fix(analytics): allow capability to offload reportExposure to async thread (SDK-80)

lib/mixpanel-ruby/flags/flags_provider.rb

@@ -12,7 +12,7 @@ module Flags
     # Base class for feature flags providers
     # Provides common HTTP handling and exposure event tracking
     class FlagsProvider
-      # @param provider_config [Hash] Configuration with :token, :api_host, :request_timeout_in_seconds
+      # @param provider_config [Hash] Configuration with :token, :api_host, :request_timeout_in_seconds, :exposure_executor
       # @param endpoint [String] API endpoint path (e.g., '/flags' or '/flags/definitions')
       # @param tracker_callback [Proc] Function used to track events (bound tracker.track method)
       # @param evaluation_mode [String] The feature flag evaluation mode. This is either 'local' or 'remote'
@@ -23,6 +23,7 @@ def initialize(provider_config, endpoint, tracker_callback, evaluation_mode, err
         @tracker_callback = tracker_callback
         @evaluation_mode = evaluation_mode
         @error_handler = error_handler
+        @exposure_executor = provider_config[:exposure_executor]
       end
 
       # Make HTTP request to flags API endpoint
@@ -104,12 +105,36 @@ def track_exposure_event(flag_key, selected_variant, context, latency_ms = nil)
         properties['$is_experiment_active'] = selected_variant.is_experiment_active unless selected_variant.is_experiment_active.nil?
         properties['$is_qa_tester'] = selected_variant.is_qa_tester unless selected_variant.is_qa_tester.nil?
 
-        begin
-          @tracker_callback.call(distinct_id, Utils::EXPOSURE_EVENT, properties)
-        rescue MixpanelError => e
-          @error_handler.handle(e)
+        dispatch_exposure(distinct_id, properties)
+      end
+
+      private
+
+      # Dispatch the tracker call inline or via the configured executor.
+      # The executor is duck-typed — anything that responds to #post(&block)
+      # works (Concurrent::ExecutorService, or a Thread.new wrapper).
+      def dispatch_exposure(distinct_id, properties)
+        if @exposure_executor
+          begin
+            @exposure_executor.post { invoke_tracker(distinct_id, properties) }
+          rescue StandardError => e
+            @error_handler.handle(MixpanelError.new("Exposure event dropped — executor refused to accept task: #{e.message}")) if @error_handler
+          end
+        else
+          invoke_tracker(distinct_id, properties)
         end
       end
+
+      def invoke_tracker(distinct_id, properties)
+        @tracker_callback.call(distinct_id, Utils::EXPOSURE_EVENT, properties)
+      rescue MixpanelError => e
+        @error_handler.handle(e) if @error_handler
+      rescue StandardError => e
+        # On the async path a bare `rescue MixpanelError` would let any
+        # other exception terminate the executor thread silently. Wrap
+        # into MixpanelError so error_handler sees a consistent type.
+        @error_handler.handle(MixpanelError.new("Exposure event failed: #{e.class}: #{e.message}")) if @error_handler
+      end
     end
   end
 end

lib/mixpanel-ruby/flags/local_flags_provider.rb

@@ -11,7 +11,8 @@ class LocalFlagsProvider < FlagsProvider
         api_host: 'api.mixpanel.com',
         request_timeout_in_seconds: 10,
         enable_polling: true,
-        polling_interval_in_seconds: 60
+        polling_interval_in_seconds: 60,
+        exposure_executor: nil
       }.freeze
 
