flysim: FLY_FEED_VIA=bus publishes every snapshot on an embedded flybus router

feed.via (FLY_FEED_VIA, default direct) and feed.bus_dir (FLY_BUS_DIR,
default /run/fly/bus). In bus mode flysim does not bind the feed port: it
starts a router on its own two-thread runtime with a closed policy (flysim
may publish fly.feed.snapshots, fly-edge may only subscribe to it), listens
for the edge on <bus_dir>/edge.sock, and a publisher task copies each snapshot
out of the watch slot onto the latest-retained topic: frame, audio and spikes
as sealed artifacts, the header as the envelope payload, or as a header
artifact past 48 KiB. The sim thread still only writes its watch slot, so a
slow bus costs snapshots on the bus and never a frame of the loop.

feedbus holds both halves of the encoding, publish and receive, and the
limits sized for the real 122,367-byte snapshot. Two counters are new:
fly_bus_published_total and fly_bus_publish_failures_total.

services/flysim/Cargo.lock

@@ -484,6 +484,7 @@ dependencies = [
  "fly-session-types",
  "flybrain-core",
  "flybrain-gb",
+ "flybus",
  "futures-util",
  "jsonschema",
  "serde",

services/flysim/crates/flysim/Cargo.toml

@@ -30,6 +30,7 @@ cuda = ["flybrain-core/cuda"]
 [dependencies]
 flybrain-core = { path = "../flybrain-core" }
 flybrain-gb = { path = "../flybrain-gb" }
+flybus = { path = "../flybus" }
 
 anyhow = "1.0"
 axum = { version = "0.8", features = ["ws"] }

services/flysim/crates/flysim/src/config.rs

@@ -9,7 +9,7 @@
 //! - the `FLY_*` names the systemd units already set (`FLY_GAME`, `FLY_ROM`, `FLY_DATASET`,
 //!   `FLY_STATE`, `FLY_STATE_HOT`, `FLY_FEED_BIND`, `FLY_CONTROL_BIND`, `FLY_METRICS_ADDR`,
 //!   `FLY_ROM_SHA256`, `FLY_ROM_PLATFORMER_SHA256`, `FLY_CHAT_ENABLED`, `FLY_CHAT_DENY_LIST`,
-//!   `FLY_MACRO_MODE`, `RAYON_NUM_THREADS`);
+//!   `FLY_MACRO_MODE`, `FLY_FEED_VIA`, `FLY_BUS_DIR`, `RAYON_NUM_THREADS`);
 //! - `FLYSIM_<SECTION>_<KEY>` for everything, e.g. `FLYSIM_LOOP_SPEED=0`.
 //!
 //! Nothing here is secret (`docs/control-api.md`: "No secrets live in this service or its
@@ -104,6 +104,35 @@ pub struct Feed {
     pub bind: SocketAddr,
     /// Audio attachment sample rate. The page wants Web Audio's native 48 kHz.
     pub audio_hz: u32,
+    /// Who serves `:7400/feed` (`docs/design/flybus.md`, "Feed over the bus").
+    pub via: FeedVia,
+    /// The bus runtime directory in `bus` mode: the router's socket and its artifact store.
+    /// Belongs on tmpfs; a store here holds a few snapshots, never history.
+    pub bus_dir: PathBuf,
+}
+
+/// Where the feed WebSocket is served from.
+///
+/// `direct` is the default and is the behaviour that predates the bus, byte for byte: flysim
+/// binds `feed.bind` itself. `bus` starts an embedded flybus router, publishes every snapshot
+/// on it, and leaves `feed.bind` to the `fly-edge` process. The control API stays in flysim
+/// either way. Nothing about the fly changes with this knob: it is outside the simulation loop
+/// and outside the compatibility string.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum FeedVia {
+    #[default]
+    Direct,
+    Bus,
+}
+
+impl FeedVia {
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            Self::Direct => "direct",
+            Self::Bus => "bus",
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
@@ -197,6 +226,8 @@ impl Default for Feed {
         Self {
             bind: "127.0.0.1:7400".parse().expect("literal address"),
             audio_hz: 48_000,
+            via: FeedVia::Direct,
+            bus_dir: PathBuf::from("/run/fly/bus"),
         }
     }
 }
@@ -255,6 +286,12 @@ impl Config {
         if let Some(value) = get("FLY_FEED_BIND") {
             self.feed.bind = parse_addr("FLY_FEED_BIND", value)?;
         }
+        if let Some(value) = get("FLY_FEED_VIA") {
+            self.feed.via = parse_feed_via("FLY_FEED_VIA", value)?;
+        }
+        if let Some(value) = get("FLY_BUS_DIR") {
+            self.feed.bus_dir = PathBuf::from(value);
+        }
         if let Some(value) = get("FLY_CONTROL_BIND") {
             self.control.bind = parse_addr("FLY_CONTROL_BIND", value)?;
         }
@@ -330,6 +367,12 @@ impl Config {
         if let Some(value) = get("FLYSIM_FEED_AUDIO_HZ") {
             self.feed.audio_hz = parse("FLYSIM_FEED_AUDIO_HZ", value)?;
         }
+        if let Some(value) = get("FLYSIM_FEED_VIA") {
+            self.feed.via = parse_feed_via("FLYSIM_FEED_VIA", value)?;
+        }
+        if let Some(value) = get("FLYSIM_FEED_BUS_DIR") {
+            self.feed.bus_dir = PathBuf::from(value);
+        }
         if let Some(value) = get("FLYSIM_CONTROL_BIND") {
             self.control.bind = parse_addr("FLYSIM_CONTROL_BIND", value)?;
         }
@@ -464,6 +507,14 @@ impl Config {
     }
 }
 
