dynamo_runtime/

distributed.rs

// SPDX-FileCopyrightText: Copyright (c) 2024-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

use crate::component::{
    self, Component, ComponentBuilder, Endpoint, EndpointDiscoverySource, Instance, Namespace,
    RoutingOccupancyState,
};
use crate::config::environment_names::tcp_response_stream;
use crate::pipeline::PipelineError;
use crate::pipeline::network::manager::NetworkManager;
use crate::service::{ServiceClient, ServiceSet};
use crate::storage::kv;
use crate::{discovery, system_status_server, transports};
use crate::{
    discovery::Discovery,
    metrics::PrometheusUpdateCallback,
    metrics::{MetricsHierarchy, MetricsRegistry},
    transports::{etcd, nats, tcp},
};

use super::utils::GracefulShutdownTracker;
use crate::SystemHealth;
use crate::runtime::Runtime;

// Used instead of std::cell::OnceCell because get_or_try_init there is nightly
use async_once_cell::OnceCell;

use std::fmt;
use std::sync::{Arc, OnceLock, Weak};
use std::time::Duration;
use tokio::sync::watch::Receiver;

use anyhow::Result;
use derive_getters::Dissolve;
use figment::error;
use std::collections::HashMap;
use tokio::sync::Mutex;
use tokio_util::sync::CancellationToken;

type EndpointDiscoverySourceMap = HashMap<Endpoint, Weak<EndpointDiscoverySource>>;
type RoutingOccupancyMap = HashMap<Endpoint, Weak<RoutingOccupancyState>>;

/// Distributed [Runtime] which provides access to shared resources across the cluster, this includes
/// communication protocols and transports.
#[derive(Clone)]
pub struct DistributedRuntime {
    // local runtime
    runtime: Runtime,

    nats_client: Option<transports::nats::Client>,
    network_manager: Arc<NetworkManager>,
    tcp_server: Arc<OnceCell<Arc<transports::tcp::server::TcpStreamServer>>>,
    system_status_server: Arc<OnceLock<Arc<system_status_server::SystemStatusServerInfo>>>,
    request_plane: RequestPlaneMode,

    // Service discovery client
    discovery_client: Arc<dyn discovery::Discovery>,

    // Discovery metadata (only used for Kubernetes backend)
    // Shared with system status server to expose via /metadata endpoint
    discovery_metadata: Option<Arc<tokio::sync::RwLock<discovery::DiscoveryMetadata>>>,

    // local registry for components
    // the registry allows us to use share runtime resources across instances of the same component object.
    // take for example two instances of a client to the same remote component. The registry allows us to use
    // a single endpoint watcher for both clients, this keeps the number background tasking watching specific
    // paths in etcd to a minimum.
    component_registry: component::Registry,

    endpoint_discovery_sources: Arc<tokio::sync::Mutex<EndpointDiscoverySourceMap>>,
    routing_occupancy_states: Arc<tokio::sync::Mutex<RoutingOccupancyMap>>,

    // Health Status
    system_health: Arc<parking_lot::Mutex<SystemHealth>>,

    // Local endpoint registry for in-process calls
    local_endpoint_registry: crate::local_endpoint_registry::LocalEndpointRegistry,

    // This hierarchy's own metrics registry
    metrics_registry: MetricsRegistry,

    // Registry for /engine/* route callbacks
    engine_routes: crate::engine_routes::EngineRouteRegistry,

    // Backs `/v1/metadata/{model_slug}/{model_suffix}/{filename}`.
    metadata_artifacts: crate::metadata_registry::MetadataArtifactRegistry,

    // Resolved event transport kind — set once at construction time from
    // DYN_EVENT_PLANE + discovery backend; returned by default_event_transport_kind().
    event_transport_kind: crate::discovery::EventTransportKind,
}

impl MetricsHierarchy for DistributedRuntime {
    fn basename(&self) -> String {
        "".to_string() // drt has no basename. Basename only begins with the Namespace.
    }

    fn parent_hierarchies(&self) -> Vec<&dyn MetricsHierarchy> {
        vec![] // drt is the root, so no parent hierarchies
    }

    fn get_metrics_registry(&self) -> &MetricsRegistry {
        &self.metrics_registry
    }

    fn connection_id(&self) -> Option<u64> {
        Some(self.discovery_client.instance_id())
    }
}

impl std::fmt::Debug for DistributedRuntime {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "DistributedRuntime")
    }
}

impl DistributedRuntime {
    pub async fn new(runtime: Runtime, config: DistributedConfig) -> Result<Self> {
        let (discovery_backend, nats_config, request_plane, event_transport_kind) =
            config.dissolve();

        let nats_client = match nats_config {
            Some(nc) => Some(nc.connect().await?),
            None => None,
        };