       # @param token [String] Mixpanel project token
@@ -24,7 +25,8 @@ def initialize(token, config, tracker_callback, error_handler)
         provider_config = {
           token: token,
           api_host: @config[:api_host],
-          request_timeout_in_seconds: @config[:request_timeout_in_seconds]
+          request_timeout_in_seconds: @config[:request_timeout_in_seconds],
+          exposure_executor: @config[:exposure_executor]
         }
 
         super(provider_config, '/flags/definitions', tracker_callback, 'local', error_handler)

lib/mixpanel-ruby/flags/remote_flags_provider.rb

@@ -7,7 +7,8 @@ module Flags
     class RemoteFlagsProvider < FlagsProvider
       DEFAULT_CONFIG = {
         api_host: 'api.mixpanel.com',
-        request_timeout_in_seconds: 10
+        request_timeout_in_seconds: 10,
+        exposure_executor: nil
       }.freeze
 
       # @param token [String] Mixpanel project token
@@ -20,7 +21,8 @@ def initialize(token, config, tracker_callback, error_handler)
         provider_config = {
           token: token,
           api_host: merged_config[:api_host],
-          request_timeout_in_seconds: merged_config[:request_timeout_in_seconds]
+          request_timeout_in_seconds: merged_config[:request_timeout_in_seconds],
+          exposure_executor: merged_config[:exposure_executor]
         }
 
         super(provider_config, '/flags', tracker_callback, 'remote', error_handler)

