[GitHub] iilyak opened a new pull request #1682: Feature flags

[GitBox <gi...@apache.org>]

iilyak opened a new pull request #1682: Feature flags
URL: https://github.com/apache/couchdb/pull/1682
 
 
   ## Overview
   
   Once in a while we merge a highly experimental features. It is beneficial to be able to have the codepath for these features effectivelly disabled by default until the implementation is stabilized. 
   This PR introduces an ability to add feature flag checks for experimental features.   
   This feature is for developers only and wouldn't need documentation.
   
   ### The feature configuration
   
   ```
   [feature_flags]
   
   my_feature||* = false
   my_feature||mytestdatabaseprefix* = true
   ```
   
   Notes:
   - The `*` is only allowed at the end of the path
   - In order to distinguish between shards and clustered db the following convention is used.
     - it is a shard if pattern starts with `/`
   - It is also possible to enable multiple flags for the path using alternative syntax
     - `mytestdatabaseprefix* = [my_featureA, my_featureB]`
   
   ### Usage of the feature in the code
   
   ```
   case couch_flags:is_enabled(my_feature, Subject) of
       true -> ...;
       false -> ...
   end
   ```
   
   Where Subject is one of `#db{}` | `#httpd{)` | `#shard{}` | `#ordered_shard{}` | `ShardName :: binary()` | `ShardName :: list()` | `DbName :: binary()` | `DbName :: list()`
   
   ### Performance
   
   There is a naive performance test in the test suite it does 1 million of calls to `is_enabled/2`. One million of calls takes "1.884 s" on the laptop.
   
   ### Security
   
   It is not safe to allow configuration of the feature flags over http since there might be a possibility of injecting arbitrary code via name of the configuration key within `feature_flags` section. Therefore the http access is disabled. 
   
   ## Testing recommendations
   
   ```
   make eunit apps=couch suites=couch_flags_tests,couch_flags_config
   ```
   
   ## Related Issues or Pull Requests
   
   N/A
   
   ## Checklist
   
   - [x] Code is written and works correctly;
   - [x] Changes are covered by tests;
   - [ ] Documentation reflects the changes;
   

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