        // Start system status server for health and metrics if enabled in configuration
        let config = crate::config::RuntimeConfig::from_settings().unwrap_or_default();
        // IMPORTANT: We must extract cancel_token from runtime BEFORE moving runtime into the struct below.
        // This is because after moving, runtime is no longer accessible in this scope (ownership rules).
        let cancel_token = if config.system_server_enabled() {
            Some(runtime.clone().child_token())
        } else {
            None
        };
        let starting_health_status = config.starting_health_status.clone();
        let use_endpoint_health_status = config.use_endpoint_health_status.clone();
        let health_endpoint_path = config.system_health_path.clone();
        let live_endpoint_path = config.system_live_path.clone();
        let system_health = Arc::new(parking_lot::Mutex::new(SystemHealth::new(
            starting_health_status,
            use_endpoint_health_status,
            config.health_check_enabled,
            health_endpoint_path,
            live_endpoint_path,
        )));

        // Initialize discovery client based on backend configuration
        let (discovery_client, discovery_metadata) = match discovery_backend {
            DiscoveryBackend::Kubernetes => {
                tracing::info!("Initializing Kubernetes discovery backend");
                let metadata = Arc::new(tokio::sync::RwLock::new(
                    crate::discovery::DiscoveryMetadata::new(),
                ));
                let client = crate::discovery::KubeDiscoveryClient::new(
                    metadata.clone(),
                    runtime.primary_token(),
                )
                .await
                .inspect_err(
                    |err| tracing::error!(%err, "Failed to initialize Kubernetes discovery client"),
                )?;
                (Arc::new(client) as Arc<dyn Discovery>, Some(metadata))
            }
            DiscoveryBackend::KvStore(kv_selector) => {
                tracing::info!("Initializing KV store discovery backend: {kv_selector}");
                let runtime_clone = runtime.clone();
                let store = match kv_selector {
                    kv::Selector::Etcd(etcd_config) => {
                        let etcd_client = etcd::Client::new(*etcd_config, runtime_clone).await.inspect_err(|err|
                            tracing::error!(%err, "Could not connect to etcd. Pass `--discovery-backend ..` to use a different backend or start etcd."))?;
                        kv::Manager::etcd(etcd_client)
                    }
                    kv::Selector::File(root) => kv::Manager::file(runtime.primary_token(), root),
                    kv::Selector::Memory => kv::Manager::memory(),
                };
                use crate::discovery::KVStoreDiscovery;
                (
                    Arc::new(KVStoreDiscovery::new(store, runtime.primary_token()))
                        as Arc<dyn Discovery>,
                    None,
                )
            }
        };

        let component_registry = component::Registry::new();

        // NetworkManager for request plane
        let network_manager = NetworkManager::new(
            runtime.child_token(),
            nats_client.clone().map(|c| c.client().clone()),
            component_registry.clone(),
            request_plane,
        );

        let distributed_runtime = Self {
            runtime,
            network_manager: Arc::new(network_manager),
            nats_client,
            tcp_server: Arc::new(OnceCell::new()),
            system_status_server: Arc::new(OnceLock::new()),
            discovery_client,
            discovery_metadata,
            component_registry,
            endpoint_discovery_sources: Arc::new(Mutex::new(HashMap::new())),
            routing_occupancy_states: Arc::new(Mutex::new(HashMap::new())),
            metrics_registry: crate::MetricsRegistry::new(),
            system_health,
            request_plane,
            local_endpoint_registry: crate::local_endpoint_registry::LocalEndpointRegistry::new(),
            engine_routes: crate::engine_routes::EngineRouteRegistry::new(),
            metadata_artifacts: crate::metadata_registry::MetadataArtifactRegistry::new(),
            event_transport_kind,
        };

