fix(runtime): connect to NATS only when a plane uses it

components/src/dynamo/common/utils/runtime.py

@@ -55,8 +55,7 @@ def create_runtime(
         discovery_backend: Discovery backend type (kubernetes, etcd, file, mem).
         request_plane: Request distribution method (nats, tcp).
         event_plane: Event publishing method (nats, zmq). When None, the Rust
-            runtime auto-detects from the discovery backend (ZMQ for file/mem,
-            NATS for etcd/kubernetes).
+            runtime reads DYN_EVENT_PLANE and defaults to ZMQ.
         use_kv_events: Deprecated. NATS enablement is now determined automatically
             from the event-plane configuration. This parameter is accepted for
             backwards compatibility but will be removed in a future release.

lib/bindings/python/rust/lib.rs

@@ -1416,7 +1416,6 @@ impl DistributedRuntime {
         };
         let request_plane: RequestPlaneMode = request_plane.parse().map_err(to_pyerr)?;
         let response_plane = resolve_response_plane_mode(response_plane.as_deref())?;
-        let explicit_event_plane = event_plane.as_deref().filter(|value| !value.is_empty());
         let event_transport_kind =
             resolve_event_transport_kind(&discovery_backend_config, event_plane.as_deref())?;
 
@@ -1448,9 +1447,7 @@ impl DistributedRuntime {
             || matches!(
                 event_transport_kind,
                 dynamo_runtime::discovery::EventTransportKind::Nats
-            )
-            || (explicit_event_plane.is_none()
-                && std::env::var(config::environment_names::nats::NATS_SERVER).is_ok());
+            );
 
         let runtime_config = DistributedConfig {
             discovery_backend: discovery_backend_config,

lib/bindings/python/tests/test_deprecated_enable_nats.py

@@ -130,6 +130,19 @@ async def _run():
     asyncio.run(_run())
 
 
+@pytest.mark.forked
+def test_default_event_plane_ignores_ambient_nats(monkeypatch):
+    """Without event_plane, file/TCP mode defaults to ZMQ and must not connect to ambient NATS."""
+
+    async def _run():
+        monkeypatch.setenv("NATS_SERVER", "nats://127.0.0.1:9")
+        monkeypatch.delenv("DYN_EVENT_PLANE", raising=False)
+        runtime = DistributedRuntime(asyncio.get_running_loop(), "file", "tcp")
+        runtime.shutdown()
+
+    asyncio.run(_run())
+
+
 @pytest.mark.forked
 def test_invalid_event_plane_errors():
     async def _run():

lib/runtime/src/distributed.rs

@@ -1012,16 +1012,9 @@ impl DistributedConfig {
             None => discovery_backend.resolve_event_transport_kind()?,
         };
 
-        // NATS is used for more than just NATS request-plane RPC:
-        // - KV router events (NATS core event plane)
-        // - 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
+        // Connect to NATS only when a plane uses it. NATS_SERVER alone says where NATS is, not
+        // that this process needs it: the operator sets it in every pod when NATS is installed.
         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
@@ -1054,7 +1047,6 @@ impl DistributedConfig {
             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
@@ -1268,6 +1260,32 @@ mod event_plane_env_tests {
     }
 }
 
+#[cfg(test)]
+mod nats_enablement_tests {
+    use super::DistributedConfig;
+    use crate::config::environment_names::{event_plane::DYN_EVENT_PLANE, nats::NATS_SERVER};
+
+    #[test]
+    fn ambient_nats_server_does_not_enable_nats_on_tcp_and_zmq() {
+        if crate::test_utils::run_isolated(
+            concat!(
+                module_path!(),
+                "::ambient_nats_server_does_not_enable_nats_on_tcp_and_zmq"
+            ),
+            &[(NATS_SERVER, "nats://127.0.0.1:9")],
+        ) {
+            return;
+        }
+
+        temp_env::with_var_unset(DYN_EVENT_PLANE, || {
+            let config =
+                DistributedConfig::from_settings_with_overrides(Some("file"), Some("tcp"), None)
+                    .unwrap();
+            assert!(config.nats_config.is_none());
+        });
+    }
+}
+
 pub mod distributed_test_utils {
     //! Common test helper functions for DistributedRuntime tests