Remove Split::Cache and make config.cache a no-op

The cache lives in each process, so a winner set from the dashboard stayed
stale in the other workers. It only saves 10 of 64 roundtrips on a dashboard
page and none on ab_test, so it is no longer worth the risk.

config.cache stays as an accessor so apps that set it keep booting.

Author: andrehjr

README.md

@@ -779,22 +779,11 @@ split_config = YAML.load_file(Rails.root.join('config', 'split.yml'))
 Split.redis = split_config[Rails.env]
 ```
 
-### Redis Caching (v4.0+)
+### Redis Caching
 
-> [!WARNING]
-> The caching mechanism has known issues in multi-process environments (e.g., Puma) and multi-server deployments with load balancers (typical production environments). Cache invalidation does not propagate across processes or servers, which can lead to inconsistent experiment data being served to users. See [#735](https://github.com/splitrb/split/issues/735) for details. Use with caution in production environments.
-
-In some high-volume usage scenarios, Redis load can be incurred by repeated
-fetches for fairly static data.  Enabling caching will reduce this load.
-
- ```ruby
-Split.configuration.cache = true
-````
-
-This currently caches:
-  - `Split::ExperimentCatalog.find`
-  - `Split::Experiment.start_time`
-  - `Split::Experiment.winner`
+`Split.configuration.cache` no longer has any effect and can be removed. The cache
+did not stay consistent across processes (see [#735](https://github.com/splitrb/split/issues/735)),
+and Split now makes few enough Redis calls that it is no longer needed.
 
 ## Namespaces
 

lib/split.rb

@@ -7,7 +7,6 @@
 require "split/algorithms/weighted_sample"
 require "split/algorithms/whiplash"
 require "split/alternative"
-require "split/cache"
 require "split/configuration"
 require "split/encapsulated_helper"
 require "split/exceptions"
@@ -69,10 +68,6 @@ def configure
     self.configuration ||= Configuration.new
     yield(configuration)
   end
-
-  def cache(namespace, key, &block)
-    Split::Cache.fetch(namespace, key, &block)
-  end
 end
 
 # Check to see if being run in a Rails application.  If so, wait until before_initialize to run configuration so Gems that create ENV variables have the chance to initialize first.

lib/split/cache.rb

@@ -17,11 +17,9 @@ class Experiment
     }
 
     def self.find(name)
-      Split.cache(:experiments, name) do
-        experiment = Experiment.new(name)
-        experiment.load_from_redis
-        experiment unless experiment.alternatives.empty?
-      end
+      experiment = Experiment.new(name)
+      experiment.load_from_redis
+      experiment unless experiment.alternatives.empty?
     end
 
     def self.find_many(names)
@@ -149,13 +147,11 @@ def alternatives=(alts)
     end
 
     def winner
-      Split.cache(:experiment_winner, name) do
-        experiment_winner = redis.hget(:experiment_winner, name)
-        if experiment_winner
-          Split::Alternative.new(experiment_winner, name)
-        else
-          nil
-        end
+      experiment_winner = redis.hget(:experiment_winner, name)
+      if experiment_winner
+        Split::Alternative.new(experiment_winner, name)
+      else
+        nil
       end
     end
 
@@ -181,7 +177,6 @@ def control
     def reset_winner
       redis.hdel(:experiment_winner, name)
       @has_winner = false
-      Split::Cache.clear_key(@name)
     end
 
     def start
@@ -191,9 +186,7 @@ def start
     def start_time
       return @start_time if defined?(@start_time)
 
-      @start_time = Split.cache(:experiment_start_times, @name) do
-        parse_start_time(redis.hget(:experiment_start_times, @name))
-      end
+      @start_time = parse_start_time(redis.hget(:experiment_start_times, @name))
     end
 
     def prefetch_trial_state!
@@ -258,7 +251,6 @@ def resettable?
 
     def reset
       Split.configuration.on_before_experiment_reset.call(self)
-      Split::Cache.clear_key(@name)
       alternatives.each(&:reset)
       reset_winner
       Split.configuration.on_experiment_reset.call(self)

lib/split/experiment.rb

@@ -247,6 +247,17 @@ def alternative(color)
     it "should have no winner initially" do
       expect(experiment.winner).to be_nil
     end
+
+    it "should see a winner changed by another process when the cache setting is on" do
+      Split.configuration.cache = true
+      experiment.save
+      experiment.winner = "red"
+      expect(experiment.winner.name).to eq("red")
+
+      Split.redis.hset(:experiment_winner, "link_color", "blue")
+
+      expect(experiment.winner.name).to eq("blue")
+    end
   end
 
   describe "winner=" do

spec/cache_spec.rb

@@ -23,7 +23,6 @@ module GlobalSharedContext
     Split.redis = Redis.new
     Split.redis.select(10)
     Split.redis.flushdb
-    Split::Cache.clear
     @ab_user = mock_user
     @params = nil
   end