        // Initialize the uptime gauge in SystemHealth
        distributed_runtime
            .system_health
            .lock()
            .initialize_uptime_gauge(&distributed_runtime)?;

        // Register an update callback so the uptime gauge is refreshed before
        // every Prometheus scrape (both system status server and frontend).
        {
            let system_health = distributed_runtime.system_health.clone();
            distributed_runtime
                .metrics_registry
                .add_update_callback(std::sync::Arc::new(move || {
                    system_health.lock().update_uptime_gauge();
                    Ok(())
                }));
        }

        // Handle system status server initialization
        if let Some(cancel_token) = cancel_token {
            // System server is enabled - start both the state and HTTP server
            let host = config.system_host.clone();
            let port = config.system_port as u16;

            // Start system status server (it creates SystemStatusState internally)
            match crate::system_status_server::spawn_system_status_server(
                &host,
                port,
                cancel_token,
                Arc::new(distributed_runtime.clone()),
                distributed_runtime.discovery_metadata.clone(),
            )
            .await
            {
                Ok((addr, handle)) => {
                    tracing::info!("System status server started successfully on {addr}");

                    // Store system status server information
                    let system_status_server_info =
                        crate::system_status_server::SystemStatusServerInfo::new(
                            addr,
                            Some(handle),
                        );

                    // Initialize the system_status_server field
                    distributed_runtime
                        .system_status_server
                        .set(Arc::new(system_status_server_info))
                        .expect("System status server info should only be set once");
                }
                Err(e) => {
                    tracing::error!("System status server startup failed: {e}");
                }
            }
        } else {
            // System server HTTP is disabled, but uptime metrics are still being tracked via SystemHealth
            tracing::debug!(
                "System status server HTTP endpoints disabled, but uptime metrics are being tracked"
            );
        }

        // Start health check manager if enabled
        if config.health_check_enabled {
            let health_check_config = crate::health_check::HealthCheckConfig {
                canary_wait_time: std::time::Duration::from_secs(config.canary_wait_time_secs),
                request_timeout: std::time::Duration::from_secs(
                    config.health_check_request_timeout_secs,
                ),
            };

            // Start the health check manager (spawns per-endpoint monitoring tasks)
            match crate::health_check::start_health_check_manager(
                distributed_runtime.clone(),
                Some(health_check_config),
            )
            .await
            {
                Ok(()) => tracing::info!(
                    "Health check manager started (canary_wait_time: {}s, request_timeout: {}s)",
                    config.canary_wait_time_secs,
                    config.health_check_request_timeout_secs
                ),
                Err(e) => tracing::error!("Health check manager failed to start: {e}"),
            }
        }

        Ok(distributed_runtime)
    }

    pub async fn from_settings(runtime: Runtime) -> Result<Self> {
        let config = DistributedConfig::from_settings();
        Self::new(runtime, config).await
    }

    pub fn runtime(&self) -> &Runtime {
        &self.runtime
    }

    pub fn primary_token(&self) -> CancellationToken {
        self.runtime.primary_token()
    }

    // TODO: Don't hand out pointers, instead have methods to use the registry in friendly ways
    // (without being aware of async locks and so on)
    pub fn component_registry(&self) -> &component::Registry {
        &self.component_registry
    }

    // TODO: Don't hand out pointers, instead provide system health related services.
    pub fn system_health(&self) -> Arc<parking_lot::Mutex<SystemHealth>> {
        self.system_health.clone()
    }

    /// Get the local endpoint registry for in-process endpoint calls
    pub fn local_endpoint_registry(
        &self,
    ) -> &crate::local_endpoint_registry::LocalEndpointRegistry {
        &self.local_endpoint_registry
    }

    /// Get the engine route registry for registering custom /engine/* routes
    pub fn engine_routes(&self) -> &crate::engine_routes::EngineRouteRegistry {
        &self.engine_routes
    }

    pub fn metadata_artifacts(&self) -> &crate::metadata_registry::MetadataArtifactRegistry {
        &self.metadata_artifacts
    }

    pub fn connection_id(&self) -> u64 {
        self.discovery_client.instance_id()
    }

    pub fn shutdown(&self) {
        self.runtime.shutdown();
        self.discovery_client.shutdown();
    }

