HLD for storage wrappers library

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

design/Kopia-kanister-integration-phase2.md

@@ -0,0 +1,179 @@
+# Problem Statement
+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`` to perform operations. The commands include: 
+- repo connect 
+- start server 
+- add users 
+- refresh server 
+
+The CLI commands executed using kube.exec can be flakey for long running commands.
+
+The goal of the proposal is
+- To replace kopia CLI with Kopia SDK wherever possible to gain more control over operations on kopia.
+
+## Scope
+1. Implement a library that provides wraps the SDK functions provided by kopia to connect to underlying storage providers - S3, Azure, GCP, Filestore
+2. Build another pkg on top of the library implemented in #1 that can be used to perform repository operations
+
+
+## High Level Design
+
+### Kopia Library
+
+#### 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
+	}
+}
+
+```
+
+
+##### Example implementation of storage interface
+
+```go
+
+package storage
+
+import (
+	"context"
+
+	"github.com/kanisterio/kanister/pkg/kopialib"
+	"github.com/kanisterio/kanister/pkg/utils"
+	"github.com/kopia/kopia/repo/blob"
+	"github.com/kopia/kopia/repo/blob/s3"
+)
+
+type s3Storage struct {
+	options *s3.Options
+	create  bool
+}
+
+// This uses `github.com/kopia/kopia/repo/blob/s3` of kopia
+// to connect to underlying storage
+func (s *s3Storage) Connect() (blob.Storage, error) {
+	return s3.New(context.Background(), s.Options, s.create)
+}
+
+// WithOptions function can be used if someone wants
+// to set the s3 configuration directly using
+// `github.com/kopia/kopia/repo/blob/s3` pkg
+func (s *s3Storage) WithOptions(opts s3.Options) {
+	s.options = &opts
+}
+
+// WithCreate function can be used
+// if someone wants to create the underlying storage
+func (s *s3Storage) WithCreate(create bool) {
+	s.create = create
+}
+
+// SetOptions function is a generic way to set the configuration
+// of underlying storage using a map
+func (s *s3Storage) SetOptions(ctx context.Context, options map[string]string) {
+	s.options = &s3.Options{
+		BucketName:      options[kopialib.BucketKey],
+		Endpoint:        options[kopialib.S3EndpointKey],
+		Prefix:          options[kopialib.PrefixKey],
+		Region:          options[kopialib.S3RegionKey],
+		SessionToken:    options[kopialib.S3TokenKey],
+		AccessKeyID:     options[kopialib.S3AccessKey],
+		SecretAccessKey: options[kopialib.S3SecretAccessKey],
+	}
+	s.options.DoNotUseTLS, _ = utils.GetBoolOrDefault(options[kopialib.DoNotUseTLS], true)
+	s.options.DoNotVerifyTLS, _ = utils.GetBoolOrDefault(options[kopialib.DoNotVerifyTLS], true)
+}
+
+```
+
+
+#### 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")
+}
+
+```