Updated Design: Persistable Context (markdown)

Design:-Persistable-Context.md

@@ -193,4 +193,24 @@ My proposal is as follows. Do you agree with this option?
   - Choose the option 2.
   - Encourage a unique key name by writing it on design note.
 
-> NOL: agreed - fall back to the `default` store, and remove the store name from the key. I think having multiple storage plugins will be rare, so this is an very much an edge case.
+> NOL: agreed - fall back to the `default` store, and remove the store name from the key. I think having multiple storage plugins will be rare, so this is an very much an edge case.
+
+#### How to get keys in the external storage
+In [Usage](#usage),
+> If you want to access the external storage, add the prefix(*) to the key of context data as follows.  
+Currently,`keys()` needs no arguments.  
+How we get all keys in the external storage?  
+  
+1. Adding a parameter to `keys()`  
+   `keys("$identifier")` returns all keys in the specified storage.  
+   If `$identifier` is `null` or `undefined`, `keys()` return all keys in the memory context.  
+   If a user uses a LocalFileSystem plugin as `$file`, `keys("$file")` returns all keys in the LocalFileSystem context.  
+
+2. No arguments same as current  
+   a. `keys()` returns all keys in all external storage.  
+      If a user uses a LocalFileSystem plugin, `keys()` returns all keys in the memory and LocalFileSystem context.  
+      If this is chosen, we have to discuss about returned keys format.  
+  
+   b. `keys()` returns all keys in the memory context.  
+       Node-RED don't provide the interface of `keys` to plugin. So a user cannot get all keys by `keys()`.  
+       If a plugin wants to returns keys, it will use `run()` to return keys.