Revert "Revert "[xDS LB] Override cluster with value from cookie"" (#33388)

Reapplying #32973

CMakeLists.txt

@@ -29618,6 +29618,10 @@ if(_gRPC_PLATFORM_LINUX OR _gRPC_PLATFORM_MAC OR _gRPC_PLATFORM_POSIX)
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/ads.grpc.pb.cc
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/ads.pb.h
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/ads.grpc.pb.h
+    ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/aggregate_cluster.pb.cc
+    ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/aggregate_cluster.grpc.pb.cc
+    ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/aggregate_cluster.pb.h
+    ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/aggregate_cluster.grpc.pb.h
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/base.pb.cc
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/base.grpc.pb.cc
     ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/testing/xds/v3/base.pb.h

build_autogenerated.yaml

@@ -16759,6 +16759,7 @@ targets:
   - src/proto/grpc/testing/simple_messages.proto
   - src/proto/grpc/testing/xds/v3/address.proto
   - src/proto/grpc/testing/xds/v3/ads.proto
+  - src/proto/grpc/testing/xds/v3/aggregate_cluster.proto
   - src/proto/grpc/testing/xds/v3/base.proto
   - src/proto/grpc/testing/xds/v3/cluster.proto
   - src/proto/grpc/testing/xds/v3/config_source.proto

src/core/BUILD

@@ -3789,6 +3789,7 @@ grpc_cc_library(
         "channel_args",
         "channel_fwd",
         "context",
+        "grpc_resolver_xds_header",
         "grpc_service_config",
         "json",
         "json_args",
@@ -5279,7 +5280,6 @@ grpc_cc_library(
         "ref_counted",
         "slice",
         "time",
-        "unique_type_name",
         "//:config",
         "//:debug_location",
         "//:gpr",

src/core/ext/filters/client_channel/resolver/xds/xds_resolver.cc

@@ -47,7 +47,6 @@
 
 #include <grpc/grpc.h>
 
-#include "src/core/lib/gprpp/unique_type_name.h"
 #include "src/core/lib/slice/slice.h"
 
 #define XXH_INLINE_ALL
@@ -102,11 +101,6 @@ namespace grpc_core {
 
 TraceFlag grpc_xds_resolver_trace(false, "xds_resolver");
 
-UniqueTypeName XdsClusterAttribute::TypeName() {
-  static UniqueTypeName::Factory kFactory("xds_cluster_name");
-  return kFactory.Create();
-}
-
 namespace {
 
 std::string GetDefaultAuthorityInternal(const URI& uri) {
@@ -246,15 +240,15 @@ class XdsResolver : public Resolver {
   // the cluster by the ConfigSelector.  The ref for each call is held
   // until the call is committed.  When the strong refs go away, we hop
   // back into the WorkSerializer to remove the entry from the map.
-  class ClusterState : public DualRefCounted<ClusterState> {
+  class ClusterRef : public DualRefCounted<ClusterRef> {
    public:
-    ClusterState(RefCountedPtr<XdsResolver> resolver,
-                 absl::string_view cluster_name)
+    ClusterRef(RefCountedPtr<XdsResolver> resolver,
+               absl::string_view cluster_name)
         : resolver_(std::move(resolver)), cluster_name_(cluster_name) {}
 
     void Orphan() override {
-      auto* resolver = resolver_.get();
-      resolver->work_serializer_->Run(
+      XdsResolver* resolver_ptr = resolver_.get();
+      resolver_ptr->work_serializer_->Run(
           [resolver = std::move(resolver_)]() {
             resolver->MaybeRemoveUnusedClusters();
           },
@@ -268,21 +262,49 @@ class XdsResolver : public Resolver {
     std::string cluster_name_;
   };
 
-  // A map containing cluster refs held by the XdsConfigSelector. A ref to
-  // this map will be taken by each call processed by the XdsConfigSelector,
-  // stored in a the call's call attributes, and later unreffed
-  // by the ClusterSelection filter.
-  class XdsClusterMap : public RefCounted<XdsClusterMap> {
+  // A routing data including cluster refs and routes table held by the
+  // XdsConfigSelector. A ref to this map will be taken by each call processed
+  // by the XdsConfigSelector, stored in a the call's call attributes, and later
+  // unreffed by the ClusterSelection filter.
+  class RouteConfigData : public RefCounted<RouteConfigData> {
    public:
-    explicit XdsClusterMap(
-        std::map<absl::string_view, RefCountedPtr<ClusterState>> clusters)
-        : clusters_(std::move(clusters)) {}
+    struct RouteEntry {
+      struct ClusterWeightState {
+        uint32_t range_end;
+        absl::string_view cluster;
+        RefCountedPtr<ServiceConfig> method_config;
+
+        bool operator==(const ClusterWeightState& other) const {
+          return range_end == other.range_end && cluster == other.cluster &&
+                 MethodConfigsEqual(method_config.get(),
+                                    other.method_config.get());
+        }
+      };
+
+      XdsRouteConfigResource::Route route;
+      RefCountedPtr<ServiceConfig> method_config;
+      std::vector<ClusterWeightState> weighted_cluster_state;
+
+      explicit RouteEntry(const XdsRouteConfigResource::Route& r) : route(r) {}
 
-    bool operator==(const XdsClusterMap& other) const {
-      return clusters_ == other.clusters_;
+      bool operator==(const RouteEntry& other) const {
+        return route == other.route &&
+               weighted_cluster_state == other.weighted_cluster_state &&
+               MethodConfigsEqual(method_config.get(),
+                                  other.method_config.get());
+      }
+    };
+
+    static absl::StatusOr<RefCountedPtr<RouteConfigData>> Create(
+        XdsResolver* resolver,
+        const std::vector<XdsRouteConfigResource::Route>& routes,
+        const Duration& default_max_stream_duration);
+
+    bool operator==(const RouteConfigData& other) const {
+      return clusters_ == other.clusters_ && routes_ == other.routes_;
     }
 
-    RefCountedPtr<ClusterState> Find(absl::string_view name) const {
+    RefCountedPtr<ClusterRef> FindClusterRef(absl::string_view name) const {
       auto it = clusters_.find(name);
       if (it == clusters_.end()) {
         return nullptr;
@@ -290,14 +312,36 @@ class XdsResolver : public Resolver {
       return it->second;
     }
 
+    RouteEntry* GetRouteForRequest(absl::string_view path,
+                                   grpc_metadata_batch* initial_metadata);
+
    private:
-    std::map<absl::string_view, RefCountedPtr<ClusterState>> clusters_;
+    class RouteListIterator;
+
+    static absl::StatusOr<RefCountedPtr<ServiceConfig>> CreateMethodConfig(
+        XdsResolver* resolver, const XdsRouteConfigResource::Route& route,
+        const XdsRouteConfigResource::Route::RouteAction::ClusterWeight*
+            cluster_weight);
+
+    static bool MethodConfigsEqual(const ServiceConfig* sc1,
+                                   const ServiceConfig* sc2) {
+      if (sc1 == nullptr) return sc2 == nullptr;
+      if (sc2 == nullptr) return false;
+      return sc1->json_string() == sc2->json_string();
+    }
+
+    absl::Status AddRouteEntry(const XdsRouteConfigResource::Route& route,
+                               const Duration& default_max_stream_duration,
+                               XdsResolver* resolver);
+
+    std::map<absl::string_view, RefCountedPtr<ClusterRef>> clusters_;
+    std::vector<RouteEntry> routes_;
   };
 
   class XdsConfigSelector : public ConfigSelector {
    public:
     XdsConfigSelector(RefCountedPtr<XdsResolver> resolver,
-                      absl::Status* status);
+                      RefCountedPtr<RouteConfigData> route_config_data);
     ~XdsConfigSelector() override;
 
     const char* name() const override { return "XdsConfigSelector"; }
@@ -305,8 +349,8 @@ class XdsResolver : public Resolver {
     bool Equals(const ConfigSelector* other) const override {
       const auto* other_xds = static_cast<const XdsConfigSelector*>(other);
       // Don't need to compare resolver_, since that will always be the same.
-      return route_table_ == other_xds->route_table_ &&
-             *cluster_map_ == *other_xds->cluster_map_;
+      return *route_config_data_ == *other_xds->route_config_data_ &&
+             filters_ == other_xds->filters_;
     }
 
     absl::Status GetCallConfig(GetCallConfigArgs args) override;
@@ -316,34 +360,28 @@ class XdsResolver : public Resolver {
     }
 
    private:
-    struct Route {
-      struct ClusterWeightState {
-        uint32_t range_end;
-        absl::string_view cluster;
-        RefCountedPtr<ServiceConfig> method_config;
-
-        bool operator==(const ClusterWeightState& other) const;
-      };
-
-      XdsRouteConfigResource::Route route;
-      RefCountedPtr<ServiceConfig> method_config;
-      std::vector<ClusterWeightState> weighted_cluster_state;
+    RefCountedPtr<XdsResolver> resolver_;
+    RefCountedPtr<RouteConfigData> route_config_data_;
+    std::vector<const grpc_channel_filter*> filters_;
+  };
 