    /// Create a [`Namespace`]
    pub fn namespace(&self, name: impl Into<String>) -> Result<Namespace> {
        Namespace::new(self.clone(), name.into())
    }

    /// Returns the discovery interface for service registration and discovery
    pub fn discovery(&self) -> Arc<dyn Discovery> {
        self.discovery_client.clone()
    }

    pub async fn tcp_server(&self) -> Result<Arc<tcp::server::TcpStreamServer>> {
        Ok(self
            .tcp_server
            .get_or_try_init(async move {
                let port = match std::env::var(tcp_response_stream::DYN_TCP_RESPONSE_STREAM_PORT) {
                    Ok(p) => p.parse::<u16>().map_err(|_| {
                        PipelineError::Generic(format!(
                            "invalid {}: '{}' is not a valid port number",
                            tcp_response_stream::DYN_TCP_RESPONSE_STREAM_PORT,
                            p
                        ))
                    })?,
                    Err(_) => 0,
                };
                let interface = std::env::var(tcp_response_stream::DYN_TCP_RESPONSE_STREAM_HOST)
                    .ok()
                    .filter(|h| !h.is_empty());

                let host_suffix = interface
                    .as_ref()
                    .map_or(String::new(), |h| format!(" on host {h}"));
                if port == 0 {
                    tracing::info!(
                        "TCP response stream server using OS-assigned port{host_suffix}"
                    );
                } else {
                    tracing::info!(
                        "TCP response stream server using fixed port {port}{host_suffix}"
                    );
                }

                let options = tcp::server::ServerOptions { port, interface };
                let server = tcp::server::TcpStreamServer::new(options).await?;
                Ok::<_, PipelineError>(server)
            })
            .await?
            .clone())
    }

    /// Get the network manager
    ///
    /// The network manager consolidates all network configuration and provides
    /// unified access to request plane servers and clients.
    pub fn network_manager(&self) -> Arc<NetworkManager> {
        self.network_manager.clone()
    }

    /// Get the request plane server (convenience method)
    ///
    /// This is a shortcut for `network_manager().await?.server().await`.
    pub async fn request_plane_server(
        &self,
    ) -> Result<Arc<dyn crate::pipeline::network::ingress::unified_server::RequestPlaneServer>>
    {
        self.network_manager().server().await
    }

    /// Get system status server information if available
    pub fn system_status_server_info(
        &self,
    ) -> Option<Arc<crate::system_status_server::SystemStatusServerInfo>> {
        self.system_status_server.get().cloned()
    }

    /// How the frontend should talk to the backend.
    pub fn request_plane(&self) -> RequestPlaneMode {
        self.request_plane
    }

    /// Returns the event transport kind this runtime was configured with.
    ///
    /// The value is resolved once at construction time by `DiscoveryBackend::resolve_event_transport_kind`:
    /// if `DYN_EVENT_PLANE` is set explicitly that value wins; otherwise the default is ZMQ.
    ///
    /// Use this instead of [`EventTransportKind::from_env_or_default`] wherever you have
    /// access to a `DistributedRuntime`.
    pub fn default_event_transport_kind(&self) -> crate::discovery::EventTransportKind {
        self.event_transport_kind
    }

    pub fn child_token(&self) -> CancellationToken {
        self.runtime.child_token()
    }

    pub(crate) fn graceful_shutdown_tracker(&self) -> Arc<GracefulShutdownTracker> {
        self.runtime.graceful_shutdown_tracker()
    }

    pub(crate) fn endpoint_discovery_sources(&self) -> Arc<Mutex<EndpointDiscoverySourceMap>> {
        self.endpoint_discovery_sources.clone()
    }

    /// Register an external long-running shutdown task with this runtime's
    /// graceful-shutdown tracker. While the returned guard is alive,
    /// `Runtime::shutdown` will keep waiting in Phase 2 (rather than
    /// advancing to Phase 3 / NATS+etcd teardown). Drop the guard once
    /// the task has finished.
    pub fn register_graceful_task(&self) -> crate::utils::GracefulTaskGuard {
        self.runtime.graceful_shutdown_tracker().register_task()
    }

    pub(crate) fn routing_occupancy_states(&self) -> Arc<Mutex<RoutingOccupancyMap>> {
        self.routing_occupancy_states.clone()
    }

