Feature flags

src/couch/src/couch_db_epi.erl

@@ -35,14 +35,15 @@ providers() ->
 
 services() ->
     [
-        {couch_db, couch_db_plugin}
+        {couch_db, couch_db_plugin},
+        {feature_flags, couch_flags}
     ].
 
 data_subscriptions() ->
     [].
 
 data_providers() ->
-    [].
+    [couch_flags_config:data_provider()].
 
 processes() ->
     [].

src/couch/src/couch_flags.erl

@@ -0,0 +1,135 @@
+% 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 relies 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").
+
+-type subject()
+    :: #db{}
+        | #httpd{}
+        | #shard{}
+        | #ordered_shard{}
+        | string()
+        | binary().
+
+-define(SERVICE_ID, feature_flags).
+
+-spec enabled(subject()) -> [atom()].
+
+enabled(Subject) ->
+    Key = maybe_handle(subject_key, [Subject], fun subject_key/1),
+    Handle = couch_epi:get_handle({flags, config}),
+    lists:usort(enabled(Handle, {<<"/", Key/binary>>})
+        ++ enabled(Handle, {couch_db:normalize_dbname(Key)})).
+
+-spec is_enabled(FlagId :: atom(), subject()) -> boolean().
+
+is_enabled(FlagId, Subject) ->
+    lists:member(FlagId, enabled(Subject)).
+
+-spec rules() ->
+    [{Key :: string(), Value :: string()}].
+
+rules() ->
+    Handle = couch_epi:get_handle(?SERVICE_ID),
+    lists:flatten(couch_epi:apply(Handle, ?SERVICE_ID, rules, [], [])).
+
+-spec enabled(Handle :: couch_epi:handle(), Key :: {binary()}) -> [atom()].
+
+enabled(Handle, Key) ->
+    case couch_epi:get_value(Handle, couch, Key) of
+        {_, _, Flags} -> Flags;
+        undefined -> []
+    end.
+
+-spec subject_key(subject()) -> binary().
+
+subject_key(#db{name = Name}) ->
+    subject_key(Name);
+subject_key(#httpd{path_parts=[Name | _Rest]}) ->
+    subject_key(Name);
+subject_key(#httpd{path_parts=[]}) ->
+    <<>>;
+subject_key(#shard{name = Name}) ->
+    subject_key(Name);
+subject_key(#ordered_shard{name = Name}) ->
+    subject_key(Name);
+subject_key(Name) when is_list(Name) ->
+    subject_key(list_to_binary(Name));
+subject_key(Name) when is_binary(Name) ->
+    Name.
+
+-spec maybe_handle(
+        Function :: atom(),
+        Args :: [term()],
+        Default :: fun((Args :: [term()]) -> term())) ->
+    term().
+
+maybe_handle(Func, Args, Default) ->
+    Handle = couch_epi:get_handle(?SERVICE_ID),
+    case couch_epi:decide(Handle, ?SERVICE_ID, Func, Args, []) of
+        no_decision when is_function(Default) ->
+            apply(Default, Args);
+        {decided, Result} ->
+            Result
+    end.