[GitHub] jaydoane commented on a change in pull request #1682: Feature flags

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jaydoane commented on a change in pull request #1682: Feature flags
URL: https://github.com/apache/couchdb/pull/1682#discussion_r232079858
 
 

 ##########
 File path: src/couch/src/couch_flags.erl
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 @@ -0,0 +1,108 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+% http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+% This module serves two functions
+% - provides public API to use to get value for a given feature flag and subject
+% - implements {feature_flags, couch_flags} service
+
+% The module rely on couch_epi_data_gen which uses the data returned by
+% `couch_flags_config:data()` to generate callback module `couch_epi_data_gen_flags_config`.
+% The generated module shouldn't be used directly. We use following APIs
+% - `couch_epi:get_handle({flags, config})` - to get handler (name of generated module)
+% - `couch_epi:get_value(Handle, Key) - to do efficient matching
+%
+% The generated module implements clauses like the following
+%  - get(couch, {binary_match_rule()}) ->
+%       {matched_pattern(), size(matched_pattern()), [flag()]} | undefined
+% For example
+%  - get(couch, {<<"/shards/test/exact">>}) ->
+%        {<<"/shards/test/exact">>,18,[baz,flag_bar,flag_foo]};
+%  - get(couch, {<<"/shards/test", _/binary>>}) ->
+%        {<<"/shards/test*">>,13,[baz,flag_bar,flag_foo]};
+%  - get(couch, {<<"/shards/exact">>}) ->
+%        {<<"/shards/exact">>,13,[flag_bar,flag_foo]};
+%  - get(couch, {<<"/shards/blacklist", _/binary>>}) ->
+%        {<<"/shards/blacklist*">>,18,[]};
+%  - get(couch, {<<"/", _/binary>>}) ->
+%        {<<"/*">>,2,[flag_foo]};
+%  - get(_, _) -> undefined.
+%
+% The `couch_epi:get/2` uses the Handler module to implement efficient matching.
+
+% In order to distinguish between shards and clustered db the following
+% convention is used.
+% - it is a shard if pattern starts with `/`
+
+-module(couch_flags).
+
+%% Public API
+-export([
+    enabled/1,
+    is_enabled/2
+]).
+
+%% For internal use
+-export([
+    rules/0
+]).
+
+%% For use from plugin
+-export([
+    subject_key/1
+]).
+
+-include_lib("couch/include/couch_db.hrl").
+-include_lib("mem3/include/mem3.hrl").
+-include("couch_db_int.hrl").
+
+-define(SERVICE_ID, feature_flags).
+
+enabled(Subject) ->
 
 Review comment:
   I would love to see type specs defined for public functions

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