Introduce Operation for clarity

extension/storage/README.md

@@ -17,14 +17,22 @@ Delete(context.Context, string) error
 Close(context.Context) error
 ```
 
-It is possible to execute several operations in a single transaction via `BatchOp`. The method takes
-two collections as arguments:
-* list of keys for which the values are retrieved and returned,
-* a map, which specifies the key/value pairs that are going to be updated; when a value is `nil`, the key is deleted.
+It is possible to execute several operations in a single transaction via `Batch`. The method takes a collection of
+`Operation` arguments (each of which contains `Key`, `Value` and `Type` properties):
 ```
-BatchOp(context.Context, []string, map[]string[]byte) ([][]byte, error)
+Batch(context.Context, ...Operation) error
 ```
 
+The elements itself can be created using:
+
+```
+SetOperation(string, []byte) Operation
+GetOperation(string) Operation
+DeleteOperation(string) Operation
+```
+
+Get operation results are stored in-place into the given Operation and can be retrieved using its `Value` property.
+
 Note: All methods should return error only if a problem occurred. (For example, if a file is no longer accessible, or if a remote service is unavailable.)
 
-Note: It is the responsibility of each component to `Close` a storage client that it has requested.
+Note: It is the responsibility of each component to `Close` a storage client that it has requested.

extension/storage/nop_client.go

@@ -46,6 +46,6 @@ func (c nopClient) Close(context.Context) error {
 }
 
 // Batch does nothing, and returns nil, nil
-func (c nopClient) Batch(ctx context.Context, strings []string, entries map[string][]byte) ([][]byte, error) {
-	return nil, nil // no result, but no problem
+func (c nopClient) Batch(context.Context, ...Operation) error {
+	return nil // no result, but no problem
 }

extension/storage/storage.go

@@ -53,11 +53,48 @@ type Client interface {
 	// Delete will delete data associated with the specified key
 	Delete(context.Context, string) error
 
-	// Batch will, respectively - get values for selected keys or upsert key/values. When the value specified
-	// is nil, the key is being deleted. It will return an array of results, where each
-	// one corresponds to a key at a given position and will be nil, if key is not found
-	Batch(context.Context, []string, map[string][]byte) ([][]byte, error)
+	// Batch handles specified operations in batch. Get operation results are put in-place
+	Batch(context.Context, ...Operation) error
 
 	// Close will release any resources held by the client
 	Close(context.Context) error
 }
+
+const (
+	Get = iota
+	Set
+	Delete
+)
+
+type operation struct {
+	// Key specifies key which is going to be get/set/deleted
+	Key string
+	// Value specifies value that is going to be set or holds result of get operation
+	Value []byte
+	// Type describes the operation type
+	Type int
+}
+
+type Operation *operation
+
+func SetOperation(key string, value []byte) Operation {
+	return &operation{
+		Key:   key,
+		Value: value,
+		Type:  Set,
+	}
+}
+
+func GetOperation(key string) Operation {
+	return &operation{
+		Key:  key,
+		Type: Get,
+	}
+}
+
+func DeleteOperation(key string) Operation {
+	return &operation{
+		Key:  key,
+		Type: Delete,
+	}
+}