Created Design: Flow Linter (markdown)

Design:-Flow-Linter.md

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+# Purpose
+Node-RED makes it easy to program by wiring nodes together.  But sometimes, because of its less restrictive nature, programmer may write flows that are hard to understand.  For example: 
+- Function node that has no name or description.  The reader need to read internal JavaScript code to understand what the node do,
+- Excessive amount of nodes in a single flow.
+
+And, some subtle differences in programming style cause readability low.
+- Size of flow on Flow Editor
+- Grid size
+- JavaScript coding style in function nodes.
+
+The flow linter provides a framework that automatically checks whether a flow is compliant with rules and conventions. 
+
+# Use cases
+- To use the consistent coding rules in a team, the tool checks a flow before submit to a repository.
+- To prevent bugs, the tool checks a flow when the programmer requests it, or continuously checks as a background task of Flow Editor. 
+  - Editor displays suggestions to user: 'This HTTP-in node doesn't have corresponding HTTP-out node.', 'These nodes in this flow must preserve message properties set by HTTP-in', etc.
+- To prevent move/edit/delete node accidentally, some node position or property change can be restricted by rules.
+
+# Requirements / ideas
+- There are various rules/conventions, and each organization/community/etc. has different policies on it.  Because of this, the rules has to be pluggable and customizable.
+- Linter is used in both batch-style (e.g. command-line interface) and on-the-fly-style (e.g. integrated in Editor).  The core service of linter can be used in both usages.
+- Rule configuration can be exported as JSON format and can be included in flow.json so that developers can distribute flow templates with their own restrictions.
+- Use compatible format with other linter/tester tools for result output.
+
+# User interface
+
+## Command-line interface (batch-style)
+![External CLI](https://github.com/node-red-hitachi/node-red/wiki/images/linter/external-cli.png)
+
+## Integrated in Editor (in Sidebar, batch-style)
+![Sidebar](https://github.com/node-red-hitachi/node-red/wiki/images/linter/sidebar.png)
+
+
+# Architecture
+
+![Functional diagram](https://github.com/node-red-hitachi/node-red/wiki/images/linter/func-diagram.png)
+
+## Flow manipulation API
+The flow manipulation API provides a highlevel interface to handle flow.json file.
+Programmers need not to know about the format of flow.json and structure of flow object.
+
+## pluggable rules
+Developer can create their own rules by writing plug-in module.
+
+Following code shows example of plug-in, that checks existance of name of function node.
+
+```javascript
+function check (afs, conf, cxt) {
+    const funcs = [...afs]               // extract all nodes 
+        .filter(e => e.type==='function')  // filter out unrelated nodes
+        .map(e => {
+            return {id:e.id, name:e.name}; // extract their node id and name
+        });
+    const verified = funcs
+        .filter(e=> e.name === undefined || e.name === "")  // check existance of name
+        .map(e=> {
+            return {rule:"no-func-name", id:e.id, result:conf}; // generate result
+        });
+
+    return verified;
+}
+
+module.exports = {
+    check: check
+};
+
+```
+- Flow-check code in plug-in is used on both CLI (runs on node.js) and Editor (runs on browser).  To use same code in both node.js and browser, there are tools for generate codes for both:
+  - [Browserify](http://browserify.org/): get all dependent npm modules and put in one script file.
+  - [Babel](https://babeljs.io/): convert modern JavaScript (ES2015+) code to (traditional) JavaScript code that can be executed on various browsers.
+
+## Configuration
+Linter reads configuration files in following order:
+1. Default configuration: nrlint.js in Node-RED install directory
+2. Project-specific configuration: $HOME/.node-red/project/projectname/nrlint.js
+3. Per-user configuration: $HOME/.node-red/nrlint.js
+4. Command-line argument: nrlint --config nrlint.js / node-red --lintconfig nrlint.js
+
+If there is a conflict between them, latter definition overrides former one.
+
+- File format:
+
+```javascript
+module.exports = {
+    "plugins": [
+        "func-style"
+    ],
+    "rules": {
+        "func-style/no-func-name": "warn",
+        "func-style/eslint": { semi: 2 }
+    }
+}
+```
+
+## Report format
+- should be similar format with other linting tools (e.g. ESlint, JShint, ...)
+
+## Other Consideration
+- When validation is invoked in Editor, validation codes should be executed in editor.  API call to server-side is expencive, and it is need to add extra adminAPI in the core.
+
+# Implementation plan
+
+## First step
+- Implement CLI version
+  - Considering future extention, e.g. use from Flow Editor, on-the-fly checking, etc. 
+
+## Next steps
+- Implement Editor-integrated version (in Sidebar, batch-style)
+- Rule configuration UI
+- Make rules be exported as JSON format and can be included in flow.json so that developers can distribute flow templates with their own restrictions.
+- On-the-fly checking in Editor
+![On the fly](https://github.com/node-red-hitachi/node-red/wiki/images/linter/on-the-fly.png)
+
+# Related works
+- [Design: Flow Manipulation API](https://github.com/node-red/node-red/wiki/Design%3A-Flow-Manipulation-API)