Updated Design: Persistable Context (markdown)

Design:-Persistable-Context.md

@@ -98,6 +98,23 @@ The value set in the config is passed to the plugin. These items are defined by
 >> but I'm worry that a user may continue to use existing memory context unintentionally.  
 >> I think Node-RED should send warn to user. (but it will be too noisy...)
 >>> NOL: rather than 'if not specified', what if they explicitly set `default` to `"memory"` as a special identifier.
+>>>> KN: I agree with supporting `"memory"` module. But I think that the case of 'if not specified' should happen when an existing user upgraded Node-RED.
+>>>> There are three options to handle such a case. I prefer the option 1.
+>>>> 1. Throw an error.
+>>>> 2. Store the context variable in memory.
+>>>> 3. Store the context variable in memory, and notify a user of the undefined module name.
+>>>> 
+>>>> I listed the actual cases below.  The above handling is related to the case 3.
+
+- There are three different scenarios when importing a flow that uses context storage.
+1. A user wants to run the flow as is by settings a context storage appropriately.
+1. A user wants to always store context variable on the same storage (e.g. memory, file).
+1. A user wants to receive an error if a context storage is used involuntarily.
+
+- To support all of the above cases, when the imported flow contains a persistable context:
+1. if the identifier is defined, use the context storage.
+1. if the identifier is not defined but `default` identifier is defined, use the default context storage.
+1. if neither the identifier nor a `default` identifier are not defined, throw an exception.
 
 ## Executing plugin specific function  
 Currently, `get` / `set` / `keys` is only methods to handle Context.