-      bool operator==(const Route& other) const;
-    };
-    using RouteTable = std::vector<Route>;
+  class XdsRouteStateAttributeImpl : public XdsRouteStateAttribute {
+   public:
+    explicit XdsRouteStateAttributeImpl(
+        RefCountedPtr<RouteConfigData> route_config_data,
+        RouteConfigData::RouteEntry* route)
+        : route_config_data_(std::move(route_config_data)), route_(route) {}
 
-    class RouteListIterator;
+    // This method can be called only once. The first call will release
+    // the reference to the cluster map, and subsequent calls will return
+    // nullptr.
+    RefCountedPtr<ClusterRef> LockAndGetCluster(absl::string_view cluster_name);
 
-    absl::StatusOr<RefCountedPtr<ServiceConfig>> CreateMethodConfig(
-        const XdsRouteConfigResource::Route& route,
-        const XdsRouteConfigResource::Route::RouteAction::ClusterWeight*
-            cluster_weight);
+    bool HasClusterForRoute(absl::string_view cluster_name) const override;
 
-    RefCountedPtr<XdsResolver> resolver_;
-    RouteTable route_table_;
-    RefCountedPtr<XdsClusterMap> cluster_map_;
-    std::vector<const grpc_channel_filter*> filters_;
+   private:
+    RefCountedPtr<RouteConfigData> route_config_data_;
+    RouteConfigData::RouteEntry* route_;
   };
 
   class ClusterSelectionFilter : public ChannelFilter {
@@ -357,29 +395,7 @@ class XdsResolver : public Resolver {
 
     // Construct a promise for one call.
     ArenaPromise<ServerMetadataHandle> MakeCallPromise(
-        CallArgs call_args, NextPromiseFactory next_promise_factory) override {
-      auto* service_config_call_data =
-          static_cast<ClientChannelServiceConfigCallData*>(
-              GetContext<grpc_call_context_element>()
-                  [GRPC_CONTEXT_SERVICE_CONFIG_CALL_DATA]
-                      .value);
-      GPR_ASSERT(service_config_call_data != nullptr);
-      auto* cluster_data = static_cast<XdsClusterMapAttribute*>(
-          service_config_call_data->GetCallAttribute(
-              XdsClusterMapAttribute::TypeName()));
-      auto* cluster_name_attribute = static_cast<XdsClusterAttribute*>(
-          service_config_call_data->GetCallAttribute(
-              XdsClusterAttribute::TypeName()));
-      if (cluster_data != nullptr && cluster_name_attribute != nullptr) {
-        auto cluster =
-            cluster_data->LockAndGetCluster(cluster_name_attribute->cluster());
-        if (cluster != nullptr) {
-          service_config_call_data->SetOnCommit(
-              [cluster = std::move(cluster)]() mutable { cluster.reset(); });
-        }
-      }
-      return next_promise_factory(std::move(call_args));
-    }
+        CallArgs call_args, NextPromiseFactory next_promise_factory) override;
 
    private:
     explicit ClusterSelectionFilter(ChannelFilter::Args filter_args)
@@ -388,46 +404,17 @@ class XdsResolver : public Resolver {
     ChannelFilter::Args filter_args_;
   };
 
-  RefCountedPtr<ClusterState> GetOrCreateClusterState(
+  RefCountedPtr<ClusterRef> GetOrCreateClusterRef(
       absl::string_view cluster_name) {
-    auto it = cluster_state_map_.find(cluster_name);
-    if (it == cluster_state_map_.end()) {
-      auto cluster = MakeRefCounted<ClusterState>(Ref(), cluster_name);
-      cluster_state_map_.emplace(cluster->cluster_name(), cluster->WeakRef());
+    auto it = cluster_ref_map_.find(cluster_name);
+    if (it == cluster_ref_map_.end()) {
+      auto cluster = MakeRefCounted<ClusterRef>(Ref(), cluster_name);
+      cluster_ref_map_.emplace(cluster->cluster_name(), cluster->WeakRef());
       return cluster;
     }
     return it->second->Ref();
   }
 
-  class XdsClusterMapAttribute
-      : public ServiceConfigCallData::CallAttributeInterface {
-   public:
-    static UniqueTypeName TypeName() {
-      static UniqueTypeName::Factory factory("xds_cluster_lb_data");
-      return factory.Create();
-    }
-
-    explicit XdsClusterMapAttribute(RefCountedPtr<XdsClusterMap> cluster_map)
-        : cluster_map_(std::move(cluster_map)) {}
-
-    // This method can be called only once. The first call will release the
-    // reference to the cluster map, and subsequent calls will return nullptr.
-    RefCountedPtr<ClusterState> LockAndGetCluster(
-        absl::string_view cluster_name) {
-      if (cluster_map_ == nullptr) {
-        return nullptr;
-      }
-      auto cluster = cluster_map_->Find(cluster_name);
-      cluster_map_.reset();
-      return cluster;
-    }
-
-    UniqueTypeName type() const override { return TypeName(); }
-
-   private:
-    RefCountedPtr<XdsClusterMap> cluster_map_;
-  };
-
   void OnListenerUpdate(XdsListenerResource listener);
   void OnRouteConfigUpdate(XdsRouteConfigResource rds_update);
   void OnError(absl::string_view context, absl::Status status);
@@ -461,188 +448,72 @@ class XdsResolver : public Resolver {
            std::string /*LB policy config*/>
       cluster_specifier_plugin_map_;
 
-  std::map<absl::string_view, WeakRefCountedPtr<ClusterState>>
-      cluster_state_map_;
+  std::map<absl::string_view, WeakRefCountedPtr<ClusterRef>> cluster_ref_map_;
 };
 
 //
-// XdsResolver::XdsConfigSelector::Route
+// XdsResolver::RouteConfigData::RouteListIterator
 //
 
-bool MethodConfigsEqual(const ServiceConfig* sc1, const ServiceConfig* sc2) {
-  if (sc1 == nullptr) return sc2 == nullptr;
-  if (sc2 == nullptr) return false;
-  return sc1->json_string() == sc2->json_string();
-}
-
-const grpc_channel_filter XdsResolver::ClusterSelectionFilter::kFilter =
-    MakePromiseBasedFilter<ClusterSelectionFilter, FilterEndpoint::kClient,
-                           kFilterExaminesServerInitialMetadata>(
-        "cluster_selection_filter");
-
-bool XdsResolver::XdsConfigSelector::Route::ClusterWeightState::operator==(
-    const ClusterWeightState& other) const {
-  return range_end == other.range_end && cluster == other.cluster &&
-         MethodConfigsEqual(method_config.get(), other.method_config.get());
-}
-
-bool XdsResolver::XdsConfigSelector::Route::operator==(
-    const Route& other) const {
-  return route == other.route &&
-         weighted_cluster_state == other.weighted_cluster_state &&
-         MethodConfigsEqual(method_config.get(), other.method_config.get());
-}
-
 // Implementation of XdsRouting::RouteListIterator for getting the matching
 // route for a request.
-class XdsResolver::XdsConfigSelector::RouteListIterator
+class XdsResolver::RouteConfigData::RouteListIterator
     : public XdsRouting::RouteListIterator {
  public:
-  explicit RouteListIterator(const RouteTable* route_table)
+  explicit RouteListIterator(const RouteConfigData* route_table)
       : route_table_(route_table) {}
 
-  size_t Size() const override { return route_table_->size(); }
+  size_t Size() const override { return route_table_->routes_.size(); }
 
   const XdsRouteConfigResource::Route::Matchers& GetMatchersForRoute(
       size_t index) const override {
-    return (*route_table_)[index].route.matchers;
+    return route_table_->routes_[index].route.matchers;
   }
 
  private:
-  const RouteTable* route_table_;
+  const RouteConfigData* route_table_;
 };
 
