feat: add FDv2 configuration builder

libs/server-sdk/include/launchdarkly/server_side/config/builders/data_system/data_system_builder.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <launchdarkly/server_side/config/builders/data_system/background_sync_builder.hpp>
+#include <launchdarkly/server_side/config/builders/data_system/fdv2_builder.hpp>
 #include <launchdarkly/server_side/config/builders/data_system/lazy_load_builder.hpp>
 #include <launchdarkly/server_side/config/built/data_system/data_system_config.hpp>
 
@@ -13,6 +14,7 @@ class DataSystemBuilder {
     DataSystemBuilder();
     using BackgroundSync = BackgroundSyncBuilder;
     using LazyLoad = LazyLoadBuilder;
+    using FDv2 = FDv2Builder;
 
     /**
      * @brief Alias for Enabled(false).
@@ -46,10 +48,19 @@ class DataSystemBuilder {
      */
     DataSystemBuilder& Method(LazyLoad lazy_load);
 
+    /**
+     * @brief Configures the FDv2 data system, which receives flag delivery
+     * updates over the new changeset-based protocol with built-in fallback
+     * and recovery semantics.
+     * @param fdv2 FDv2 configuration.
+     * @return Reference to this.
+     */
+    DataSystemBuilder& Method(FDv2 fdv2);
+
     [[nodiscard]] tl::expected<built::DataSystemConfig, Error> Build() const;
 
    private:
-    std::optional<std::variant<BackgroundSync, LazyLoad>> method_builder_;
+    std::optional<std::variant<BackgroundSync, LazyLoad, FDv2>> method_builder_;
     built::DataSystemConfig config_;
 };
 

libs/server-sdk/include/launchdarkly/server_side/config/builders/data_system/fdv2_builder.hpp

@@ -0,0 +1,145 @@
+#pragma once
+
+#include <launchdarkly/config/shared/builders/data_source_builder.hpp>
+#include <launchdarkly/config/shared/sdks.hpp>
+#include <launchdarkly/server_side/config/built/data_system/fdv2_config.hpp>
+
+#include <chrono>
+#include <optional>
+#include <string>
+
+namespace launchdarkly::server_side::config::builders {
+
+class FDv2Builder {
+   public:
+    class Streaming {
+       public:
+        Streaming& InitialReconnectDelay(std::chrono::milliseconds delay);
+        Streaming& Filter(std::string filter_key);
+        Streaming& BaseUrl(std::string base_url);
+        [[nodiscard]] built::FDv2Config::StreamingConfig Build() const;
+
+       private:
+        std::chrono::milliseconds initial_reconnect_delay_{1000};
+        std::optional<std::string> filter_key_;
+        std::optional<std::string> base_url_override_;
+    };
+
+    class Polling {
+       public:
+        Polling& PollInterval(std::chrono::seconds interval);
+        Polling& Filter(std::string filter_key);
+        Polling& BaseUrl(std::string base_url);
+        [[nodiscard]] built::FDv2Config::PollingConfig Build() const;
+
+       private:
+        std::chrono::seconds poll_interval_{30};
+        std::optional<std::string> filter_key_;
+        std::optional<std::string> base_url_override_;
+    };
+
+    /**
+     * @return A builder pre-populated with the spec-recommended initializers,
+     *     synchronizers, and FDv1 fallback. Equivalent to calling
+     *     Initializer(), Synchronizer(), and FDv1Fallback() with the
+     *     standard sources.
+     */
+    static FDv2Builder Default();
+
+    /**
+     * @return A builder with no initializers, no synchronizers, and no
+     *     FDv1 fallback. Callers must add sources explicitly.
+     */
+    static FDv2Builder Custom();
+
+    /**
+     * @brief Appends a polling initializer to the initializers list.
+     * @param source Polling source configuration for the initializer.
+     * @return Reference to this.
+     */
+    FDv2Builder& Initializer(Polling source);
+
+    /**
+     * @brief Appends a streaming synchronizer to the synchronizers list.
+     * Order in the list determines preference: the first entry is the
+     * primary synchronizer, subsequent entries are fallbacks.
+     * @param source Streaming source configuration.
+     * @return Reference to this.
+     */
+    FDv2Builder& Synchronizer(Streaming source);
+
+    /**
+     * @brief Appends a polling synchronizer to the synchronizers list. See
+     * Synchronizer(Streaming) for ordering semantics.
+     * @param source Polling source configuration.
+     * @return Reference to this.
+     */
+    FDv2Builder& Synchronizer(Polling source);
+
+    using FDv1Streaming =
+        launchdarkly::config::shared::builders::StreamingBuilder<
+            launchdarkly::config::shared::ServerSDK>;
+    using FDv1Polling = launchdarkly::config::shared::builders::PollingBuilder<
+        launchdarkly::config::shared::ServerSDK>;
+
+    /**
+     * @brief Configures the FDv1 streaming source used as a last-resort
+     * fallback when the LaunchDarkly service signals (via the
+     * X-LD-FD-Fallback header) that the SDK should switch to FDv1. The
+     * fallback reads its endpoint from the top-level ServiceEndpoints; to
+     * point the fallback at a custom URL, configure ServiceEndpoints
+     * accordingly.
+     * @param source FDv1 streaming source configuration.
+     * @return Reference to this.
+     */
+    FDv2Builder& FDv1Fallback(FDv1Streaming source);
+
+    /**
+     * @brief Configures the FDv1 polling source used as a last-resort
+     * fallback when the LaunchDarkly service signals (via the
+     * X-LD-FD-Fallback header) that the SDK should switch to FDv1. The
+     * fallback reads its endpoint from the top-level ServiceEndpoints; to
+     * point the fallback at a custom URL, configure ServiceEndpoints
+     * accordingly.
+     * @param source FDv1 polling source configuration.
+     * @return Reference to this.
+     */
+    FDv2Builder& FDv1Fallback(FDv1Polling source);
+
+    /**
+     * @brief Disables the FDv1 fallback. After this call, an FDv1
+     * fallback directive from the service leaves the data source in
+     * the interrupted state and schedules an FDv2 retry on the
+     * directive's TTL.
+     * @return Reference to this.
+     */
+    FDv2Builder& DisableFDv1Fallback();
+
+    /**
+     * @brief Sets how long the active synchronizer may remain interrupted
+     * before the orchestrator falls back to the next-preferred synchronizer.
+     * @param timeout Duration the synchronizer must be continuously
+     *     interrupted for before fallback fires.
+     * @return Reference to this.
+     */
+    FDv2Builder& FallbackTimeout(std::chrono::milliseconds timeout);
+
+    /**
+     * @brief Sets how long a fallback synchronizer must run successfully
+     * before the orchestrator attempts to recover to the primary
+     * synchronizer.
+     * @param timeout Duration the fallback synchronizer must run before a
+     *     recovery attempt is made.
+     * @return Reference to this.
+     */
+    FDv2Builder& RecoveryTimeout(std::chrono::milliseconds timeout);
+
+    [[nodiscard]] built::FDv2Config Build() const;
+
+   private:
+    FDv2Builder();
+
+    built::FDv2Config config_;
+};
+
+}  // namespace launchdarkly::server_side::config::builders

