diff --git a/accounting/storage/__init__.py b/accounting/storage/__init__.py
index 1403245..bbaa16d 100644
--- a/accounting/storage/__init__.py
+++ b/accounting/storage/__init__.py
@@ -1,19 +1,35 @@
+from abc import ABCMeta, abstractmethod
 
-class Storage:
+
+class Storage():
     '''
     ABC for accounting storage
     '''
+    __metaclass__ = ABCMeta
+
     def __init__(self, *args, **kw):
-        raise NotImplementedError()
+        pass
 
+    @abstractmethod
     def get_transactions(self, *args, **kw):
-        raise NotImplementedError()
+        raise NotImplementedError
 
+    @abstractmethod
     def get_transaction(self, *args, **kw):
-        raise NotImplementedError()
+        raise NotImplementedError
 
+    @abstractmethod
     def get_account(self, *args, **kw):
-        raise NotImplementedError()
+        raise NotImplementedError
 
+    @abstractmethod
     def get_accounts(self, *args, **kw):
-        raise NotImplementedError()
+        raise NotImplementedError
+
+    @abstractmethod
+    def update_transaction(self, transaction):
+        raise NotImplementedError
+
+    @abstractmethod
+    def reverse_transaction(self, transaction_id):
+        raise NotImplementedError