 //
-// XdsResolver::XdsConfigSelector
+// XdsResolver::RouteConfigData
 //
 
-XdsResolver::XdsConfigSelector::XdsConfigSelector(
-    RefCountedPtr<XdsResolver> resolver, absl::Status* status)
-    : resolver_(std::move(resolver)) {
-  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
-    gpr_log(GPR_INFO, "[xds_resolver %p] creating XdsConfigSelector %p",
-            resolver_.get(), this);
-  }
-  // 1. Construct the route table
-  // 2  Update resolver's cluster state map
-  // 3. Construct cluster list to hold on to entries in the cluster state
-  // map.
+absl::StatusOr<RefCountedPtr<XdsResolver::RouteConfigData>>
+XdsResolver::RouteConfigData::Create(
+    XdsResolver* resolver,
+    const std::vector<XdsRouteConfigResource::Route>& routes,
+    const Duration& default_max_stream_duration) {
+  auto data = MakeRefCounted<RouteConfigData>();
   // Reserve the necessary entries up-front to avoid reallocation as we add
   // elements. This is necessary because the string_view in the entry's
   // weighted_cluster_state field points to the memory in the route field, so
   // moving the entry in a reallocation will cause the string_view to point to
   // invalid data.
-  route_table_.reserve(resolver_->current_virtual_host_->routes.size());
-  std::map<absl::string_view, RefCountedPtr<ClusterState>> clusters;
-  auto maybe_add_cluster = [&](absl::string_view cluster_name) {
-    if (clusters.find(cluster_name) != clusters.end()) return;
-    auto cluster_state = resolver_->GetOrCreateClusterState(cluster_name);
-    absl::string_view name = cluster_state->cluster_name();
-    clusters.emplace(name, std::move(cluster_state));
-  };
-  for (auto& route : resolver_->current_virtual_host_->routes) {
-    if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
-      gpr_log(GPR_INFO, "[xds_resolver %p] XdsConfigSelector %p: route: %s",
-              resolver_.get(), this, route.ToString().c_str());
-    }
-    route_table_.emplace_back();
-    auto& route_entry = route_table_.back();
-    route_entry.route = route;
-    auto* route_action =
-        absl::get_if<XdsRouteConfigResource::Route::RouteAction>(
-            &route_entry.route.action);
-    if (route_action != nullptr) {
-      // If the route doesn't specify a timeout, set its timeout to the global
-      // one.
-      if (!route_action->max_stream_duration.has_value()) {
-        route_action->max_stream_duration =
-            resolver_->current_listener_.http_max_stream_duration;
-      }
-      Match(
-          route_action->action,
-          // cluster name
-          [&](const XdsRouteConfigResource::Route::RouteAction::ClusterName&
-                  cluster_name) {
-            auto result = CreateMethodConfig(route_entry.route, nullptr);
-            if (!result.ok()) {
-              *status = result.status();
-              return;
-            }
-            route_entry.method_config = std::move(*result);
-            maybe_add_cluster(
-                absl::StrCat("cluster:", cluster_name.cluster_name));
-          },
-          // WeightedClusters
-          [&](const std::vector<
-              XdsRouteConfigResource::Route::RouteAction::ClusterWeight>&
-                  weighted_clusters) {
-            uint32_t end = 0;
-            for (const auto& weighted_cluster : weighted_clusters) {
-              Route::ClusterWeightState cluster_weight_state;
-              auto result =
-                  CreateMethodConfig(route_entry.route, &weighted_cluster);
-              if (!result.ok()) {
-                *status = result.status();
-                return;
-              }
-              cluster_weight_state.method_config = std::move(*result);
-              end += weighted_cluster.weight;
-              cluster_weight_state.range_end = end;
-              cluster_weight_state.cluster = weighted_cluster.name;
-              route_entry.weighted_cluster_state.push_back(
-                  std::move(cluster_weight_state));
-              maybe_add_cluster(
-                  absl::StrCat("cluster:", weighted_cluster.name));
-            }
-          },
-          // ClusterSpecifierPlugin
-          [&](const XdsRouteConfigResource::Route::RouteAction::
-                  ClusterSpecifierPluginName& cluster_specifier_plugin_name) {
-            auto result = CreateMethodConfig(route_entry.route, nullptr);
-            if (!result.ok()) {
-              *status = result.status();
-              return;
-            }
-            route_entry.method_config = std::move(*result);
-            maybe_add_cluster(absl::StrCat(
-                "cluster_specifier_plugin:",
-                cluster_specifier_plugin_name.cluster_specifier_plugin_name));
-          });
-      if (!status->ok()) return;
-    }
-  }
-  cluster_map_ = MakeRefCounted<XdsClusterMap>(std::move(clusters));
-  // Populate filter list.
-  const auto& http_filter_registry =
-      static_cast<const GrpcXdsBootstrap&>(resolver_->xds_client_->bootstrap())
-          .http_filter_registry();
-  for (const auto& http_filter : resolver_->current_listener_.http_filters) {
-    // Find filter.  This is guaranteed to succeed, because it's checked
-    // at config validation time in the XdsApi code.
-    const XdsHttpFilterImpl* filter_impl =
-        http_filter_registry.GetFilterForType(
-            http_filter.config.config_proto_type_name);
-    GPR_ASSERT(filter_impl != nullptr);
-    // Add C-core filter to list.
-    if (filter_impl->channel_filter() != nullptr) {
-      filters_.push_back(filter_impl->channel_filter());
+  data->routes_.reserve(routes.size());
+  for (auto& route : routes) {
+    absl::Status status =
+        data->AddRouteEntry(route, default_max_stream_duration, resolver);
+    if (!status.ok()) {
+      return status;
     }
   }
-  filters_.push_back(&ClusterSelectionFilter::kFilter);
+  return data;
 }
 
-XdsResolver::XdsConfigSelector::~XdsConfigSelector() {
-  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
-    gpr_log(GPR_INFO, "[xds_resolver %p] destroying XdsConfigSelector %p",
-            resolver_.get(), this);
+XdsResolver::RouteConfigData::RouteEntry*
+XdsResolver::RouteConfigData::GetRouteForRequest(
+    absl::string_view path, grpc_metadata_batch* initial_metadata) {
+  auto route_index = XdsRouting::GetRouteForRequest(RouteListIterator(this),
+                                                    path, initial_metadata);
+  if (!route_index.has_value()) {
+    return nullptr;
   }
-  cluster_map_.reset();
-  resolver_->MaybeRemoveUnusedClusters();
+  return &routes_[*route_index];
 }
 
 absl::StatusOr<RefCountedPtr<ServiceConfig>>
-XdsResolver::XdsConfigSelector::CreateMethodConfig(
-    const XdsRouteConfigResource::Route& route,
+XdsResolver::RouteConfigData::CreateMethodConfig(
+    XdsResolver* resolver, const XdsRouteConfigResource::Route& route,
     const XdsRouteConfigResource::Route::RouteAction::ClusterWeight*
         cluster_weight) {
   std::vector<std::string> fields;
@@ -694,11 +565,11 @@ XdsResolver::XdsConfigSelector::CreateMethodConfig(
   }
   // Handle xDS HTTP filters.
   auto result = XdsRouting::GeneratePerHTTPFilterConfigs(
-      static_cast<const GrpcXdsBootstrap&>(resolver_->xds_client_->bootstrap())
+      static_cast<const GrpcXdsBootstrap&>(resolver->xds_client_->bootstrap())
           .http_filter_registry(),
-      resolver_->current_listener_.http_filters,
-      resolver_->current_virtual_host_.value(), route, cluster_weight,
-      resolver_->args_);
+      resolver->current_listener_.http_filters,
+      resolver->current_virtual_host_.value(), route, cluster_weight,
+      resolver->args_);
   if (!result.ok()) return result.status();
   for (const auto& p : result->per_filter_configs) {
     fields.emplace_back(absl::StrCat("    \"", p.first, "\": [\n",
@@ -722,6 +593,128 @@ XdsResolver::XdsConfigSelector::CreateMethodConfig(
   return nullptr;
 }
 