libs/server-sdk/include/launchdarkly/server_side/config/built/data_system/data_system_config.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <launchdarkly/server_side/config/built/data_system/background_sync_config.hpp>
+#include <launchdarkly/server_side/config/built/data_system/fdv2_config.hpp>
 #include <launchdarkly/server_side/config/built/data_system/lazy_load_config.hpp>
 
 #include <variant>
@@ -9,7 +10,7 @@ namespace launchdarkly::server_side::config::built {
 
 struct DataSystemConfig {
     bool disabled;
-    std::variant<LazyLoadConfig, BackgroundSyncConfig> system_;
+    std::variant<LazyLoadConfig, BackgroundSyncConfig, FDv2Config> system_;
 };
 
 }  // namespace launchdarkly::server_side::config::built

libs/server-sdk/include/launchdarkly/server_side/config/built/data_system/fdv2_config.hpp

@@ -0,0 +1,56 @@
+#pragma once
+
+#include <launchdarkly/config/shared/built/data_source_config.hpp>
+#include <launchdarkly/config/shared/sdks.hpp>
+
+#include <chrono>
+#include <optional>
+#include <string>
+#include <variant>
+#include <vector>
+
+namespace launchdarkly::server_side::config::built {
+
+struct FDv2Config {
+    struct StreamingConfig {
+        std::chrono::milliseconds initial_reconnect_delay;
+        std::optional<std::string> filter_key;
+        std::optional<std::string> base_url_override;
+
+        friend bool operator==(StreamingConfig const& lhs,
+                               StreamingConfig const& rhs) {
+            return lhs.initial_reconnect_delay == rhs.initial_reconnect_delay &&
+                   lhs.filter_key == rhs.filter_key &&
+                   lhs.base_url_override == rhs.base_url_override;
+        }
+    };
+
+    struct PollingConfig {
+        std::chrono::seconds poll_interval;
+        std::optional<std::string> filter_key;
+        std::optional<std::string> base_url_override;
+
+        friend bool operator==(PollingConfig const& lhs,
+                               PollingConfig const& rhs) {
+            return lhs.poll_interval == rhs.poll_interval &&
+                   lhs.filter_key == rhs.filter_key &&
+                   lhs.base_url_override == rhs.base_url_override;
+        }
+    };
+
+    using FDv1StreamingConfig =
+        launchdarkly::config::shared::built::StreamingConfig<
+            launchdarkly::config::shared::ServerSDK>;
+    using FDv1PollingConfig =
+        launchdarkly::config::shared::built::PollingConfig<
+            launchdarkly::config::shared::ServerSDK>;
+
+    std::vector<PollingConfig> initializers;
+    std::vector<std::variant<StreamingConfig, PollingConfig>> synchronizers;
+    std::optional<std::variant<FDv1StreamingConfig, FDv1PollingConfig>>
+        fdv1_fallback;
+    std::chrono::milliseconds fallback_timeout;
+    std::chrono::milliseconds recovery_timeout;
+};
+
+}  // namespace launchdarkly::server_side::config::built

libs/server-sdk/src/CMakeLists.txt

@@ -30,6 +30,7 @@ target_sources(${LIBNAME}
         config/builders/data_system/background_sync_builder.cpp
         config/builders/data_system/bootstrap_builder.cpp
         config/builders/data_system/data_system_builder.cpp
+        config/builders/data_system/fdv2_builder.cpp
         config/builders/data_system/lazy_load_builder.cpp
         config/builders/data_system/data_destination_builder.cpp
         config/builders/big_segments_builder.cpp
@@ -81,6 +82,10 @@ target_sources(${LIBNAME}
         data_systems/fdv2/fdv2_data_system.cpp
         data_systems/fdv2/fdv1_adapter_synchronizer.hpp
         data_systems/fdv2/fdv1_adapter_synchronizer.cpp
+        data_systems/fdv2/synchronizer_factories.hpp
+        data_systems/fdv2/synchronizer_factories.cpp
+        data_systems/fdv2/initializer_factories.hpp
+        data_systems/fdv2/initializer_factories.cpp
         data_systems/background_sync/sources/streaming/streaming_data_source.hpp
         data_systems/background_sync/sources/streaming/streaming_data_source.cpp
         data_systems/background_sync/sources/streaming/event_handler.hpp