+fn parse_feed_via(name: &str, value: &str) -> Result<FeedVia> {
+    match value.to_ascii_lowercase().as_str() {
+        "direct" => Ok(FeedVia::Direct),
+        "bus" => Ok(FeedVia::Bus),
+        _ => bail!("{name}: {value:?} is not a feed path; expected \"direct\" or \"bus\""),
+    }
+}
+
 fn parse_addr(name: &str, value: &str) -> Result<SocketAddr> {
     value
         .parse()
@@ -687,6 +738,29 @@ mod tests {
         assert_eq!(config, Config::default());
     }
 
+    #[test]
+    fn the_feed_path_is_direct_unless_the_environment_says_bus() {
+        let config = Config::default();
+        assert_eq!(config.feed.via, FeedVia::Direct);
+        assert_eq!(config.feed.bus_dir, PathBuf::from("/run/fly/bus"));
+
+        let mut config = Config::default();
+        config
+            .apply_env(&env(&[("FLY_FEED_VIA", "bus"), ("FLY_BUS_DIR", "/tmp/fly-bus")]))
+            .unwrap();
+        assert_eq!(config.feed.via, FeedVia::Bus);
+        assert_eq!(config.feed.bus_dir, PathBuf::from("/tmp/fly-bus"));
+
+        let mut config = Config::default();
+        config.apply_env(&env(&[("FLYSIM_FEED_VIA", "DIRECT")])).unwrap();
+        assert_eq!(config.feed.via, FeedVia::Direct);
+
+        // A typo is a refusal, not a silent fallback to one of the two.
+        let error = Config::default().apply_env(&env(&[("FLY_FEED_VIA", "buss")])).unwrap_err();
+        assert!(error.to_string().contains("FLY_FEED_VIA"), "{error}");
+        assert_eq!(toml::from_str::<Config>("[feed]\nvia = \"bus\"\n").unwrap().feed.via, FeedVia::Bus);
+    }
+
     #[test]
     fn the_example_file_parses_and_validates() {
         let path = concat!(env!("CARGO_MANIFEST_DIR"), "/../../flysim.toml.example");

