High level design for starting kopia server in kanister using kopia S…

…DK (#2597)

* HLD for storage wrappers library

Signed-off-by: Amruta Kale <amruta.kale@veeam.com>

* add diagrams for CLI and SDK approach

* address review comments

* cosmetic changes

* address review comments

Signed-off-by: Amruta Kale <amruta.kale@veeam.com>

---------

Signed-off-by: Amruta Kale <amruta.kale@veeam.com>

design/kanister-kopia-integration.md

@@ -15,6 +15,7 @@
     - [Client-side Setup](#client-side-setup)
     - [Server Access Users Management](#server-access-users-management)
   - [Secrets Management](#secrets-management)
+  - [Replace Kopia CLI with SDK](#replace-kopia-CLI-with-SDK)
 <!-- /toc -->
 
 This document proposes all the high-level changes required within Kanister to
@@ -599,3 +600,17 @@ credentials. This model ensures Kanister remains free from a hard dependency on
 any crypto packages, and vault-like functionalities.
 
 If misplaced, Kanister will not be able to recover these credentials.
+
+
+### Replace Kopia CLI with SDK
+
+Currently, we are using Kopia CLI to perform the repository and kopia repository server operations in Kanister.
+The repository controller creates a pod, executes commands through `kube.exec` on the pod to perform
+repository operations. The commands include: 
+- repo connect 
+- start server 
+- add users 
+- refresh server 
+
+Kopia provides an SDK to perform repository operations which can be used instead of CLI. The detailed design is explained in the document
+[Replace Kopia CLI with Kopia SDK](https://github.com/kanisterio/kanister/blob/master/design/replace-CLI-with-SDK.md).

design/replace-CLI-with-SDK.md

@@ -0,0 +1,164 @@
+
+<!-- toc -->
+- [Motivation](#motivation)
+- [Scope](#scope)
+- [High Level Design](#high-level-design)
+  - [Kopia SDK wrappers](#kopia-sdk-wrappers)
+    - [Storage pkg](#storage-pkg)
+    - [Repository pkg](#repository-pkg)
+  - [Repository server controller changes](#repository-server-controller-changes)
+	- [Kopia CLI Approach](#kopia-cli-approach)
+	- [Kopia SDK Approach](#kopia-sdk-approach)
+<!-- /toc -->
+
+
+# Motivation
+
+Kanister uses a Kubernetes custom controller that makes use of Kopia as a primary backup and restore tool. 
+The detailed design of the customer controller can be found [here](https://github.com/kanisterio/kanister/blob/master/design/kanister-kopia-integration.md)
+
+The custom controller called as repository server controller currently uses Kopia CLI to perform the Kopia operations. All the
+operations are executed inside a pod using the `kubectl exec` function. 
+
+We can get following benefits if we start using Kopia SDK instead of CLI:
+- Better error handling
+- Dependency management
+
+The goal over here is to start the Kopia server by executing a pre-built binary. The binary would execute most of the 
+operations to start the server using kopia SDK and reduce the dependency on Kopia CLI and gain more flexibility over the operations
+
+## Scope
+1. Implement a library that wraps the SDK functions provided by Kopia to connect to underlying storage providers - S3, Azure, GCP, Filestore
+2. Build another library on top of the library implemented in #1 that can be used to perform repository operations
+3. Modify the repository server controller to run a pod that executes a custom image. The binary would take all the
+necessary steps to make the Kopia server ready using Kopia SDK and Kopia CLI. 
+
+## High Level Design
+
+### Kopia SDK Wrappers
+
+#### Storage pkg
+
+```go
+
+package storage
+
+import (
+	"context"
+
+	"github.com/kopia/kopia/repo/blob"
+)
+
+type StorageType string
+
+const (
+	TypeS3        StorageType = "S3"
+	TypeAzure     StorageType = "Azure"
+	TypeFileStore StorageType = "FileStore"
+	TypeGCP       StorageType = "GCP"
+)
+
+type Storage interface {
+	Connect() (blob.Storage, error)
+	SetOptions(context.Context, map[string]string)
+	WithCreate(bool)
+}
+
+func New(storageType StorageType) Storage {
+	switch storageType {
+	case TypeS3:
+		return &s3Storage{}
+	case TypeFileStore:
+		return &fileSystem{}
+	case TypeAzure:
+		return &azureStorage{}
+	case TypeGCP:
+		return &gcpStorage{}
+	default:
+		return nil
+	}
+}
+
+```
+
+#### Repository pkg
+
+This pkg would be a wrapper over the `storage pkg` built above and the Kopia repositoy pkg `github.com/kopia/kopia/repo` 
+provided by Kopia SDK
+
+```go
+
+package repository
+
+type Repository struct {
+	st          storage.Storage
+	password    string
+	configFile  string
+	storageType storage.StorageType
+}
+
+// Create repository using Kopia SDK
+func (r *Repository) Create(opts *repo.NewRepositoryOptions) (err error) {
+	storage, err := r.st.Connect()
+	if err != nil {
+		return err
+	}
+	return repo.Initialize(context.Background(), storage, opts, r.password)
+}
+
+// Connect to the repository using Kopia SDK
+func (r *Repository) Connect(opts *repo.ConnectOptions) (err error) {
+	storage, err := r.st.Connect()
+	if err != nil {
+		return err
+	}
+	return repo.Connect(context.Background(), r.configFile, storage, r.password, opts)
+}
+
+// Connect to the repository by providing a config file
+func (r *Repository) ConnectUsingFile() error {
+	repoConfig := repositoryConfigFileName(r.configFile)
+	if _, err := os.Stat(repoConfig); os.IsNotExist(err) {
+		return errors.New("failed find kopia configuration file")
+	}
+
+	_, err := repo.Open(context.Background(), repoConfig, r.password, &repo.Options{})
+	return err
+}
+
+
+func repositoryConfigFileName(configFile string) string {
+	if configFile != "" {
+		return configFile
+	}
+	return filepath.Join(os.Getenv("HOME"), ".config", "kopia", "repository.config")
+}
+
+```
+
+### Repository server controller changes
+
+#### Kopia CLI Approach
+
+![Alt text](images/kopia-CLI-workflow.png?raw=true "Kanister Kopia Integration using Kopia CLI")
+
+Above diagram explains the current workflow repository server controller
+uses to start the Kopia repository server. All the commands are executed from the controller pod inside 
+the respoitory server pod using `kube.exec`. The repository server pod that is created by controller
+uses `kanister-tools` image
+
+
+#### Kopia SDK Approach
+
+![Alt text](images/kopia-SDK-workflow.png?raw=true "Kanister Kopia Integration using Kopia SDK")
+
+
+As shown in the figure we will be building a custom image which is going have this workflow:
+1. Start the Kopia repository server in `--async-repo-connect` mode that means the server would be started without
+connecting to the repository in an async mode. Existing approach starts the server only after the connection to Kopia repository
+is successful. Kopia SDK currently does not have an exported function to start the Kopia server. So we would still be using Kopia
+CLI to start the server
+2. Check Kopia server status using Kopia SDK wrappers explained in section [Kopia SDK wrappers](#kopia-sdk-wrappers)
+2. Connect to Kopia repository using Kopia SDK wrappers
+3. Add or Update server users using Kopia SDK wrappers
+4. Refresh server using Kopia SDK wrappers