[DOC] Design doc for option values inheriting from prop in config.yaml (#483)

Author: dennisseah

technical-docs/designs/commands/inheritFieldInOnBoardDoc.png

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+# Software Design Document
+
+Reference: Simplification of command option values inheriting configuration
+values from spk config yaml command<br> Authors: Andre Briggs, Dennis Seah
+
+| Revision | Date         | Author      | Remarks                                                 |
+| -------: | ------------ | ----------- | ------------------------------------------------------- |
+|      0.1 | Mar-28, 2020 | Dennis Seah | Initial Draft                                           |
+|      1.0 | Mar-31, 2020 | Dennis Seah | Set revision as 1.0, doc is presented in review meeting |
+
+## 1. Overview
+
+By Design, `spk` command option values inherit from configuration values from
+spk config yaml. This is to help the user from having to provide these values in
+the command line. However, in our generate documents and `commander` generated
+help, users are not informed about these inheritance. In this design doc, we
+propose a way to add this inheritance information in our generated operation
+document; and also a helper function to automatically populate values from
+`config.yaml` to the command option values.
+
+## 2. Out of Scope
+
+This design shall only target making user experience better by providing
+inheritance information in operation help document; and the helper function to
+populate values from `config.yaml` to the command option values. It will
+succinct explain how this function works.
+
+## 3. Design Details
+
+### 3.1 New property in command json file.
+
+Taking this option from `spk deployment onboard` command for example.
+
+```
+  "options": [
+    {
+      "arg": "-s, --storage-account-name <storage-account-name>",
+      "description": "Azure storage account name",
+      "required": true
+    },
+```
+
+We introduce a new optional property `inherit`
+
+```
+  "options": [
+    {
+      "arg": "-s, --storage-account-name <storage-account-name>",
+      "description": "Azure storage account name",
+      "inherit": "introspection.azure.account_name",
+      "required": true
+    },
+```
+
+With this new property, we have the opportunities to
+
+1. add inheritance information in operation help document;
+1. create helper function to populate values from `config.yaml` to the command
+   option values.
+1. developers can have a clear understand of what values in `config.yaml` are
+   inherited in which commands.
+
+### 3.2 Add inheritance information in operation help document
+
+In the
+[spk.js](https://github.com/CatalystCode/spk/blob/master/docs/commands/spk.js)
+javascript, we can add code to insert inheritance information like this.
+
+<p style="text-align:center">
+<img src="inheritFieldInOnBoardDoc.png" width="600px">
+</p>
+
+### 3.3 Helper function to populate values from config.yaml
+
+the pseudo code is like this
+
+```
+interface ConfigValues {
+  [key:string]: string | ConfigValues | undefined
+}
+
+interface CommandValues {
+  [key:string]: string | undefined
+}
+
+export const populateInheritValueFromConfig = (
+  options: CommandOption[], # from json decorator
+  config: ConfigValues,     # from config.yaml
+  values: CommandValues     # user input
+): void => {
+  if [config && options] # do nothing if these are absent
+    get options that have inherit property set
+    for each of these options
+      if values do not contain value for the option
+        find the value in config and set it to values object
+  }
+};
+```
+
+## 4. Dependencies
+
+None
+
+## 5. Risks & Mitigation
+
+None
+
+## 6. Documentation
+
+Document how to add `inherit` property in json file in our developer guide.
+
+\- end -