FFM-9927 Update MM3 Hash / Fallback to identifier for BB (#60)

CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [3.0.0] - 2023-12-04
+## ** Breaking for Erlang applications (not affecting Elixir applications) **
+
+- The percentage rollout hash algorithm was slightly different compared to other Feature Flags SDKs, which resulted
+in a different bucket allocation for the same target. The overall distribution was the same, but this change ensures
+that the same target will get the same allocation per SDK
+- if a custom BucketBy field is set on the web app, but it can't be found in a target, then the SDK will fall back
+to bucketing by target identifier for that target a warning will be logged
+
 ## [2.0.1] - 2023-07-02
 ### Fixes
 - Some SDK dependencies were not getting included in releases created by `mix `
@@ -150,4 +159,3 @@ Evaluation Rules not yet complete:
 - Pre-requisites
 - Percentage Rollout
 
-

rebar.config

@@ -53,10 +53,4 @@
   ]
 }.
 
-{
-  shell,
-  [
-    % {config, "config/sys.config"},
-    {apps, [cfclient]}
-  ]
-}.
+{shell, [{config, "config/sys.config"}, {apps, [cfclient]}]}.

src/cfclient.app.src

@@ -4,7 +4,7 @@
   [
     {description, "Harness Feature Flags Server SDK"},
     {pkg_name, "harness_ff_erlang_server_sdk"},
-    {vsn, "2.0.1"},
+    {vsn, "3.0.0"},
     {registered, []},
     {mod, {cfclient_app, []}},
     {applications, [kernel, stdlib, cfapi, base64url, murmur, jsx, mochiweb]},

src/cfclient_evaluator.erl

@@ -387,9 +387,19 @@ search_rules_for_inclusion([Rule | Tail], Target, Config) ->
         Distribution when Distribution /= null ->
           #{bucketBy := BucketBy, variations := Variations} = Distribution,
           #{identifier := Id, name := Name} = Target,
-          Attributes = maps:get(attributes, Target, #{}),
-          TargetAttributeValue = get_attribute_value(Attributes, BucketBy, Id, Name),
-          apply_percentage_rollout(Variations, BucketBy, TargetAttributeValue, 0)
+          TargetAttributes = maps:get(attributes, Target, #{}),
+          TargetAttributeValue = get_attribute_value(TargetAttributes, BucketBy, Id, Name),
+          {FinalTargetAttributeValue, FinalBucketBy} =
+            case TargetAttributeValue of
+              <<>> ->
+                % If the BucketBy field isn't found in the target, then fall back to the ID and log a warning
+                ?LOG_WARNING("BucketBy attribute '~p' not found in target attributes, falling back to 'identifier' ~p", [BucketBy, Id]),
+                {Id, <<"identifier">>};
+
+              _ ->
+                {TargetAttributeValue, BucketBy}
+            end,
+          apply_percentage_rollout(Variations, FinalBucketBy, FinalTargetAttributeValue, 0)
       end;
 
     _ -> search_rules_for_inclusion(Tail, Target, Config)
@@ -557,7 +567,7 @@ apply_percentage_rollout([], _, _, _) -> excluded.
 
 -spec should_rollout(binary(), binary(), integer()) -> boolean().
 should_rollout(BucketBy, TargetValue, Percentage) ->
-  Concatenated = <<TargetValue/binary, ":", BucketBy/binary>>,
+  Concatenated = <<BucketBy/binary, ":", TargetValue/binary>>,
   % Using a pure Elixir library for murmur3
   Hash = 'Elixir.Murmur':hash_x86_32(Concatenated),
   BucketID = (Hash rem 100) + 1,

src/cfclient_metrics_attributes.hrl

@@ -18,6 +18,6 @@
 %% Constants for Metrics Data Attribute Values
 
 -define(SDK_TYPE_ATTRIBUTE_VALUE, <<"server">>).
--define(SDK_VERSION_ATTRIBUTE_VALUE, <<"1.1.0">>).
+-define(SDK_VERSION_ATTRIBUTE_VALUE, <<"3.0.0">>).
 -define(SDK_LANGUAGE_ATTRIBUTE_VALUE, <<"erlang">>).
 -define(TARGET_GLOBAL_IDENTIFIER, <<"__global__cf_target">>).

src/murmur_example.erl

@@ -0,0 +1,9 @@
+-module(murmur_example).
+
+-export([run_hash/0]).
+
+run_hash() ->
+  Concatenated = "identifier:test",
+  % Using a pure Elixir library for murmur3
+  Hash = 'Elixir.Murmur':hash_x86_32(Concatenated),
+  BucketID = (Hash rem 100) + 1.

test/cfclient_evaluator_test_data.erl

@@ -29,8 +29,8 @@
     prerequisite_matches_flag_2/0,
     prerequisite_matches_flag_3/0,
     generate_targets/2,
-    percentage_rollout_boolean/2,
-    percentage_rollout_multi_variate/3
+    percentage_rollout_boolean/3,
+    percentage_rollout_multi_variate/4
   ]
 ).
 
@@ -905,7 +905,7 @@ target_group_for_percentage_rollout() ->
   }.
 
 
-percentage_rollout_boolean(Weight1, Weight2) ->
+percentage_rollout_boolean(Weight1, Weight2, BucketBy) ->
   #{
     defaultServe => #{variation => <<"true">>},
     environment => <<"dev">>,
@@ -937,7 +937,7 @@ percentage_rollout_boolean(Weight1, Weight2) ->
           distribution
           =>
           #{
-            bucketBy => <<"identifier">>,
+            bucketBy => BucketBy,
             variations
             =>
             [
@@ -959,7 +959,7 @@ percentage_rollout_boolean(Weight1, Weight2) ->
     version => 4
   }.
 
-percentage_rollout_multi_variate(Weight1, Weight2, Weight3) ->
+percentage_rollout_multi_variate(Weight1, Weight2, Weight3, BucketBy) ->
   #{
     defaultServe => #{variation => <<"variation3">>},
     environment => <<"dev">>,
@@ -991,7 +991,7 @@ percentage_rollout_multi_variate(Weight1, Weight2, Weight3) ->
           distribution
           =>
           #{
-            bucketBy => <<"identifier">>,
+            bucketBy => BucketBy,
             variations
             =>
             [