSH using `paramiko`.
+    - Executes commands to fetch:
+        - CPU Usage: `top`
+        - RAM Usage: `free`
+        - Disk Space: `df`
+        - Temperature: `/sys/class/thermal/thermal_zone0/temp` or `vcgencmd`.
+- **`check_web_service()`**:
+    - Performs an HTTP GET request.
+    - Calculates latency in milliseconds.
+
+## Environment Variables
+- `PC_PASS`: SSH password for the Ubuntu Server.
+- `PI_PASS`: SSH password for the Raspberry Pi.