    /// TODO: This is a temporary KV router measure for component/component.rs EventPublisher impl for
    /// Component, to allow it to publish to NATS. KV Router is the only user.
    ///
    /// When NATS is not available (e.g., running in approximate mode with --no-kv-events),
    /// this function returns Ok(()) silently since publishing is optional in that mode.
    pub async fn kv_router_nats_publish(
        &self,
        subject: String,
        payload: bytes::Bytes,
    ) -> anyhow::Result<()> {
        self.kv_router_nats_publish_subject(subject.into(), payload)
            .await
    }

    pub(crate) async fn kv_router_nats_publish_subject(
        &self,
        subject: async_nats::Subject,
        payload: bytes::Bytes,
    ) -> anyhow::Result<()> {
        let Some(nats_client) = self.nats_client.as_ref() else {
            // NATS not available - this is expected in approximate mode (--no-kv-events)
            tracing::trace!("Skipping NATS publish (NATS not configured): {subject}");
            return Ok(());
        };
        Ok(nats_client.client().publish(subject, payload).await?)
    }

    /// TODO: This is a temporary KV router measure for component/component.rs EventSubscriber impl for
    /// Component, to allow it to subscribe to NATS. KV Router is the only user.
    pub(crate) async fn kv_router_nats_subscribe(
        &self,
        subject: String,
    ) -> Result<async_nats::Subscriber> {
        let Some(nats_client) = self.nats_client.as_ref() else {
            anyhow::bail!("KV router's EventSubscriber requires NATS");
        };
        Ok(nats_client.client().subscribe(subject).await?)
    }

    /// TODO (karenc): This is a temporary KV router measure for worker query requests.
    /// Allows KV Router to perform request/reply with workers. (versus the pub/sub pattern above)
    /// KV Router is the only user, made public for use in dynamo-llm crate
    pub async fn kv_router_nats_request(
        &self,
        subject: String,
        payload: bytes::Bytes,
        timeout: std::time::Duration,
    ) -> anyhow::Result<async_nats::Message> {
        let Some(nats_client) = self.nats_client.as_ref() else {
            anyhow::bail!("KV router's request requires NATS");
        };
        let response =
            tokio::time::timeout(timeout, nats_client.client().request(subject, payload))
                .await
                .map_err(|_| anyhow::anyhow!("Request timed out after {:?}", timeout))??;
        Ok(response)
    }

    /// DEPRECATED: This method exists only for NATS request plane support.
    /// Once everything uses the TCP request plane, this can be removed along with
    /// the NATS service registration infrastructure.
    ///
    /// Returns a receiver that signals when the NATS service registration is complete.
    /// The caller should use `blocking_recv()` to wait for completion.
    pub fn register_nats_service(
        &self,
        component: Component,
    ) -> tokio::sync::mpsc::Receiver<Result<(), String>> {
        // Create a oneshot-style channel (capacity 1) to signal completion
        let (tx, rx) = tokio::sync::mpsc::channel::<Result<(), String>>(1);

        let drt = self.clone();
        self.runtime().secondary().spawn(async move {
            let service_name = component.service_name();

            // Pre-check to save cost of creating the service, but don't hold the lock
            if drt
                .component_registry()
                .inner
                .lock()
                .await
                .services
                .contains_key(&service_name)
            {
                // The NATS service is per component, but it is called from `serve_endpoint`, and there
                // are often multiple endpoints for a component (e.g. `clear_kv_blocks` and `generate`).
                tracing::trace!("Service {service_name} already exists");
                // Signal success - service already exists
                let _ = tx.send(Ok(())).await;
                return;
            }

            let Some(nats_client) = drt.nats_client.as_ref() else {
                tracing::error!("Cannot create NATS service without NATS.");
                let _ = tx
                    .send(Err("Cannot create NATS service without NATS".to_string()))
                    .await;
                return;
            };
            let description = None;
            let nats_service = match crate::component::service::build_nats_service(
                nats_client,
                &component,
                description,
            )
            .await
            {
                Ok(service) => service,
                Err(err) => {
                    tracing::error!(error = %err, component = service_name, "Failed to build NATS service");
                    let _ = tx.send(Err(format!("Failed to build NATS service: {err}"))).await;
                    return;
                }
            };

            let mut guard = drt.component_registry().inner.lock().await;
            if !guard.services.contains_key(&service_name) {
                // Normal case
                guard.services.insert(service_name.clone(), nats_service);

                tracing::info!("Added NATS service {service_name}");

                drop(guard);
            } else {
                drop(guard);
                let _ = nats_service.stop().await;
                // The NATS service is per component, but it is called from `serve_endpoint`, and there
                // are often multiple endpoints for a component (e.g. `clear_kv_blocks` and `generate`).
                // TODO: Is this still true?
            }

