feat: Add initial support for hooks

lib/ldclient-rb/config.rb

@@ -43,6 +43,7 @@ class Config
     # @option opts [BigSegmentsConfig] :big_segments See {#big_segments}.
     # @option opts [Hash] :application See {#application}
     # @option opts [String] :payload_filter_key See {#payload_filter_key}
+    # @option hooks [Array<Interfaces::Hooks::Hook]
     #
     def initialize(opts = {})
       @base_uri = (opts[:base_uri] || Config.default_base_uri).chomp("/")
@@ -75,6 +76,7 @@ def initialize(opts = {})
       @big_segments = opts[:big_segments] || BigSegmentsConfig.new(store: nil)
       @application = LaunchDarkly::Impl::Util.validate_application_info(opts[:application] || {}, @logger)
       @payload_filter_key = opts[:payload_filter_key]
+      @hooks = (opts[:hooks] || []).keep_if { |hook| hook.is_a? Interfaces::Hooks::Hook }
       @data_source_update_sink = nil
     end
 
@@ -372,6 +374,17 @@ def diagnostic_opt_out?
     #
     attr_reader :socket_factory
 
+    #
+    # Initial set of hooks for the client.
+    #
+    # Hooks provide entrypoints which allow for observation of SDK functions.
+    #
+    # LaunchDarkly provides integration packages, and most applications will not
+    # need to implement their own hooks. Refer to the `launchdarkly-server-sdk-otel` gem
+    # for instrumentation.
+    #
+    attr_reader :hooks
+
     #
     # The default LaunchDarkly client configuration. This configuration sets
     # reasonable defaults for most users.

lib/ldclient-rb/interfaces.rb

@@ -885,5 +885,89 @@ def build
         end
       end
     end
+
+    module Hooks
+      #
+      # Mixin for extending SDK functionality via hooks.
+      #
+      # All provided hook implementations **MUST** include this mixin. Hooks without this mixin will be ignored.
+      #
+      # This mixin includes default implementations for all hook handlers. This allows LaunchDarkly to expand the list
+      # of hook handlers without breaking customer integrations.
+      #
+      module Hook
+        #
+        # Get metadata about the hook implementation.
+        #
+        # @return [Metadata]
+        #
+        def metadata
+          Metadata('UNDEFINED')
+        end
+
+        #
+        # The before method is called during the execution of a variation method before the flag value has been
+        # determined. The method is executed synchronously.
+        #
+        # @param hook_context [EvaluationContext] Contains information about the evaluation being performed. This is not
+        # mutable.
+        # @param data [Hash] A record associated with each stage of hook invocations. Each stage is called with the data
+        # of the previous stage for a series. The input record should not be modified.
+        # @return [Hash] Data to use when executing the next state of the hook in the evaluation series.
+        #
+        def before_evaluation(hook_context, data)
+          {}
+        end
+
+        #
+        # The after method is called during the execution of the variation method
+        # after the flag value has been determined. The method is executed synchronously.
+        #
+        # @param hook_context [EvaluationContext] Contains read-only information about the evaluation being performed.
+        # @param data [Hash] A record associated with each stage of hook invocations. Each stage is called with the data
+        # of the previous stage for a series.
+        # @param detail [LaunchDarkly::EvaluationDetail] The result of the evaluation. This value should not be
+        # modified.
+        # @return [Hash] Data to use when executing the next state of the hook in the evaluation series.
+        #
+        def after_evaluation(hook_context, data, detail)
+          data
+        end
+      end
+
+      #
+      # Metadata data class used for annotating hook implementations.
+      #
+      class Metadata
+        attr_reader :name
+
+        def initialize(name)
+          @name = name
+        end
+      end
+
+      #
+      # Contextual information that will be provided to handlers during evaluation series.
+      #
+      class EvaluationContext
+        attr_reader :key
+        attr_reader :context
+        attr_reader :value
+        attr_reader :method
+
+        #
+        # @param key [String]
+        # @param context [LaunchDarkly::LDContext]
+        # @param value [any]
+        # @param method [Symbol]
+        #
+        def initialize(key, context, value, method)
+          @key = key
+          @context = context
+          @value = value
+          @method = method
+        end
+      end
+    end
   end
 end

lib/ldclient-rb/ldclient.rb

@@ -54,6 +54,7 @@ def initialize(sdk_key, config = Config.default, wait_for_sec = 5)
       end
 
       @sdk_key = sdk_key
+      @hooks = config.hooks
 
       @shared_executor = Concurrent::SingleThreadExecutor.new
 
@@ -131,6 +132,23 @@ def initialize(sdk_key, config = Config.default, wait_for_sec = 5)
       end
     end
 