+absl::Status XdsResolver::RouteConfigData::AddRouteEntry(
+    const XdsRouteConfigResource::Route& route,
+    const Duration& default_max_stream_duration, XdsResolver* resolver) {
+  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
+    gpr_log(GPR_INFO, "[xds_resolver %p] XdsConfigSelector %p: route: %s",
+            resolver, this, route.ToString().c_str());
+  }
+  routes_.emplace_back(route);
+  auto* route_entry = &routes_.back();
+  auto maybe_add_cluster = [&](absl::string_view cluster_name) {
+    if (clusters_.find(cluster_name) != clusters_.end()) return;
+    auto cluster_state = resolver->GetOrCreateClusterRef(cluster_name);
+    absl::string_view name = cluster_state->cluster_name();
+    clusters_.emplace(name, std::move(cluster_state));
+  };
+  auto* route_action = absl::get_if<XdsRouteConfigResource::Route::RouteAction>(
+      &route_entry->route.action);
+  if (route_action != nullptr) {
+    // If the route doesn't specify a timeout, set its timeout to the global
+    // one.
+    if (!route_action->max_stream_duration.has_value()) {
+      route_action->max_stream_duration = default_max_stream_duration;
+    }
+    absl::Status status = Match(
+        route_action->action,
+        // cluster name
+        [&](const XdsRouteConfigResource::Route::RouteAction::ClusterName&
+                cluster_name) {
+          auto result =
+              CreateMethodConfig(resolver, route_entry->route, nullptr);
+          if (!result.ok()) {
+            return result.status();
+          }
+          route_entry->method_config = std::move(*result);
+          maybe_add_cluster(
+              absl::StrCat("cluster:", cluster_name.cluster_name));
+          return absl::OkStatus();
+        },
+        // WeightedClusters
+        [&](const std::vector<
+            XdsRouteConfigResource::Route::RouteAction::ClusterWeight>&
+                weighted_clusters) {
+          uint32_t end = 0;
+          for (const auto& weighted_cluster : weighted_clusters) {
+            auto result = CreateMethodConfig(resolver, route_entry->route,
+                                             &weighted_cluster);
+            if (!result.ok()) {
+              return result.status();
+            }
+            RouteEntry::ClusterWeightState cluster_weight_state;
+            cluster_weight_state.method_config = std::move(*result);
+            end += weighted_cluster.weight;
+            cluster_weight_state.range_end = end;
+            cluster_weight_state.cluster = weighted_cluster.name;
+            route_entry->weighted_cluster_state.push_back(
+                std::move(cluster_weight_state));
+            maybe_add_cluster(absl::StrCat("cluster:", weighted_cluster.name));
+          }
+          return absl::OkStatus();
+        },
+        // ClusterSpecifierPlugin
+        [&](const XdsRouteConfigResource::Route::RouteAction::
+                ClusterSpecifierPluginName& cluster_specifier_plugin_name) {
+          auto result =
+              CreateMethodConfig(resolver, route_entry->route, nullptr);
+          if (!result.ok()) {
+            return result.status();
+          }
+          route_entry->method_config = std::move(*result);
+          maybe_add_cluster(absl::StrCat(
+              "cluster_specifier_plugin:",
+              cluster_specifier_plugin_name.cluster_specifier_plugin_name));
+          return absl::OkStatus();
+        });
+    if (!status.ok()) {
+      return status;
+    }
+  }
+  return absl::OkStatus();
+}
+
+//
+// XdsResolver::XdsConfigSelector
+//
+
+XdsResolver::XdsConfigSelector::XdsConfigSelector(
+    RefCountedPtr<XdsResolver> resolver,
+    RefCountedPtr<RouteConfigData> route_config_data)
+    : resolver_(std::move(resolver)),
+      route_config_data_(std::move(route_config_data)) {
+  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
+    gpr_log(GPR_INFO, "[xds_resolver %p] creating XdsConfigSelector %p",
+            resolver_.get(), this);
+  }
+  // Populate filter list.
+  const auto& http_filter_registry =
+      static_cast<const GrpcXdsBootstrap&>(resolver_->xds_client_->bootstrap())
+          .http_filter_registry();
+  for (const auto& http_filter : resolver_->current_listener_.http_filters) {
+    // Find filter.  This is guaranteed to succeed, because it's checked
+    // at config validation time in the XdsApi code.
+    const XdsHttpFilterImpl* filter_impl =
+        http_filter_registry.GetFilterForType(
+            http_filter.config.config_proto_type_name);
+    GPR_ASSERT(filter_impl != nullptr);
+    // Add C-core filter to list.
+    if (filter_impl->channel_filter() != nullptr) {
+      filters_.push_back(filter_impl->channel_filter());
+    }
+  }
+  filters_.push_back(&ClusterSelectionFilter::kFilter);
+}
+
+XdsResolver::XdsConfigSelector::~XdsConfigSelector() {
+  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
+    gpr_log(GPR_INFO, "[xds_resolver %p] destroying XdsConfigSelector %p",
+            resolver_.get(), this);
+  }
+  route_config_data_.reset();
+  resolver_->MaybeRemoveUnusedClusters();
+}
+
 absl::optional<uint64_t> HeaderHashHelper(
     const XdsRouteConfigResource::Route::RouteAction::HashPolicy::Header&
         header_policy,
@@ -747,18 +740,16 @@ absl::Status XdsResolver::XdsConfigSelector::GetCallConfig(
     GetCallConfigArgs args) {
   Slice* path = args.initial_metadata->get_pointer(HttpPathMetadata());
   GPR_ASSERT(path != nullptr);
-  auto route_index = XdsRouting::GetRouteForRequest(
-      RouteListIterator(&route_table_), path->as_string_view(),
-      args.initial_metadata);
-  if (!route_index.has_value()) {
+  auto* entry = route_config_data_->GetRouteForRequest(path->as_string_view(),
+                                                       args.initial_metadata);
+  if (entry == nullptr) {
     return absl::UnavailableError(
         "No matching route found in xDS route config");
   }
-  auto& entry = route_table_[*route_index];
   // Found a route match
   const auto* route_action =
       absl::get_if<XdsRouteConfigResource::Route::RouteAction>(
-          &entry.route.action);
+          &entry->route.action);
   if (route_action == nullptr) {
     return absl::UnavailableError("Matching route has inappropriate action");
   }
@@ -771,24 +762,24 @@ absl::Status XdsResolver::XdsConfigSelector::GetCallConfig(
               action_cluster_name) {
         cluster_name =
             absl::StrCat("cluster:", action_cluster_name.cluster_name);
-        method_config = entry.method_config;
+        method_config = entry->method_config;
       },
       // WeightedClusters
       [&](const std::vector<
           XdsRouteConfigResource::Route::RouteAction::ClusterWeight>&
           /*weighted_clusters*/) {
         const uint32_t key = absl::Uniform<uint32_t>(
-            absl::BitGen(), 0, entry.weighted_cluster_state.back().range_end);
+            absl::BitGen(), 0, entry->weighted_cluster_state.back().range_end);
         // Find the index in weighted clusters corresponding to key.
         size_t mid = 0;
         size_t start_index = 0;
-        size_t end_index = entry.weighted_cluster_state.size() - 1;
+        size_t end_index = entry->weighted_cluster_state.size() - 1;
         size_t index = 0;
         while (end_index > start_index) {
           mid = (start_index + end_index) / 2;
-          if (entry.weighted_cluster_state[mid].range_end > key) {
+          if (entry->weighted_cluster_state[mid].range_end > key) {
             end_index = mid;
-          } else if (entry.weighted_cluster_state[mid].range_end < key) {
+          } else if (entry->weighted_cluster_state[mid].range_end < key) {
             start_index = mid + 1;
           } else {
             index = mid + 1;
@@ -796,10 +787,10 @@ absl::Status XdsResolver::XdsConfigSelector::GetCallConfig(
           }
         }
         if (index == 0) index = start_index;
-        GPR_ASSERT(entry.weighted_cluster_state[index].range_end > key);
+        GPR_ASSERT(entry->weighted_cluster_state[index].range_end > key);
         cluster_name = absl::StrCat(
-            "cluster:", entry.weighted_cluster_state[index].cluster);
-        method_config = entry.weighted_cluster_state[index].method_config;
+            "cluster:", entry->weighted_cluster_state[index].cluster);
+        method_config = entry->weighted_cluster_state[index].method_config;
       },
       // ClusterSpecifierPlugin
       [&](const XdsRouteConfigResource::Route::RouteAction::
@@ -807,9 +798,9 @@ absl::Status XdsResolver::XdsConfigSelector::GetCallConfig(
         cluster_name = absl::StrCat(
             "cluster_specifier_plugin:",
             cluster_specifier_plugin_name.cluster_specifier_plugin_name);
-        method_config = entry.method_config;
+        method_config = entry->method_config;
       });
-  auto cluster = cluster_map_->Find(cluster_name);
+  auto cluster = route_config_data_->FindClusterRef(cluster_name);
   GPR_ASSERT(cluster != nullptr);
   // Generate a hash.
   absl::optional<uint64_t> hash;
@@ -857,10 +848,85 @@ absl::Status XdsResolver::XdsConfigSelector::GetCallConfig(
   args.service_config_call_data->SetCallAttribute(
       args.arena->New<RequestHashAttribute>(hash_value));
   args.service_config_call_data->SetCallAttribute(
-      args.arena->ManagedNew<XdsClusterMapAttribute>(cluster_map_));
+      args.arena->ManagedNew<XdsRouteStateAttributeImpl>(route_config_data_,
+                                                         entry));
   return absl::OkStatus();
 }
 
+//
+// XdsResolver::XdsRouteStateAttributeImpl
+//
+
+bool XdsResolver::XdsRouteStateAttributeImpl::HasClusterForRoute(
+    absl::string_view cluster_name) const {
+  // Found a route match
+  const auto* route_action =
+      absl::get_if<XdsRouteConfigResource::Route::RouteAction>(
+          &static_cast<RouteConfigData::RouteEntry*>(route_)->route.action);
+  if (route_action == nullptr) return false;
+  return Match(
+      route_action->action,
+      [&](const XdsRouteConfigResource::Route::RouteAction::ClusterName& name) {
+        return name.cluster_name == cluster_name;
+      },
+      [&](const std::vector<
+          XdsRouteConfigResource::Route::RouteAction::ClusterWeight>&
+              clusters) {
+        for (const auto& cluster : clusters) {
+          if (cluster.name == cluster_name) {
+            return true;
+          }
+        }
+        return false;
+      },
+      [&](const XdsRouteConfigResource::Route::RouteAction::
+              ClusterSpecifierPluginName& /* name */) { return false; });
+}
+
+RefCountedPtr<XdsResolver::ClusterRef>
+XdsResolver::XdsRouteStateAttributeImpl::LockAndGetCluster(
+    absl::string_view cluster_name) {
+  if (route_config_data_ == nullptr) {
+    return nullptr;
+  }
+  auto cluster = route_config_data_->FindClusterRef(cluster_name);
+  route_config_data_.reset();
+  return cluster;
+}
+
+//
+// XdsResolver::ClusterSelectionFilter
+//
+
+const grpc_channel_filter XdsResolver::ClusterSelectionFilter::kFilter =
+    MakePromiseBasedFilter<ClusterSelectionFilter, FilterEndpoint::kClient,
+                           kFilterExaminesServerInitialMetadata>(
+        "cluster_selection_filter");
+
+ArenaPromise<ServerMetadataHandle>
+XdsResolver::ClusterSelectionFilter::MakeCallPromise(
+    CallArgs call_args, NextPromiseFactory next_promise_factory) {
+  auto* service_config_call_data =
+      static_cast<ClientChannelServiceConfigCallData*>(
+          GetContext<grpc_call_context_element>()
+              [GRPC_CONTEXT_SERVICE_CONFIG_CALL_DATA]
+                  .value);
+  GPR_ASSERT(service_config_call_data != nullptr);
+  auto* route_state_attribute = static_cast<XdsRouteStateAttributeImpl*>(
+      service_config_call_data->GetCallAttribute<XdsRouteStateAttribute>());
+  auto* cluster_name_attribute =
+      service_config_call_data->GetCallAttribute<XdsClusterAttribute>();
+  if (route_state_attribute != nullptr && cluster_name_attribute != nullptr) {
+    auto cluster = route_state_attribute->LockAndGetCluster(
+        cluster_name_attribute->cluster());
+    if (cluster != nullptr) {
+      service_config_call_data->SetOnCommit(
+          [cluster = std::move(cluster)]() mutable { cluster.reset(); });
+    }
+  }
+  return next_promise_factory(std::move(call_args));
+}
+
 //
 // XdsResolver
 //
@@ -1016,7 +1082,6 @@ void XdsResolver::OnListenerUpdate(XdsListenerResource listener) {
       });
 }
 