            // Signal completion - service registered successfully
            let _ = tx.send(Ok(())).await;
        });

        rx
    }
}

/// Selects which discovery backend to use and, for KV store backends, which KV store.
#[derive(Clone, Debug)]
pub enum DiscoveryBackend {
    /// Use Kubernetes API for service discovery (no KV store needed)
    Kubernetes,
    /// Use a KV store (etcd, file, or memory) for service discovery
    KvStore(kv::Selector),
}

impl DiscoveryBackend {
    /// Returns true if this backend requires no external services (file or in-memory).
    ///
    /// Local backends do not need etcd, NATS, or any other infrastructure daemon.
    pub fn is_local(&self) -> bool {
        matches!(
            self,
            DiscoveryBackend::KvStore(kv::Selector::File(_))
                | DiscoveryBackend::KvStore(kv::Selector::Memory)
        )
    }

    /// Resolve the event transport kind for this backend.
    ///
    /// This is the single authoritative mapping of `DYN_EVENT_PLANE` →
    /// `EventTransportKind`. ZMQ is the default event plane for all backends
    /// (`file`/`mem`/`etcd`/`kubernetes`); NATS is an explicit opt-in via
    /// `DYN_EVENT_PLANE=nats`.
    ///
    /// Call this once at startup and store the result; do not call it repeatedly.
    pub fn resolve_event_transport_kind(&self) -> crate::discovery::EventTransportKind {
        use crate::config::environment_names::event_plane::DYN_EVENT_PLANE;
        use crate::discovery::EventTransportKind;
        match std::env::var(DYN_EVENT_PLANE).as_deref() {
            Ok("nats") => EventTransportKind::Nats,
            Ok("zmq") => EventTransportKind::Zmq,
            // Unset or empty: ZMQ is the default for every backend.
            Ok("") | Err(_) => EventTransportKind::Zmq,
            Ok(other) => {
                tracing::warn!(
                    "Invalid DYN_EVENT_PLANE value '{}'. Valid values: 'nats', 'zmq'. \
                     Defaulting to ZMQ.",
                    other
                );
                EventTransportKind::Zmq
            }
        }
    }
}

#[derive(Dissolve)]
pub struct DistributedConfig {
    pub discovery_backend: DiscoveryBackend,
    pub nats_config: Option<nats::ClientOptions>,
    pub request_plane: RequestPlaneMode,
    /// Resolved event transport kind — computed once at config time from
    /// `DYN_EVENT_PLANE` and the discovery backend, then stored on the runtime
    /// so callers always get the same answer regardless of which other services
    /// happen to be reachable.
    pub event_transport_kind: crate::discovery::EventTransportKind,
}

impl DistributedConfig {
    pub fn from_settings() -> DistributedConfig {
        let request_plane = RequestPlaneMode::from_env();

        // Determine the discovery backend first — we need it to compute the NATS default below.
        // Valid values for DYN_DISCOVERY_BACKEND: "kubernetes", "etcd" (default), "file", "mem"
        let backend_str =
            std::env::var("DYN_DISCOVERY_BACKEND").unwrap_or_else(|_| "etcd".to_string());

        let discovery_backend = match backend_str.as_str() {
            "kubernetes" => {
                tracing::info!("Using Kubernetes discovery backend");
                DiscoveryBackend::Kubernetes
            }
            other => {
                let selector: kv::Selector = other.parse().unwrap_or_else(|_| {
                    panic!(
                        "Unknown DYN_DISCOVERY_BACKEND value: '{other}'. \
                         Valid options: kubernetes, etcd, file, mem"
                    )
                });
                DiscoveryBackend::KvStore(selector)
            }
        };