+    #
+    # Add a hook to the client. In order to register a hook before the client starts, please use the `hooks` property of
+    # {#LDConfig}.
+    #
+    # Hooks provide entrypoints which allow for observation of SDK functions.
+    #
+    # @param hook [Interfaces::Hooks::Hook]
+    #
+    def add_hook(hook)
+      unless hook.is_a?(Interfaces::Hooks::Hook)
+        @config.logger.error { "[LDClient] Attempted to add a hook that does not include the LaunchDarkly::Intefaces::Hooks:Hook mixin. Ignoring." }
+        return
+      end
+
+      @hooks.push(hook)
+    end
+
     #
     # Tells the client that all pending analytics events should be delivered as soon as possible.
     #
@@ -226,10 +244,116 @@ def variation(key, context, default)
     # @return [EvaluationDetail] an object describing the result
     #
     def variation_detail(key, context, default)
-      detail, _, _ = evaluate_internal(key, context, default, true)
+      detail, _, _ = evaluate_with_hooks(key, context, default, :variation_detail) do
+        evaluate_internal(key, context, default, true)
+      end
+
       detail
     end
 
+    #
+    # evaluate_with_hook will run the provided block, wrapping it with evaluation hook support.
+    #
+    # Example:
+    #
+    # ```ruby
+    # evaluate_with_hooks(key, context, default, method) do
+    #   puts 'This is being wrapped with evaluation hooks'
+    # end
+    # ```
+    #
+    # @param key [String]
+    # @param context [String]
+    # @param default [String]
+    # @param method [Symbol]
+    # @param &block [#call] Implicit passed block
+    #
+    private def evaluate_with_hooks(key, context, default, method)
+      return yield if @hooks.empty?
+
+      hooks, hook_context = prepare_hooks(key, context, default, method)
+      hook_data = execute_before_evaluation(hooks, hook_context)
+      evaluation_detail, flag, error = yield
+      execute_after_evaluation(hooks, hook_context, hook_data, evaluation_detail)
+
+      [evaluation_detail, flag, error]
+    end
+
+    #
+    # Execute the :before_evaluation stage of the evaluation series.
+    #
+    # This method will return the results of each hook, indexed into an array in the same order as the hooks. If a hook
+    # raised an uncaught exception, the value will be nil.
+    #
+    # @param hooks [Array<Interfaces::Hooks::Hook>]
+    # @param hook_context [EvaluationContext]
+    #
+    # @return [Array<any>]
+    #
+    private def execute_before_evaluation(hooks, hook_context)
+      hooks.map do |hook|
+        try_execute_stage(:before_evaluation, hook.metadata.name) do
+          hook.before_evaluation(hook_context, {})
+        end
+      end
+    end
+
+    #
+    # Execute the :after_evaluation stage of the evaluation series.
+    #
+    # This method will return the results of each hook, indexed into an array in the same order as the hooks. If a hook
+    # raised an uncaught exception, the value will be nil.
+    #
+    # @param hooks [Array<Interfaces::Hooks::Hook>]
+    # @param hook_context [EvaluationContext]
+    # @param hook_data [Array<any>]
+    # @param evaluation_detail [EvaluationDetail]
+    #
+    # @return [Array<any>]
+    #
+    private def execute_after_evaluation(hooks, hook_context, hook_data, evaluation_detail)
+      hooks.zip(hook_data).reverse.map do |(hook, data)|
+        try_execute_stage(:after_evaluation, hook.metadata.name) do
+          hook.after_evaluation(hook_context, data, evaluation_detail)
+        end
+      end
+    end
+
+    #
+    # Try to execute the provided block. If execution raises an exception, catch and log it, then move on with
+    # execution.
+    #
+    # @return [any]
+    #
+    private def try_execute_stage(method, hook_name)
+      begin
+        yield
+      rescue => e
+        @config.logger.error { "[LDClient] An error occurred in #{method} of the hook #{hook_name}: #{e}" }
+        nil
+      end
+    end
+
+    #
+    # Return a copy of the existing hooks and a few instance of the EvaluationContext used for the evaluation series.
+    #
+    # @param key [String]
+    # @param context [LDContext]
+    # @param default [any]
+    # @param method [Symbol]
+    # @return [Array[Array<Interfaces::Hooks::Hook>, Interfaces::Hooks::EvaluationContext]]
+    #
+    private def prepare_hooks(key, context, default, method)
+      # Copy the hooks to use a consistent set during the evaluation series.
+      #
+      # Hooks can be added and we want to ensure all correct stages for a given hook execute. For example, we do not
+      # want to trigger the after_evaluation method without also triggering the before_evaluation method.
+      hooks = @hooks.dup
+      hook_context = Interfaces::Hooks::EvaluationContext.new(key, context, default, method)
+
+      [hooks, hook_context]
+    end
+
     #
     # This method returns the migration stage of the migration feature flag for the given evaluation context.
     #
@@ -508,7 +632,9 @@ def create_default_data_source(sdk_key, config, diagnostic_accumulator)
     # @return [Array<EvaluationDetail, [LaunchDarkly::Impl::Model::FeatureFlag, nil], [String, nil]>]
     #
     def variation_with_flag(key, context, default)
-      evaluate_internal(key, context, default, false)
+      evaluate_with_hooks(key, context, default, :variation_detail) do
+        evaluate_internal(key, context, default, false)
+      end
     end
 
     #