services/flysim/crates/flysim/src/feedbus.rs

@@ -0,0 +1,347 @@
+//! The feed over flybus (`FLY_FEED_VIA=bus`, `docs/design/flybus.md` "Feed over the bus").
+//!
+//! Both halves of the bus encoding live here, so the publisher in flysim and the subscriber in
+//! `fly-edge` cannot drift apart:
+//!
+//! - [`publish`] turns one [`Snapshot`] into one publication on [`TOPIC`]: every attachment the
+//!   header lists as a sealed artifact named after its kind (`frame`, `audio`, `spikes`), and the
+//!   header itself as the envelope payload `{"header": {...}}`. A header too large for an
+//!   envelope travels as a `header` artifact instead, so no snapshot is ever unpublishable.
+//! - [`receive`] turns that publication back into the same [`Snapshot`], which `fly-edge` hands to
+//!   [`crate::feed`] exactly as flysim does. The WebSocket bytes are therefore produced by the same
+//!   `Snapshot::encode` on both paths.
+//!
+//! The simulation thread never sees any of this. It publishes into its `watch` slot as it
+//! always has; [`run_publisher`] is a task on the bus's own runtime that reads that slot and
+//! skips whatever it was too slow to see, the same drop-oldest rule every feed client gets.
+//! A stalled router, a full store or an absent edge can cost snapshots on the bus, never a
+//! frame of the loop.
+
+use std::io::Write as _;
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+use flybus::{
+    Artifact, BusError, Client, ClientConfig, ErrorCode, Grants, Limits, Message, Pattern, Policy,
+    PublishReceipt, Retained, Router, RouterConfig, UnixListenerHandle,
+};
+use serde_json::{Map, Value};
+use tokio::sync::watch;
+
+use crate::metrics::Metrics;
+use crate::snapshot::{AttachmentKind, FeedHeader, Snapshot};
+
+/// The one topic: `latest` retention, so a subscriber that joins late starts from the newest
+/// snapshot and one that falls behind is coalesced rather than queued.
+pub const TOPIC: &str = "fly.feed.snapshots";
+/// The publisher's participant id (in-process, launcher-bound).
+pub const PUBLISHER: &str = "flysim";
+/// The edge's participant id; the Unix socket is bound to it.
+pub const EDGE: &str = "fly-edge";
+/// Socket file under `feed.bus_dir`, bound to [`EDGE`] only.
+pub const SOCKET: &str = "edge.sock";
+/// Store root under `feed.bus_dir`; the router makes its per-incarnation directory inside it.
+pub const STORE: &str = "store";
+/// Headers up to this many JSON bytes ride in the envelope; anything larger becomes an artifact.
+/// Well under flybus's 65,536-byte envelope limit, leaving room for the attachment references
+/// and the router's ids. A real header is 2 to 8 KB.
+pub const HEADER_INLINE_MAX: usize = 48 * 1024;
+/// The attachment name of an out-of-line header.
+pub const HEADER_ARTIFACT: &str = "header";
+
+/// `<bus_dir>/edge.sock`.
+pub fn socket_path(bus_dir: &Path) -> PathBuf {
+    bus_dir.join(SOCKET)
+}
+
+/// `<bus_dir>/store`.
+pub fn store_root(bus_dir: &Path) -> PathBuf {
+    bus_dir.join(STORE)
+}
+
+/// The router limits for the feed (`docs/design/flybus.md`, amendment "Feed sizing").
+///
+/// One snapshot with attachments is 122,367 bytes on the live fly: a 92,160-byte 160x144 RGBA
+/// frame, a 17,407-byte spike bitset (139,255 neurons) and about 12,800 bytes of audio (1,600
+/// stereo f32 frames at 48 kHz per 30 Hz snapshot). A `latest` subscriber pins at most its one
+/// queued slot plus its in-flight credits, the topic pins one retained value, and the publisher
+/// holds one snapshot of staging plus the sealed copy while it seals. With [`Limits::max_clients`]
+/// at 8 and in-flight credits capped at 2, the worst case is 7 subscribers that never consume:
+/// `7 * 3 + 1 + 2 = 24` snapshots, about 3 MB. The store cap is ten times that so a burst of
+/// catch-up audio after a stall still fits, and it is RAM (tmpfs), so it is kept small on purpose.
+pub fn limits() -> Limits {
+    Limits {
+        max_clients: 8,
+        max_services: 8,
+        max_topics: 8,
+        max_subscriptions_per_client: 4,
+        max_subscriptions: 16,
+        max_latest_in_flight: 2,
+        max_owners_per_client: 64,
+        reserved_owners_per_client: 8,
+        // Audio accumulates while the loop is behind its publish deadline; 4 MiB is ten seconds
+        // of it, far past anything the pacer allows before it logs lag.
+        max_artifact_bytes: 4 << 20,
+        max_store_bytes: 32 << 20,
+        max_retained_bytes: 8 << 20,
+        ..Limits::default()