        // Resolve event transport kind once — the single source of truth used both to
        // decide whether to open a NATS connection and to answer
        // `DistributedRuntime::default_event_transport_kind()` later.
        let event_transport_kind = discovery_backend.resolve_event_transport_kind();

        // NATS is used for more than just NATS request-plane RPC:
        // - KV router events (JetStream or NATS core + local indexer)
        // - inter-router replica sync (NATS core)
        //
        // Enable the NATS client when any of these hold:
        // 1. Request plane is NATS
        // 2. NATS_SERVER is explicitly configured by the user
        // 3. The resolved event transport kind is NATS
        let nats_enabled = request_plane.is_nats()
            || std::env::var(crate::config::environment_names::nats::NATS_SERVER).is_ok()
            || matches!(
                event_transport_kind,
                crate::discovery::EventTransportKind::Nats
            );

        DistributedConfig {
            discovery_backend,
            nats_config: if nats_enabled {
                Some(nats::ClientOptions::default())
            } else {
                None
            },
            request_plane,
            event_transport_kind,
        }
    }

    pub fn for_cli() -> DistributedConfig {
        let etcd_config = etcd::ClientOptions {
            attach_lease: false,
            ..Default::default()
        };
        let request_plane = RequestPlaneMode::from_env();
        let discovery_backend =
            DiscoveryBackend::KvStore(kv::Selector::Etcd(Box::new(etcd_config)));
        let event_transport_kind = discovery_backend.resolve_event_transport_kind();
        let nats_enabled = request_plane.is_nats()
            || std::env::var(crate::config::environment_names::nats::NATS_SERVER).is_ok()
            || matches!(
                event_transport_kind,
                crate::discovery::EventTransportKind::Nats
            );
        DistributedConfig {
            discovery_backend,
            nats_config: if nats_enabled {
                Some(nats::ClientOptions::default())
            } else {
                None
            },
            request_plane,
            event_transport_kind,
        }
    }

    /// A DistributedConfig that isn't distributed, for when the frontend and backend are in the
    /// same process.
    pub fn process_local() -> DistributedConfig {
        DistributedConfig {
            discovery_backend: DiscoveryBackend::KvStore(kv::Selector::Memory),
            nats_config: None,
            // This won't be used in process local, so we likely need a "none" option to
            // communicate that and avoid opening the ports.
            request_plane: RequestPlaneMode::Tcp,
            event_transport_kind: crate::discovery::EventTransportKind::Zmq,
        }
    }
}

/// Request plane transport mode configuration
///
/// This determines how requests are distributed from routers to workers:
/// - `Nats`: Use NATS for request distribution (legacy)
/// - `Tcp`: Use raw TCP for request distribution with msgpack support (default)
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum RequestPlaneMode {
    /// Use NATS for request plane
    Nats,
    /// Use raw TCP for request plane with msgpack support
    #[default]
    Tcp,
}

impl fmt::Display for RequestPlaneMode {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Nats => write!(f, "nats"),
            Self::Tcp => write!(f, "tcp"),
        }
    }
}

impl std::str::FromStr for RequestPlaneMode {
    type Err = anyhow::Error;

    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
        match s.to_lowercase().as_str() {
            "nats" => Ok(Self::Nats),
            "tcp" => Ok(Self::Tcp),
            _ => Err(anyhow::anyhow!(
                "Invalid request plane mode: '{}'. Valid options are: 'nats', 'tcp'",
                s
            )),
        }
    }
}

impl RequestPlaneMode {
    /// Get the request plane mode from environment variable (uncached)
    /// Reads from `DYN_REQUEST_PLANE` environment variable.
    fn from_env() -> Self {
        std::env::var("DYN_REQUEST_PLANE")
            .ok()
            .and_then(|s| s.parse().ok())
            .unwrap_or_default()
    }

    pub fn is_nats(&self) -> bool {
        matches!(self, RequestPlaneMode::Nats)
    }
}

pub mod distributed_test_utils {
    //! Common test helper functions for DistributedRuntime tests