-namespace {
 class VirtualHostListIterator : public XdsRouting::VirtualHostListIterator {
  public:
   explicit VirtualHostListIterator(
@@ -1033,7 +1098,6 @@ class VirtualHostListIterator : public XdsRouting::VirtualHostListIterator {
  private:
   const std::vector<XdsRouteConfigResource::VirtualHost>* virtual_hosts_;
 };
-}  // namespace
 
 void XdsResolver::OnRouteConfigUpdate(XdsRouteConfigResource rds_update) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_resolver_trace)) {
@@ -1100,7 +1164,7 @@ void XdsResolver::OnResourceDoesNotExist(std::string context) {
 absl::StatusOr<RefCountedPtr<ServiceConfig>>
 XdsResolver::CreateServiceConfig() {
   std::vector<std::string> clusters;
-  for (const auto& cluster : cluster_state_map_) {
+  for (const auto& cluster : cluster_ref_map_) {
     absl::string_view child_name = cluster.first;
     if (absl::ConsumePrefix(&child_name, "cluster_specifier_plugin:")) {
       clusters.push_back(absl::StrFormat(
@@ -1142,13 +1206,16 @@ void XdsResolver::GenerateResult() {
   if (!current_virtual_host_.has_value()) return;
   // First create XdsConfigSelector, which may add new entries to the cluster
   // state map, and then CreateServiceConfig for LB policies.
-  absl::Status status;
-  auto config_selector = MakeRefCounted<XdsConfigSelector>(Ref(), &status);
-  if (!status.ok()) {
+  auto route_config_data =
+      RouteConfigData::Create(this, current_virtual_host_->routes,
+                              current_listener_.http_max_stream_duration);
+  if (!route_config_data.ok()) {
     OnError("could not create ConfigSelector",
-            absl::UnavailableError(status.message()));
+            absl::UnavailableError(route_config_data.status().message()));
     return;
   }
+  auto config_selector =
+      MakeRefCounted<XdsConfigSelector>(Ref(), std::move(*route_config_data));
   Result result;
   result.addresses.emplace();
   result.service_config = CreateServiceConfig();
@@ -1169,13 +1236,13 @@ void XdsResolver::GenerateResult() {
 
 void XdsResolver::MaybeRemoveUnusedClusters() {
   bool update_needed = false;
-  for (auto it = cluster_state_map_.begin(); it != cluster_state_map_.end();) {
-    RefCountedPtr<ClusterState> cluster_state = it->second->RefIfNonZero();
+  for (auto it = cluster_ref_map_.begin(); it != cluster_ref_map_.end();) {
+    RefCountedPtr<ClusterRef> cluster_state = it->second->RefIfNonZero();
     if (cluster_state != nullptr) {
       ++it;
     } else {
       update_needed = true;
-      it = cluster_state_map_.erase(it);
+      it = cluster_ref_map_.erase(it);
     }
   }
   if (update_needed && xds_client_ != nullptr) {
@@ -1185,7 +1252,7 @@ void XdsResolver::MaybeRemoveUnusedClusters() {
 }
 
 //
-// Factory
+// XdsResolverFactory
 //
 
 class XdsResolverFactory : public ResolverFactory {
@@ -1210,7 +1277,6 @@ class XdsResolverFactory : public ResolverFactory {
     return MakeOrphanable<XdsResolver>(std::move(args));
   }
 };
-
 }  // namespace
 
 void RegisterXdsResolver(CoreConfiguration::Builder* builder) {

src/core/ext/filters/client_channel/resolver/xds/xds_resolver.h

@@ -29,17 +29,33 @@ namespace grpc_core {
 class XdsClusterAttribute
     : public ServiceConfigCallData::CallAttributeInterface {
  public:
-  static UniqueTypeName TypeName();
+  static UniqueTypeName TypeName() {
+    static UniqueTypeName::Factory kFactory("xds_cluster_name");
+    return kFactory.Create();
+  }
 
   explicit XdsClusterAttribute(absl::string_view cluster) : cluster_(cluster) {}
 
   absl::string_view cluster() const { return cluster_; }
+  void set_cluster(absl::string_view cluster) { cluster_ = cluster; }
 
  private:
   UniqueTypeName type() const override { return TypeName(); }
 
   absl::string_view cluster_;
 };
+
+class XdsRouteStateAttribute
+    : public ServiceConfigCallData::CallAttributeInterface {
+ public:
+  static UniqueTypeName TypeName() {
+    static UniqueTypeName::Factory factory("xds_cluster_lb_data");
+    return factory.Create();
+  }
+
+  virtual bool HasClusterForRoute(absl::string_view cluster_name) const = 0;
+  UniqueTypeName type() const override { return TypeName(); }
+};
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_EXT_FILTERS_CLIENT_CHANNEL_RESOLVER_XDS_XDS_RESOLVER_H

src/core/ext/filters/stateful_session/stateful_session_filter.cc

@@ -33,11 +33,13 @@
 #include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
+#include "absl/strings/strip.h"
 #include "absl/types/optional.h"
 
 #include <grpc/support/log.h>
 #include <grpc/support/time.h>
 
+#include "src/core/ext/filters/client_channel/resolver/xds/xds_resolver.h"
 #include "src/core/ext/filters/stateful_session/stateful_session_service_config_parser.h"
 #include "src/core/lib/channel/channel_stack.h"
 #include "src/core/lib/channel/context.h"
@@ -83,35 +85,142 @@ StatefulSessionFilter::StatefulSessionFilter(ChannelFilter::Args filter_args)
 
