LEP for Azure Blob Storage Backup Store Support

Ref: 1309

Signed-off-by: James Lu <james.lu@suse.com>

enhancements/20230418-azure-blob-storage-backup-store-support.md

@@ -0,0 +1,68 @@
+# Azure Blob Storage Backup Store Support
+
+## Summary
+
+Longhorn supports Azure Blob Storage as a backup storage.
+
+### Related Issues
+
+https://github.com/longhorn/longhorn/issues/1309
+
+## Motivation
+
+### Goals
+
+- Support Azure Blob Storage as a backup storage.
+
+## Proposal
+
+- Introduce Azure Blob Storage client for supporting Azure Blob Storage as a backup storage.
+
+## User Stories
+
+Longhorn already supports NFSv4, CIFS and S3 servers as backup storage. However, certain users may still want to be able to utilize Azure blob storage to push/pull backups to/from.
+
+### User Experience In Details
+
+- Users can configure a Azure Blob Storage as a backup storage
+  - Set **Backup Target**. The path to a Azure Blob Storage is like
+
+    ```bash
+    azblob://${container}@blob.core.windows.net/${path name}
+    ```
+
+- Set **Backup Target Credential Secret**
+  - Create a secret and deploy it
+
+    ```yaml
+    apiVersion: v1
+    kind: Secret
+    metadata:
+    name: azblob-secret
+      namespace: longhorn-system
+    type: Opaque
+    data:
+      AZBLOB_ACCOUNT_NAME: ${AZBLOB_ACCOUNT_NAME}
+      AZBLOB_ACCOUNT_KEY: ${AZBLOB_ACCOUNT_KEY}
+    ```
+
+  - Set the setting **Backup Target Credential Secret** to `azblob-secret`
+
+## Design
+
+### Implementation Overview
+
+- longhorn-manager
+  - Introduce the fields `AZBLOB_ACCOUNT_NAME` and `AZBLOB_ACCOUNT_KEY` in credentials. The two fields are passed to engine and replica processes for volume backup and restore operations.
+- backupstore
+  - Implement Azure Blob Storage register/unregister and basic CRUD functions.
+
+## Test Plan
+
+### Integration Tests
+
+1. Set a Azure Blob Storage as backup storage.
+2. Create volumes and write some data.
+3. Back up volumes to the backup storage and the operation should succeed.
+4. Restore backups and operations should succeed.
+5. All data is not corrupted.