    /// Helper function to create a DRT instance for integration-only tests.
    /// Uses from_current to leverage existing tokio runtime
    /// Note: Settings are read from environment variables inside DistributedRuntime::from_settings
    #[cfg(feature = "integration")]
    pub async fn create_test_drt_async() -> super::DistributedRuntime {
        use crate::transports::nats;

        let rt = crate::Runtime::from_current().unwrap();
        let config = super::DistributedConfig {
            discovery_backend: super::DiscoveryBackend::KvStore(
                crate::storage::kv::Selector::Memory,
            ),
            nats_config: Some(nats::ClientOptions::default()),
            request_plane: crate::distributed::RequestPlaneMode::default(),
            event_transport_kind: crate::discovery::EventTransportKind::Nats,
        };
        super::DistributedRuntime::new(rt, config).await.unwrap()
    }

    /// Helper function to create a DRT instance which points at
    /// a (shared) file-backed KV store and ephemeral NATS transport so that
    /// multiple DRT instances may observe the same registration state.
    /// NOTE: This gets around the fact that create_test_drt_async() is
    /// hardcoded to spin up a memory-backed discovery store
    /// which means we can't share discovery state across runtimes.
    pub async fn create_test_shared_drt_async(
        store_path: &std::path::Path,
    ) -> super::DistributedRuntime {
        use crate::transports::nats;

        let rt = crate::Runtime::from_current().unwrap();
        let config = super::DistributedConfig {
            discovery_backend: super::DiscoveryBackend::KvStore(
                crate::storage::kv::Selector::File(store_path.to_path_buf()),
            ),
            nats_config: Some(nats::ClientOptions::default()),
            request_plane: crate::distributed::RequestPlaneMode::default(),
            event_transport_kind: crate::discovery::EventTransportKind::Nats,
        };
        super::DistributedRuntime::new(rt, config).await.unwrap()
    }
}

#[cfg(all(test, feature = "integration"))]
mod tests {
    use super::RequestPlaneMode;
    use super::distributed_test_utils::create_test_drt_async;

    #[tokio::test]
    async fn test_drt_uptime_after_delay_system_disabled() {
        use crate::config::environment_names::runtime::system as env_system;
        // Test uptime with system status server disabled
        temp_env::async_with_vars([(env_system::DYN_SYSTEM_PORT, None::<&str>)], async {
            // Start a DRT
            let drt = create_test_drt_async().await;

            // Wait 50ms
            tokio::time::sleep(tokio::time::Duration::from_millis(50)).await;

            // Check that uptime is 50+ ms
            let uptime = drt.system_health.lock().uptime();
            assert!(
                uptime >= std::time::Duration::from_millis(50),
                "Expected uptime to be at least 50ms, but got {:?}",
                uptime
            );

            println!(
                "✓ DRT uptime test passed (system disabled): uptime = {:?}",
                uptime
            );
        })
        .await;
    }

    #[tokio::test]
    async fn test_drt_uptime_after_delay_system_enabled() {
        use crate::config::environment_names::runtime::system as env_system;
        // Test uptime with system status server enabled
        temp_env::async_with_vars([(env_system::DYN_SYSTEM_PORT, Some("8081"))], async {
            // Start a DRT
            let drt = create_test_drt_async().await;

            // Wait 50ms
            tokio::time::sleep(tokio::time::Duration::from_millis(50)).await;

            // Check that uptime is 50+ ms
            let uptime = drt.system_health.lock().uptime();
            assert!(
                uptime >= std::time::Duration::from_millis(50),
                "Expected uptime to be at least 50ms, but got {:?}",
                uptime
            );

            println!(
                "✓ DRT uptime test passed (system enabled): uptime = {:?}",
                uptime
            );
        })
        .await;
    }

    #[test]
    fn test_request_plane_mode_from_str() {
        assert_eq!(
            "nats".parse::<RequestPlaneMode>().unwrap(),
            RequestPlaneMode::Nats
        );
        assert_eq!(
            "tcp".parse::<RequestPlaneMode>().unwrap(),
            RequestPlaneMode::Tcp
        );
        assert_eq!(
            "NATS".parse::<RequestPlaneMode>().unwrap(),
            RequestPlaneMode::Nats
        );
        assert_eq!(
            "TCP".parse::<RequestPlaneMode>().unwrap(),
            RequestPlaneMode::Tcp
        );
        assert!("invalid".parse::<RequestPlaneMode>().is_err());
    }
}