 namespace {
 
+absl::string_view AllocateStringOnArena(
+    absl::string_view src1, absl::string_view src2 = absl::string_view()) {
+  if (src1.empty() && src2.empty()) {
+    return absl::string_view();
+  }
+  char* arena_allocated_value =
+      static_cast<char*>(GetContext<Arena>()->Alloc(src1.size() + src2.size()));
+  memcpy(arena_allocated_value, src1.data(), src1.size());
+  if (!src2.empty()) {
+    memcpy(arena_allocated_value + src1.size(), src2.data(), src2.size());
+  }
+  return absl::string_view(arena_allocated_value, src1.size() + src2.size());
+}
+
 // Adds the set-cookie header to the server initial metadata if needed.
 void MaybeUpdateServerInitialMetadata(
     const StatefulSessionMethodParsedConfig::CookieConfig* cookie_config,
-    absl::optional<absl::string_view> cookie_value,
-    ServerMetadata* server_initial_metadata) {
+    bool cluster_changed, absl::string_view host_override,
+    absl::string_view actual_cluster, ServerMetadata* server_initial_metadata) {
   // Get peer string.
   Slice* peer_string = server_initial_metadata->get_pointer(PeerString());
-  if (peer_string == nullptr) return;  // Nothing we can do.
-  // If there was no cookie or if the address changed, set the cookie.
-  if (!cookie_value.has_value() ||
-      peer_string->as_string_view() != *cookie_value) {
-    std::vector<std::string> parts = {absl::StrCat(
-        *cookie_config->name, "=",
-        absl::Base64Escape(peer_string->as_string_view()), "; HttpOnly")};
-    if (!cookie_config->path.empty()) {
-      parts.emplace_back(absl::StrCat("Path=", cookie_config->path));
-    }
-    if (cookie_config->ttl > Duration::Zero()) {
-      parts.emplace_back(
-          absl::StrCat("Max-Age=", cookie_config->ttl.as_timespec().tv_sec));
-    }
-    server_initial_metadata->Append(
-        "set-cookie", Slice::FromCopiedString(absl::StrJoin(parts, "; ")),
-        [](absl::string_view error, const Slice&) {
-          Crash(absl::StrCat("ERROR ADDING set-cookie METADATA: ", error));
-        });
+  if (peer_string == nullptr) {
+    // No changes, keep the same set-cookie header.
+    return;
+  }
+  if (host_override == peer_string->as_string_view() && !cluster_changed) {
+    return;
+  }
+  std::string new_value(peer_string->as_string_view());
+  if (!actual_cluster.empty()) {
+    absl::StrAppend(&new_value, ";", actual_cluster);
+  }
+  std::vector<std::string> parts = {absl::StrCat(
+      *cookie_config->name, "=", absl::Base64Escape(new_value), "; HttpOnly")};
+  if (!cookie_config->path.empty()) {
+    parts.emplace_back(absl::StrCat("Path=", cookie_config->path));
+  }
+  if (cookie_config->ttl > Duration::Zero()) {
+    parts.emplace_back(
+        absl::StrCat("Max-Age=", cookie_config->ttl.as_timespec().tv_sec));
+  }
+  server_initial_metadata->Append(
+      "set-cookie", Slice::FromCopiedString(absl::StrJoin(parts, "; ")),
+      [](absl::string_view error, const Slice&) {
+        Crash(absl::StrCat("ERROR ADDING set-cookie METADATA: ", error));
+      });
+}
+
+// Returns an arena-allocated string containing the cluster name
+// to use for this RPC, which will live long enough to use when modifying
+// the server's initial metadata. If cluster_from_cookie is non-empty and
+// points to a cluster present in the selected route, uses that; otherwise,
+// uses the cluster selected by the XdsConfigSelector.
+// Returns the empty string if cluster override cannot be used (i.e., the route
+// uses a cluster specifier plugin).
+absl::string_view GetClusterToUse(
+    absl::string_view cluster_from_cookie,
+    ServiceConfigCallData* service_config_call_data) {
+  // Get cluster assigned by the XdsConfigSelector.
+  auto cluster_attribute =
+      service_config_call_data->GetCallAttribute<XdsClusterAttribute>();
+  GPR_ASSERT(cluster_attribute != nullptr);
+  auto current_cluster = cluster_attribute->cluster();
+  static constexpr absl::string_view kClusterPrefix = "cluster:";
+  // If prefix is not "cluster:", then we can't use cluster override.
+  if (!absl::ConsumePrefix(&current_cluster, kClusterPrefix)) {
+    return absl::string_view();
+  }
+  // No cluster in cookie, use the cluster from the attribute
+  if (cluster_from_cookie.empty()) {
+    return AllocateStringOnArena(current_cluster);
+  }
+  // Use cluster from the cookie if it is configured for the route.
+  auto route_data =
+      service_config_call_data->GetCallAttribute<XdsRouteStateAttribute>();
+  GPR_ASSERT(route_data != nullptr);
+  // Cookie cluster was not configured for route - use the one from the
+  // attribute
+  if (!route_data->HasClusterForRoute(cluster_from_cookie)) {
+    return AllocateStringOnArena(current_cluster);
+  }
+  auto arena_allocated_cluster =
+      AllocateStringOnArena(kClusterPrefix, cluster_from_cookie);
+  // Update the cluster name attribute with an arena allocated value.
+  cluster_attribute->set_cluster(arena_allocated_cluster);
+  return absl::StripPrefix(arena_allocated_cluster, kClusterPrefix);
+}
+
+std::string GetCookieValue(const ClientMetadataHandle& client_initial_metadata,
+                           absl::string_view cookie_name) {
+  // Check to see if the cookie header is present.
+  std::string buffer;
+  auto header_value =
+      client_initial_metadata->GetStringValue("cookie", &buffer);
+  if (!header_value.has_value()) return "";
+  // Parse cookie header.
+  std::vector<absl::string_view> values;
+  for (absl::string_view cookie : absl::StrSplit(*header_value, "; ")) {
+    std::pair<absl::string_view, absl::string_view> kv =
+        absl::StrSplit(cookie, absl::MaxSplits('=', 1));
+    if (kv.first == cookie_name) values.push_back(kv.second);
   }
+  if (values.empty()) return "";
+  // TODO(roth): Figure out the right behavior for multiple cookies.
+  // For now, just choose the first value.
+  std::string decoded;
+  if (absl::Base64Unescape(values.front(), &decoded)) {
+    return decoded;
+  }
+  return "";
 }
 
+bool IsConfiguredPath(absl::string_view configured_path,
+                      const ClientMetadataHandle& client_initial_metadata) {
+  // No path configured meaning all paths match
+  if (configured_path.empty()) {
+    return true;
+  }
+  // Check to see if the configured path matches the request path.
+  Slice* path_slice = client_initial_metadata->get_pointer(HttpPathMetadata());
+  GPR_ASSERT(path_slice != nullptr);
+  absl::string_view path = path_slice->as_string_view();
+  // Matching criteria from
+  // https://www.rfc-editor.org/rfc/rfc6265#section-5.1.4.
+  // The cookie-path is a prefix of the request-path (and)
+  if (!absl::StartsWith(path, configured_path)) {
+    return false;
+  }
+  // One of
+  // 1. The cookie-path and the request-path are identical.
+  // 2. The last character of the cookie-path is %x2F ("/").
+  // 3. The first character of the request-path that is not included
+  //    in the cookie-path is a %x2F ("/") character.
+  return path.length() == configured_path.length() ||
+         configured_path.back() == '/' || path[configured_path.length()] == '/';
+}
 }  // namespace
 
 // Construct a promise for one call.
@@ -129,90 +238,57 @@ ArenaPromise<ServerMetadataHandle> StatefulSessionFilter::MakeCallPromise(
   GPR_ASSERT(method_params != nullptr);
   auto* cookie_config = method_params->GetConfig(index_);
   GPR_ASSERT(cookie_config != nullptr);
-  if (!cookie_config->name.has_value()) {
+  if (!cookie_config->name.has_value() ||
+      !IsConfiguredPath(cookie_config->path,
+                        call_args.client_initial_metadata)) {
     return next_promise_factory(std::move(call_args));
   }
-  // We have a config.
-  // If the config has a path, check to see if it matches the request path.
-  if (!cookie_config->path.empty()) {
-    Slice* path_slice =
-        call_args.client_initial_metadata->get_pointer(HttpPathMetadata());
-    GPR_ASSERT(path_slice != nullptr);
-    absl::string_view path = path_slice->as_string_view();
-    // Matching criteria from
-    // https://www.rfc-editor.org/rfc/rfc6265#section-5.1.4.
-    if (!absl::StartsWith(path, cookie_config->path) ||
-        (path.size() != cookie_config->path.size() &&
-         cookie_config->path.back() != '/' &&
-         path[cookie_config->path.size() + 1] != '/')) {
-      return next_promise_factory(std::move(call_args));
-    }
-  }
-  // Check to see if we have a host override cookie.
-  auto cookie_value = GetOverrideHostFromCookie(
-      call_args.client_initial_metadata, *cookie_config->name);
-  if (cookie_value.has_value()) {
-    if (GRPC_TRACE_FLAG_ENABLED(grpc_stateful_session_filter_trace)) {
-      gpr_log(GPR_INFO,
-              "chand=%p: stateful session filter found cookie %s value %s",
-              this, cookie_config->name->c_str(),
-              std::string(*cookie_value).c_str());
-    }
-    // We have a valid cookie, so add the call attribute to be used by the
-    // xds_override_host LB policy.
+  // Base64-decode cookie value.
+  std::string cookie_value =
+      GetCookieValue(call_args.client_initial_metadata, *cookie_config->name);
+  // Cookie format is "host;cluster"
+  std::pair<absl::string_view, absl::string_view> host_cluster =
+      absl::StrSplit(cookie_value, absl::MaxSplits(';', 1));
+  absl::string_view host_override;
+  // Set override host attribute.  Allocate the string on the
+  // arena, so that it has the right lifetime.
+  if (!host_cluster.first.empty()) {
+    host_override = AllocateStringOnArena(host_cluster.first);
     service_config_call_data->SetCallAttribute(
-        GetContext<Arena>()->New<XdsOverrideHostAttribute>(*cookie_value));
+        GetContext<Arena>()->New<XdsOverrideHostAttribute>(host_override));
   }
+  // Check if the cluster override is valid, and apply it if necessary.
+  // Note that cluster_name will point to an arena-allocated string
+  // that will still be alive when we see the server initial metadata.
+  // If the cluster name is empty, that means we cannot use a
+  // cluster override (i.e., the route uses a cluster specifier plugin).
+  absl::string_view cluster_name =
+      GetClusterToUse(host_cluster.second, service_config_call_data);
+  bool cluster_changed = cluster_name != host_cluster.second;
   // Intercept server initial metadata.
   call_args.server_initial_metadata->InterceptAndMap(
-      [cookie_config, cookie_value](ServerMetadataHandle md) {
+      [cookie_config, cluster_changed, host_override,
+       cluster_name](ServerMetadataHandle md) {
         // Add cookie to server initial metadata if needed.
-        MaybeUpdateServerInitialMetadata(cookie_config, cookie_value, md.get());
+        MaybeUpdateServerInitialMetadata(cookie_config, cluster_changed,
+                                         host_override, cluster_name, md.get());
         return md;
       });
   return Map(next_promise_factory(std::move(call_args)),
-             [cookie_config, cookie_value](ServerMetadataHandle md) {
+             [cookie_config, cluster_changed, host_override,
+              cluster_name](ServerMetadataHandle md) {
                // If we got a Trailers-Only response, then add the
                // cookie to the trailing metadata instead of the
                // initial metadata.
                if (md->get(GrpcTrailersOnly()).value_or(false)) {
-                 MaybeUpdateServerInitialMetadata(cookie_config, cookie_value,
-                                                  md.get());
+                 MaybeUpdateServerInitialMetadata(
+                     cookie_config, cluster_changed, host_override,
+                     cluster_name, md.get());
                }
                return md;
              });
 }
 
