Config: Implement deferred clusters on worker.

api/envoy/config/bootstrap/v3/bootstrap.proto

@@ -438,6 +438,7 @@ message Admin {
 }
 
 // Cluster manager :ref:`architecture overview <arch_overview_cluster_manager>`.
+// [#next-free-field: 6]
 message ClusterManager {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.bootstrap.v2.ClusterManager";
@@ -478,6 +479,11 @@ message ClusterManager {
   // <envoy_v3_api_field_config.core.v3.ApiConfigSource.api_type>` :ref:`GRPC
   // <envoy_v3_api_enum_value_config.core.v3.ApiConfigSource.ApiType.GRPC>`.
   core.v3.ApiConfigSource load_stats_config = 4;
+
+  // Whether the ClusterManager will create clusters on the worker threads
+  // inline during requests. This will save memory and CPU cycles in cases where
+  // there are lots of inactive clusters and > 1 worker thread.
+  bool enable_deferred_cluster_creation = 5;
 }
 
 // Allows you to specify different watchdog configs for different subsystems.

changelogs/current.yaml

@@ -30,6 +30,14 @@ new_features:
   change: |
     added %RESPONSE_FLAGS_LONG% substitution string, that will output a pascal case string representing the resonse flags.
     The output response flags will correspond with %RESPONSE_FLAGS%, only with a long textual string representation.
+- area: config
+  change: |
+    Added the capability to defer broadcasting of certain cluster (CDS, EDS) to
+    worker threads from the main thread. This optimization can save significant
+    amount of memory in cases where there are (1) a large number of workers and
+    (2) a large amount of config, most of which is unused. This capability is
+    guarded by :ref:`enable_deferred_cluster_creation
+    <envoy_v3_api_field_config.bootstrap.v3.ClusterManager.enable_deferred_cluster_creation>`.
 - area: extension_discovery_service
   change: |
     added ECDS support for :ref:` downstream network filters<envoy_v3_api_field_config.listener.v3.Filter.config_discovery>`.

docs/root/configuration/upstream/cluster_manager/cluster_stats.rst

@@ -28,6 +28,17 @@ upstreams and control plane xDS clusters.
   active_clusters, Gauge, Number of currently active (warmed) clusters
   warming_clusters, Gauge, Number of currently warming (not active) clusters
 
+
+In addition to the cluster manager stats, there are per worker thread local
+cluster manager statistics tree rooted at
+*thread_local_cluster_manager.<worker_id>.* with the following statistics.
+
+.. csv-table::
+  :header: Name, Type, Description
+  :widths: 1, 1, 2
+
+  clusters_inflated, Gauge, Number of clusters the worker has initialized. If using cluster deferral this number should be <= (cluster_added - clusters_removed).
+
 .. _config_cluster_stats:
 
 Every cluster has a statistics tree rooted at *cluster.<name>.* with the following statistics:

envoy/upstream/cluster_manager.h

@@ -43,18 +43,20 @@ namespace Upstream {
  * ClusterUpdateCallbacks provide a way to expose Cluster lifecycle events in the
  * ClusterManager.
  */
+using ThreadLocalClusterCommand = std::function<ThreadLocalCluster&()>;
 class ClusterUpdateCallbacks {
 public:
   virtual ~ClusterUpdateCallbacks() = default;
 
   /**
    * onClusterAddOrUpdate is called when a new cluster is added or an existing cluster
    * is updated in the ClusterManager.
-   * @param cluster is the ThreadLocalCluster that represents the updated
-   * cluster.
+   * @param cluster_name the name of the changed cluster.
+   * @param get_cluster is a callable that will provide the ThreadLocalCluster that represents the
+   * updated cluster. It should be used within the call or discarded.
    */
-  virtual void onClusterAddOrUpdate(ThreadLocalCluster& cluster) PURE;
-
+  virtual void onClusterAddOrUpdate(absl::string_view cluster_name,
+                                    ThreadLocalClusterCommand& get_cluster) PURE;
   /**
    * onClusterRemoval is called when a cluster is removed; the argument is the cluster name.
    * @param cluster_name is the name of the removed cluster.

source/common/upstream/cluster_discovery_manager.cc

@@ -9,13 +9,15 @@ namespace Upstream {
 
 namespace {
 
-using ClusterAddedCb = std::function<void(ThreadLocalCluster&)>;
+using ClusterAddedCb = std::function<void(absl::string_view)>;
 
 class ClusterCallbacks : public ClusterUpdateCallbacks {
 public:
   ClusterCallbacks(ClusterAddedCb cb) : cb_(std::move(cb)) {}
 
-  void onClusterAddOrUpdate(ThreadLocalCluster& cluster) override { cb_(cluster); };
+  void onClusterAddOrUpdate(absl::string_view cluster_name, ThreadLocalClusterCommand&) override {
+    cb_(cluster_name);
+  };
 
   void onClusterRemoval(const std::string&) override {}
 
@@ -28,12 +30,12 @@ class ClusterCallbacks : public ClusterUpdateCallbacks {
 ClusterDiscoveryManager::ClusterDiscoveryManager(
     std::string thread_name, ClusterLifecycleCallbackHandler& lifecycle_callbacks_handler)
     : thread_name_(std::move(thread_name)) {
-  callbacks_ = std::make_unique<ClusterCallbacks>([this](ThreadLocalCluster& cluster) {
+  callbacks_ = std::make_unique<ClusterCallbacks>([this](absl::string_view cluster_name) {
     ENVOY_LOG(trace,
               "cm cdm: starting processing cluster name {} (status {}) from cluster lifecycle "
               "callback in {}",
-              cluster.info()->name(), enumToInt(ClusterDiscoveryStatus::Available), thread_name_);
-    processClusterName(cluster.info()->name(), ClusterDiscoveryStatus::Available);
+              cluster_name, enumToInt(ClusterDiscoveryStatus::Available), thread_name_);
+    processClusterName(cluster_name, ClusterDiscoveryStatus::Available);
   });
   callbacks_handle_ = lifecycle_callbacks_handler.addClusterUpdateCallbacks(*callbacks_);
 }