openfeature-provider/README.md

@@ -201,6 +201,32 @@ When you are done using the provider, shut it down to stop any background pollin
 provider.shutdown
 ```
 
+## Async Exposure Tracking
+
+By default, every flag evaluation tracks an exposure event inline — the `/track` HTTP round trip happens on the calling thread before `fetch_*_value` returns. For latency-sensitive code paths, pass an `:exposure_executor` so exposure tracking runs off-thread. The executor is duck-typed — anything that responds to `#post(&block)` works:
+
+```ruby
+# With concurrent-ruby (recommended)
+require 'concurrent'
+exposure_executor = Concurrent::FixedThreadPool.new(1)
+
+provider = Mixpanel::OpenFeature::Provider.from_local(
+  'YOUR_PROJECT_TOKEN',
+  { exposure_executor: exposure_executor },
+)
+
+# Without concurrent-ruby (minimal wrapper)
+class ThreadPerCall
+  def post(&block) = Thread.new(&block)
+end
+provider = Mixpanel::OpenFeature::Provider.from_local(
+  'YOUR_PROJECT_TOKEN',
+  { exposure_executor: ThreadPerCall.new },
+)
+```
+
+Available on both local and remote flag configs. Defaults to `nil` (inline behavior); existing setups are unaffected.
+
 ## Error Handling
 
 The provider uses OpenFeature's standard error codes to indicate issues during flag evaluation:

spec/mixpanel-ruby/flags/local_flags_spec.rb

@@ -1,4 +1,5 @@
 require 'json'
+require 'timeout'
 require 'mixpanel-ruby/flags/local_flags_provider'
 require 'mixpanel-ruby/flags/types'
 require 'webmock/rspec'
@@ -718,6 +719,84 @@ def user_context_with_properties(properties)
 
       provider.send(:track_exposure_event, 'test_flag', variant, test_context)
     end
+
+    it 'runs the tracker inline by default (no executor configured)' do
+      flag = create_test_flag
+      stub_flag_definitions([flag])
+      provider.start_polling_for_definitions!
+
+      variant = Mixpanel::Flags::SelectedVariant.new(
+        variant_key: 'treatment', variant_value: 'treatment'
+      )
+
+      calling_thread = Thread.current
+      tracker_thread = nil
+      allow(mock_tracker).to receive(:call) { tracker_thread = Thread.current }
+
+      provider.send(:track_exposure_event, 'test_flag', variant, test_context)
+      expect(tracker_thread).to be(calling_thread)
+    end
+
+    it 'dispatches the tracker via :exposure_executor when configured' do
+      executor = Object.new
+      def executor.post(&block)
+        Thread.new(&block)
+      end
+
+      tracker_thread = nil
+      tracker_ran = Queue.new
+      tracker = ->(_distinct_id, _event, _properties) {
+        tracker_thread = Thread.current
+        tracker_ran << :done
+      }
+
+      provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false, exposure_executor: executor },
+        tracker,
+        mock_error_handler
+      )
+
+      variant = Mixpanel::Flags::SelectedVariant.new(
+        variant_key: 'treatment', variant_value: 'treatment'
+      )
+      provider.send(:track_exposure_event, 'test_flag', variant, test_context)
+
+      # Bounded wait — a bare Queue#pop would hang CI forever if the
+      # tracker block raised before pushing :done.
+      Timeout.timeout(2) { tracker_ran.pop }
+      expect(tracker_thread).not_to be(Thread.current)
+    end
+
+    it 'reports non-MixpanelError exceptions from the async tracker to error_handler' do
+      handler_called = Queue.new
+      handler = double('error_handler')
+      allow(handler).to receive(:handle) { |e| handler_called << e }
+
+      executor = Object.new
+      def executor.post(&block)
+        Thread.new(&block)
+      end
+
+      tracker = ->(_distinct_id, _event, _properties) { raise NoMethodError, 'boom' }
+
+      provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false, exposure_executor: executor },
+        tracker,
+        handler
+      )
+
+      variant = Mixpanel::Flags::SelectedVariant.new(
+        variant_key: 'treatment', variant_value: 'treatment'
+      )
+      provider.send(:track_exposure_event, 'test_flag', variant, test_context)
+
+      err = Timeout.timeout(2) { handler_called.pop }
+      expect(err).to be_a(Mixpanel::MixpanelError)
+      expect(err.message).to include('NoMethodError')
+      expect(err.message).to include('boom')
+    end
   end
 
   describe 'polling' do

spec/mixpanel-ruby/flags/remote_flags_spec.rb

@@ -1,4 +1,5 @@
 require 'json'
+require 'timeout'
 require 'mixpanel-ruby/flags/remote_flags_provider'
 require 'mixpanel-ruby/flags/types'
 require 'webmock/rspec'
@@ -116,6 +117,61 @@ def stub_flags_request_error(error)
       provider.get_variant_value('test_flag', 'control', test_context)
     end
 
+    it 'runs the tracker inline by default (no executor configured)' do
+      response = create_success_response({
+        'test_flag' => {
+          'variant_key' => 'treatment',
+          'variant_value' => 'treatment'
+        }
+      })
+      stub_flags_request(response)
+
+      calling_thread = Thread.current
+      tracker_thread = nil
+      allow(mock_tracker).to receive(:call) { tracker_thread = Thread.current }
+
+      provider.get_variant_value('test_flag', 'control', test_context)
+      expect(tracker_thread).to be(calling_thread)
+    end
+
+    it 'dispatches the tracker via the configured exposure_executor off the calling thread' do
+      response = create_success_response({
+        'test_flag' => {
+          'variant_key' => 'treatment',
+          'variant_value' => 'treatment'
+        }
+      })
+      stub_flags_request(response)
+
+      calling_thread = Thread.current
+      tracker_thread = nil
+      tracker_ran = Queue.new
+      tracker = ->(_distinct_id, _event, _properties) {
+        tracker_thread = Thread.current
+        tracker_ran << :done
+      }
+
+      # Minimal duck-typed executor: spawn a thread per call.
+      executor = Object.new
+      def executor.post(&block)
+        Thread.new(&block)
+      end
+
+      provider = Mixpanel::Flags::RemoteFlagsProvider.new(
+        test_token,
+        { exposure_executor: executor },
+        tracker,
+        mock_error_handler
+      )
+
+      provider.get_variant_value('test_flag', 'control', test_context)
+
+      # Bounded wait — a bare Queue#pop would hang CI forever if the
+      # tracker block raised before pushing :done.
+      Timeout.timeout(2) { tracker_ran.pop }
+      expect(tracker_thread).not_to be(calling_thread)
+    end
+
     it 'does not track exposure event when report_exposure is false' do
       response = create_success_response({
         'test_flag' => {