-absl::optional<absl::string_view>
-StatefulSessionFilter::GetOverrideHostFromCookie(
-    const ClientMetadataHandle& client_initial_metadata,
-    absl::string_view cookie_name) {
-  // Check to see if the cookie header is present.
-  std::string buffer;
-  auto header_value =
-      client_initial_metadata->GetStringValue("cookie", &buffer);
-  if (!header_value.has_value()) return absl::nullopt;
-  // Parse cookie header.
-  std::vector<absl::string_view> values;
-  for (absl::string_view cookie : absl::StrSplit(*header_value, "; ")) {
-    std::pair<absl::string_view, absl::string_view> kv =
-        absl::StrSplit(cookie, absl::MaxSplits('=', 1));
-    if (kv.first == cookie_name) values.push_back(kv.second);
-  }
-  if (values.empty()) return absl::nullopt;
-  // TODO(roth): Figure out the right behavior for multiple cookies.
-  // For now, just choose the first value.
-  absl::string_view value = values.front();
-  // Base64-decode it.
-  std::string decoded_value;
-  if (!absl::Base64Unescape(value, &decoded_value)) return absl::nullopt;
-  // Copy it into the arena, since it will need to persist until the LB pick.
-  char* arena_value =
-      static_cast<char*>(GetContext<Arena>()->Alloc(decoded_value.size()));
-  memcpy(arena_value, decoded_value.c_str(), decoded_value.size());
-  return absl::string_view(arena_value, decoded_value.size());
-}
-
 void StatefulSessionFilterRegister(CoreConfiguration::Builder* builder) {
   StatefulSessionServiceConfigParser::Register(builder);
 }

src/core/ext/filters/stateful_session/stateful_session_filter.h

@@ -23,7 +23,6 @@
 
 #include "absl/status/statusor.h"
 #include "absl/strings/string_view.h"
-#include "absl/types/optional.h"
 
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/channel/channel_fwd.h"
@@ -65,11 +64,6 @@ class StatefulSessionFilter : public ChannelFilter {
 
  private:
   explicit StatefulSessionFilter(ChannelFilter::Args filter_args);
-
-  absl::optional<absl::string_view> GetOverrideHostFromCookie(
-      const ClientMetadataHandle& initial_metadata,
-      absl::string_view cookie_name);
-
   // The relative index of instances of the same filter.
   const size_t index_;
   // Index of the service config parser.

src/core/lib/service_config/service_config_call_data.h

@@ -88,6 +88,11 @@ class ServiceConfigCallData {
     call_attributes_.EmplaceBack(value);
   }
 
+  template <typename A>
+  A* GetCallAttribute() const {
+    return static_cast<A*>(GetCallAttribute(A::TypeName()));
+  }
+
   CallAttributeInterface* GetCallAttribute(UniqueTypeName type) const {
     for (CallAttributeInterface* attribute : call_attributes_) {
       if (attribute->type() == type) return attribute;

test/cpp/end2end/xds/BUILD

@@ -440,6 +440,7 @@ grpc_cc_test(
         "//:gpr",
         "//:grpc",
         "//:grpc++",
+        "//src/proto/grpc/testing/xds/v3:aggregate_cluster_proto",
         "//src/proto/grpc/testing/xds/v3:router_proto",
         "//src/proto/grpc/testing/xds/v3:stateful_session_cookie_proto",
         "//src/proto/grpc/testing/xds/v3:stateful_session_proto",

test/cpp/end2end/xds/xds_end2end_test_lib.cc

@@ -631,7 +631,7 @@ XdsEnd2endTest::CreateEndpointsForBackends(size_t start_index,
 }
 
 ClusterLoadAssignment XdsEnd2endTest::BuildEdsResource(
-    const EdsResourceArgs& args, const char* eds_service_name) {
+    const EdsResourceArgs& args, absl::string_view eds_service_name) {
   ClusterLoadAssignment assignment;
   assignment.set_cluster_name(eds_service_name);
   for (const auto& locality : args.locality_list) {

test/cpp/end2end/xds/xds_end2end_test_lib.h

@@ -654,7 +654,7 @@ class XdsEnd2endTest : public ::testing::TestWithParam<XdsTestType> {
   // Constructs an EDS resource.
   ClusterLoadAssignment BuildEdsResource(
       const EdsResourceArgs& args,
-      const char* eds_service_name = kDefaultEdsServiceName);
+      absl::string_view eds_service_name = kDefaultEdsServiceName);
 
   //
   // Backend management

test/cpp/end2end/xds/xds_override_host_end2end_test.cc

@@ -27,6 +27,7 @@
 #include "src/core/ext/filters/client_channel/backup_poller.h"
 #include "src/core/lib/config/config_vars.h"
 #include "src/core/lib/gprpp/match.h"
+#include "src/proto/grpc/testing/xds/v3/aggregate_cluster.grpc.pb.h"
 #include "src/proto/grpc/testing/xds/v3/cluster.grpc.pb.h"
 #include "src/proto/grpc/testing/xds/v3/outlier_detection.grpc.pb.h"
 #include "src/proto/grpc/testing/xds/v3/router.grpc.pb.h"
@@ -118,19 +119,24 @@ class OverrideHostTest : public XdsEnd2endTest {
     return listener;
   }
 
+  // Send requests until a desired backend is hit and returns cookie name/value
+  // pairs. Empty collection is returned if the backend was never hit.
+  // For weighted clusters, more than one request per backend may be necessary
+  // to obtain the cookie. max_requests_per_backend argument specifies
+  // the number of requests per backend to send.
   std::vector<std::pair<std::string, std::string>>
   GetAffinityCookieHeaderForBackend(grpc_core::DebugLocation debug_location,
                                     size_t backend_index,
-                                    RpcOptions rpc_options = RpcOptions()) {
+                                    size_t max_requests_per_backend = 1) {
     EXPECT_LT(backend_index, backends_.size());
     if (backend_index >= backends_.size()) {
       return {};
     }
     const auto& backend = backends_[backend_index];
-    for (size_t i = 0; i < backends_.size(); ++i) {
+    for (size_t i = 0; i < max_requests_per_backend * backends_.size(); ++i) {
       std::multimap<std::string, std::string> server_initial_metadata;
       grpc::Status status =
-          SendRpc(rpc_options, nullptr, &server_initial_metadata);
+          SendRpc(RpcOptions(), nullptr, &server_initial_metadata);
       EXPECT_TRUE(status.ok())
           << "code=" << status.error_code()
           << ", message=" << status.error_message() << "\n"
@@ -150,6 +156,44 @@ class OverrideHostTest : public XdsEnd2endTest {
         << "Desired backend had not been hit";
     return {};
   }
+
+  void SetClusterResource(absl::string_view cluster_name,
+                          absl::string_view eds_resource_name) {
+    Cluster cluster = default_cluster_;
+    cluster.set_name(cluster_name);
+    cluster.mutable_eds_cluster_config()->set_service_name(eds_resource_name);
+    balancer_->ads_service()->SetCdsResource(cluster);
+  }
+
+  RouteConfiguration BuildRouteConfigurationWithWeightedClusters(
+      const std::map<absl::string_view, uint32_t> clusters) {
+    RouteConfiguration new_route_config = default_route_config_;
+    auto* route1 = new_route_config.mutable_virtual_hosts(0)->mutable_routes(0);
+    for (const auto& cluster : clusters) {
+      auto* weighted_cluster =
+          route1->mutable_route()->mutable_weighted_clusters()->add_clusters();
+      weighted_cluster->set_name(cluster.first);
+      weighted_cluster->mutable_weight()->set_value(cluster.second);
+    }
+    return new_route_config;
+  }
+
+  void SetCdsAndEdsResources(absl::string_view cluster_name,
+                             absl::string_view eds_service_name,
+                             size_t start_index, size_t end_index) {
+    balancer_->ads_service()->SetEdsResource(BuildEdsResource(
+        EdsResourceArgs({{"locality0",
+                          CreateEndpointsForBackends(start_index, end_index)}}),
+        eds_service_name));
+    SetClusterResource(cluster_name, eds_service_name);
+  }
+
+  static double BackendRequestPercentage(
+      const std::unique_ptr<BackendServerThread>& backend,
+      size_t num_requests) {
+    return static_cast<double>(backend->backend_service()->request_count()) /
+           num_requests;
+  }
 };
 
