[NP] add platform main principles

src/core/PRINCIPLES.md

@@ -0,0 +1,47 @@
+## Common dictionary
+Plugin - a piece of software expressing specific subject of the business unit within the code.
+
+Platform - functionality required to run all the Kibana plugins.
+
+## New platform principles
+### Explicit business domains separation
+The plugins code should be structured on the top level to reflect business units.
+```js
+// GOOD
+src/plugins
+ - apm
+  - server
+  - public
+ - search
+ ..
+// BAD
+src/plugins
+ - server
+  - apm
+  - search
+```
+### Explicit dependencies
+Each plugin should declare dependencies on the other plugins explicitly. Plugins cannot have circular dependencies. Plugins shouldn't access runtime objects, HTTP endpoints, DOM nodes, etc. created by a third party plugin without declaring a dependency on this plugin.
+```json
+"requiredPlugins": ["search"],
+"optionalPlugins": ["apm"],
+```
+### Explicit API declaration
+Each plugin has to define an explicit API. Any other API's that has not been declared explicitly as public should be considered private. HTTP endpoints belonging to other plugins are considered private. Plugins should expose a JavaScript client on top of these HTTP endpoints for other plugins to consume.
+```js
+// GOOD
+deps.plugin.getData();
+// BAD.
+`GET /api/plugin/data`
+```
+### Encapsulated state
+Each plugin encapsulates its internal state. It doesn't rely on any kind of global state. Plugins provide an internal state via explicit API, reflecting the dynamic nature of the state (an event bus, observables, getter/setter functions). A plugin can change other plugin state by calling its public API method.
+```js
+// GOOD
+deps.plugin.getData();
+deps.plugin.data$.subscribe();
+deps.plugin.setAddress('...');
+// BAD.
+deps.plugin._data;
+deps.plugin.data = '...';
+```

src/core/TESTING.md

@@ -247,8 +247,3 @@ _How to test against specific plugin APIs (eg. data plugin)_
 ## Plugin Contracts
 
 _How to test your plugin's exposed API_
-
-Guidelines:
-- Plugins should never interact with other plugins' REST API directly
-- Plugins should interact with other plugins via JavaScript contracts
-- Exposed contracts need to be well tested to ensure breaking changes are detected easily