Add documentation for sync agents

blog/2025-12-31-backup-sync-agents.md

@@ -0,0 +1,13 @@
+---
+author: Joakim Sørensen
+authorURL: https://github.com/ludeeus
+authorImageURL: /img/profile/ludeeus.jpg
+authorTwitter: ludeeus
+title: "Backup sync agents"
+---
+
+As of Home Assistant Core 2025.12, integrations can provide [sync agents](/docs/core/platform/backup#sync-agents) to allow users to back up and restore their data to and from external services.
+
+Sync agents allows integrations to store backups in a remote location, such as a cloud service or a network-attached storage (NAS) device.
+
+More details can be found in the [backup platform documentation](/docs/core/platform/backup#sync-agents).

docs/core/platform/backup.md

@@ -2,11 +2,15 @@
 title: "Backup"
 ---
 
+The backup platform consists of two parts; [Pre/Post backup hooks](#prepost-backup-hooks) and [Sync agents](#sync-agents).
+
+## Pre/Post backup hooks
+
 When Home Assistant is creating a backup, there might be a need to pause certain operations in the integration, or dumping data so it can properly be restored.
 
 This is done by adding 2 functions (`async_pre_backup` and `async_post_backup`) to `backup.py`
 
-## Adding support
+### Adding support
 
 The quickest way to add backup support to a new integration is by using our built-in scaffold template. From a Home Assistant dev environment, run `python3 -m script.scaffold backup` and follow the instructions.
 
@@ -22,3 +26,32 @@ async def async_pre_backup(hass: HomeAssistant) -> None:
 async def async_post_backup(hass: HomeAssistant) -> None:
     """Perform operations after a backup finishes."""
 ```
+
+## Sync agents
+
+Sync agents are used to dispatch a backup to a remote location. This is done by implementing the a `BackupSyncAgent` class.
+
+To register your sync agent, you need to add a `async_get_backup_sync_agents` function to your integrations backup platform.
+
+```python
+async def async_get_backup_sync_agents(
+    hass: HomeAssistant,
+) -> list[BackupSyncAgent]:
+    """Register the backup sync agents."""
+    return [LoremIpsumBackupSyncAgent("syncer")]
+
+class LoremIpsumBackupSyncAgent(BackupSyncAgent):
+    ...
+```
+
+### Sync agent methods
+
+The base class `BackupSyncAgent` has several methods that needs to be implemented.
+
+Method | Description
+:--- | :---
+`async_download_backup` | Download a backup from the remote location. The `id` parameter is the ID of the synced backup that was returned in `async_list_backups`. The `path` parameter is the full file path to download the synced backup to.
+`async_upload_backup` | Upload a backup. The `path` parameter is the full file path to the backup that should be synced. The `metadata` parameter contains metadata about the backup that should be synced.
+`async_list_backups` | List backups.
+
+When a user creates a backup, Home Assistant will call the `async_upload_backup` method on the sync agent to store the backup in the remote location.