 INSTANTIATE_TEST_SUITE_P(XdsTest, OverrideHostTest,
@@ -267,6 +311,132 @@ TEST_P(OverrideHostTest, DrainingExcludedFromOverrideSet) {
   EXPECT_EQ(2, backends_[2]->backend_service()->request_count());
   ResetBackendCounters();
 }
+
+TEST_P(OverrideHostTest, OverrideWithWeightedClusters) {
+  CreateAndStartBackends(3);
+  const char* kNewCluster1Name = "new_cluster_1";
+  const char* kNewEdsService1Name = "new_eds_service_name_1";
+  const char* kNewCluster2Name = "new_cluster_2";
+  const char* kNewEdsService2Name = "new_eds_service_name_2";
+  const uint32_t kWeight1 = 1;
+  const uint32_t kWeight2 = 3;
+  const double kErrorTolerance = 0.025;
+  const size_t kNumEchoRpcs = ComputeIdealNumRpcs(
+      static_cast<double>(kWeight1) / (kWeight1 + kWeight2), kErrorTolerance);
+  // Populate EDS and CDS resources.
+  SetCdsAndEdsResources(kNewCluster1Name, kNewEdsService1Name, 0, 1);
+  SetCdsAndEdsResources(kNewCluster2Name, kNewEdsService2Name, 1, 3);
+  // Populating Route Configurations for LDS.
+  SetListenerAndRouteConfiguration(
+      balancer_.get(), BuildListenerWithStatefulSessionFilter(),
+      BuildRouteConfigurationWithWeightedClusters(
+          {{kNewCluster1Name, kWeight1}, {kNewCluster2Name, kWeight2}}));
+  WaitForAllBackends(DEBUG_LOCATION, 0, 3);
+  // Get cookie
+  auto session_cookie = GetAffinityCookieHeaderForBackend(DEBUG_LOCATION, 1, 3);
+  ASSERT_FALSE(session_cookie.empty());
+  // All requests go to the backend we requested.
+  CheckRpcSendOk(DEBUG_LOCATION, kNumEchoRpcs,
+                 RpcOptions().set_metadata(session_cookie));
+  EXPECT_EQ(backends_[0]->backend_service()->request_count(), 0);
+  EXPECT_EQ(backends_[1]->backend_service()->request_count(), kNumEchoRpcs);
+  EXPECT_EQ(backends_[2]->backend_service()->request_count(), 0);
+}
+
+TEST_P(OverrideHostTest, ClusterOverrideHonoredButHostGone) {
+  CreateAndStartBackends(4);
+  const char* kNewCluster1Name = "new_cluster_1";
+  const char* kNewEdsService1Name = "new_eds_service_name_1";
+  const char* kNewCluster2Name = "new_cluster_2";
+  const char* kNewEdsService2Name = "new_eds_service_name_2";
+  const uint32_t kWeight1 = 1;
+  const uint32_t kWeight2 = 3;
+  const double kErrorTolerance = 0.025;
+  const double kWeight2Percent =
+      static_cast<double>(kWeight2) / (kWeight1 + kWeight2);
+  const size_t kNumEchoRpcs =
+      ComputeIdealNumRpcs(kWeight2Percent, kErrorTolerance);
+  // Populate EDS and CDS resources.
+  SetCdsAndEdsResources(kNewCluster1Name, kNewEdsService1Name, 0, 1);
+  SetCdsAndEdsResources(kNewCluster2Name, kNewEdsService2Name, 1, 3);
+  // Populating Route Configurations for LDS.
+  SetListenerAndRouteConfiguration(
+      balancer_.get(), BuildListenerWithStatefulSessionFilter(),
+      BuildRouteConfigurationWithWeightedClusters(
+          {{kNewCluster1Name, kWeight1}, {kNewCluster2Name, kWeight2}}));
+  WaitForAllBackends(DEBUG_LOCATION, 0, 3);
+  auto session_cookie = GetAffinityCookieHeaderForBackend(DEBUG_LOCATION, 1, 3);
+  ASSERT_FALSE(session_cookie.empty());
+  // Remove backends[1] from cluster2
+  balancer_->ads_service()->SetEdsResource(BuildEdsResource(
+      EdsResourceArgs({{"locality0", CreateEndpointsForBackends(2, 4)}}),
+      kNewEdsService2Name));
+  WaitForAllBackends(DEBUG_LOCATION, 3, 4);
+  CheckRpcSendOk(DEBUG_LOCATION, kNumEchoRpcs,
+                 RpcOptions().set_metadata(session_cookie));
+  // Traffic goes to a second cluster, where it is equally distributed between
+  // the two remaining hosts
+  EXPECT_THAT(BackendRequestPercentage(backends_[2], kNumEchoRpcs),
+              ::testing::DoubleNear(.5, kErrorTolerance));
+  EXPECT_THAT(BackendRequestPercentage(backends_[3], kNumEchoRpcs),
+              ::testing::DoubleNear(.5, kErrorTolerance));
+  EXPECT_NE(session_cookie,
+            GetAffinityCookieHeaderForBackend(DEBUG_LOCATION, 2, 3));
+}
+
+TEST_P(OverrideHostTest, ClusterGoneHostStays) {
+  CreateAndStartBackends(3);
+  const char* kNewCluster1Name = "new_cluster_1";
+  const char* kNewEdsService1Name = "new_eds_service_name_1";
+  const char* kNewCluster2Name = "new_cluster_2";
+  const char* kNewEdsService2Name = "new_eds_service_name_2";
+  const char* kNewCluster3Name = "new_cluster_3";
+  const char* kNewEdsService3Name = "new_eds_service_name_3";
+  const uint32_t kWeight1 = 3;
+  const uint32_t kWeight2 = 1;
+  const double kErrorTolerance = 0.025;
+  const double kPercentage1 =
+      static_cast<double>(kWeight1) / (kWeight1 + kWeight2);
+  const size_t kNumEchoRpcs =
+      ComputeIdealNumRpcs(kPercentage1, kErrorTolerance);
+  // Populate EDS and CDS resources.
+  SetCdsAndEdsResources(kNewCluster1Name, kNewEdsService1Name, 0, 1);
+  SetCdsAndEdsResources(kNewCluster2Name, kNewEdsService2Name, 1, 2);
+  // Populating Route Configurations for LDS.
+  SetListenerAndRouteConfiguration(
+      balancer_.get(), BuildListenerWithStatefulSessionFilter(),
+      BuildRouteConfigurationWithWeightedClusters(
+          {{kNewCluster1Name, kWeight1}, {kNewCluster2Name, kWeight2}}));
+  WaitForAllBackends(DEBUG_LOCATION, 0, 2);
+  auto backend1_in_cluster2_cookie =
+      GetAffinityCookieHeaderForBackend(DEBUG_LOCATION, 1, 5);
+  ASSERT_FALSE(backend1_in_cluster2_cookie.empty());
+  // Create a new cluster, cluster 3, containing a new backend, backend 2.
+  SetCdsAndEdsResources(kNewCluster3Name, kNewEdsService3Name, 2, 3);
+  // Send an EDS update for cluster 1 that adds backend 1. (Now cluster 1 has
+  // backends 0 and 1.)
+  balancer_->ads_service()->SetEdsResource(BuildEdsResource(
+      EdsResourceArgs({{"locality0", CreateEndpointsForBackends(0, 2)}}),
+      kNewEdsService1Name));
+  SetListenerAndRouteConfiguration(
+      balancer_.get(), BuildListenerWithStatefulSessionFilter(),
+      BuildRouteConfigurationWithWeightedClusters(
+          {{kNewCluster1Name, kWeight1}, {kNewCluster3Name, kWeight2}}));
+  WaitForAllBackends(DEBUG_LOCATION, 2);
+  CheckRpcSendOk(DEBUG_LOCATION, kNumEchoRpcs,
+                 RpcOptions().set_metadata(backend1_in_cluster2_cookie));
+  // Traffic is split between clusters. Cluster1 traffic is sent to backends_[1]
+  EXPECT_THAT(BackendRequestPercentage(backends_[0], kNumEchoRpcs),
+              ::testing::DoubleNear(0, kErrorTolerance));
+  EXPECT_THAT(BackendRequestPercentage(backends_[1], kNumEchoRpcs),
+              ::testing::DoubleNear(kPercentage1, kErrorTolerance));
+  EXPECT_THAT(BackendRequestPercentage(backends_[2], kNumEchoRpcs),
+              ::testing::DoubleNear(1 - kPercentage1, kErrorTolerance));
+  // backends_[1] cookie is updated with a new cluster
+  EXPECT_NE(backend1_in_cluster2_cookie,
+            GetAffinityCookieHeaderForBackend(DEBUG_LOCATION, 1, 5));
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace grpc

test/cpp/end2end/xds/xds_routing_end2end_test.cc

@@ -402,7 +402,9 @@ INSTANTIATE_TEST_SUITE_P(
     ::testing::Values(XdsTestType(), XdsTestType().set_enable_rds_testing()),
     &XdsTestType::Name);
 
-MATCHER_P2(AdjustedClockInRange, t1, t2, "equals time") {
+MATCHER_P2(AdjustedClockInRange, t1, t2,
+           absl::StrFormat("time between %s and %s", t1.ToString().c_str(),
+                           t2.ToString().c_str())) {
   gpr_cycle_counter cycle_now = gpr_get_cycle_counter();
   grpc_core::Timestamp cycle_time =
       grpc_core::Timestamp::FromCycleCounterRoundDown(cycle_now);