+    }
+}
+
+/// flysim may declare and publish the feed topic; the edge may only subscribe to it.
+pub fn policy() -> Policy {
+    Policy::closed()
+        .client(
+            PUBLISHER,
+            Grants {
+                publish: vec![Pattern::exact(TOPIC)],
+                manage_topics: vec![Pattern::exact(TOPIC)],
+                ..Grants::default()
+            },
+        )
+        .client(
+            EDGE,
+            Grants {
+                subscribe: vec![Pattern::exact(TOPIC)],
+                ..Grants::default()
+            },
+        )
+}
+
+/// A running router and the edge's socket. Dropping it stops listening; the router stops with
+/// the runtime it was started on.
+pub struct BusFeed {
+    pub router: Router,
+    _listener: UnixListenerHandle,
+}
+
+/// Start the embedded router under `bus_dir` and listen for the edge on `<bus_dir>/edge.sock`.
+///
+/// Must run inside a Tokio runtime. `Router::new` removes store directories a previous flysim
+/// left behind (their `flock` is free once that process is gone); a stale socket file is removed
+/// here, because a socket outlives its listener on disk.
+pub async fn start_router(bus_dir: &Path) -> anyhow::Result<BusFeed> {
+    use anyhow::Context as _;
+    use std::os::unix::fs::DirBuilderExt as _;
+    std::fs::DirBuilder::new()
+        .recursive(true)
+        .mode(0o700)
+        .create(bus_dir)
+        .with_context(|| format!("creating the bus directory {}", bus_dir.display()))?;
+    let root = store_root(bus_dir);
+    std::fs::DirBuilder::new()
+        .recursive(true)
+        .mode(0o700)
+        .create(&root)
+        .with_context(|| format!("creating the bus store root {}", root.display()))?;
+    let socket = socket_path(bus_dir);
+    match std::fs::remove_file(&socket) {
+        Ok(()) => tracing::info!(socket = %socket.display(), "removed a stale bus socket"),
+        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
+        Err(error) => {
+            return Err(error).with_context(|| format!("removing {}", socket.display()));
+        }
+    }
+    let mut config = RouterConfig::new(root);
+    config.limits = limits();
+    config.policy = policy();
+    let router = Router::new(config).context("starting the flybus router")?;
+    let listener = router
+        .listen_unix_as(&socket, EDGE)
+        .await
+        .with_context(|| format!("listening on {}", socket.display()))?;
+    tracing::info!(
+        socket = %socket.display(),
+        store = %router.store_dir().display(),
+        router = router.router_id(),
+        "feed bus listening"
+    );
+    Ok(BusFeed {
+        router,
+        _listener: listener,
+    })
+}
+
+fn attachment_name(kind: AttachmentKind) -> &'static str {
+    match kind {
+        AttachmentKind::Frame => "frame",
+        AttachmentKind::Audio => "audio",
+        AttachmentKind::Spikes => "spikes",
+    }
+}
+
+fn content_type(kind: AttachmentKind) -> &'static str {
+    match kind {
+        AttachmentKind::Frame => "image/x-rgba",
+        AttachmentKind::Audio => "audio/x-f32le",
+        AttachmentKind::Spikes => "application/x-spike-bitset",
+    }
+}
+
+fn bytes_of(snapshot: &Snapshot, kind: AttachmentKind) -> &[u8] {
+    match kind {
+        AttachmentKind::Frame => &snapshot.frame,
+        AttachmentKind::Audio => &snapshot.audio,
+        AttachmentKind::Spikes => &snapshot.spikes,
+    }
+}
+
+async fn seal(client: &Client, bytes: &[u8], content_type: &str) -> Result<Artifact, BusError> {
+    let mut writer = client
+        .artifacts()
+        .allocate(bytes.len() as u64, content_type)
+        .await?;
+    writer
+        .write_all(bytes)
+        .map_err(|error| BusError::new(ErrorCode::StoreFailure, format!("staging: {error}")))?;
+    writer.seal().await
+}
+
+/// Publish one snapshot: its attachments as artifacts, its header as the payload.
+pub async fn publish(client: &Client, snapshot: &Snapshot) -> Result<PublishReceipt, BusError> {
+    let header = &snapshot.header;
+    let json = serde_json::to_vec(header).expect("a FeedHeader always serializes");
+
+    let mut kinds: Vec<AttachmentKind> = Vec::with_capacity(3);
+    for kind in header.attachments.iter().copied() {
+        if !kinds.contains(&kind) {
+            kinds.push(kind);
+        }
+    }
+    let mut artifacts: Vec<(&'static str, Artifact)> = Vec::with_capacity(4);
+    for kind in kinds {
+        let artifact = seal(client, bytes_of(snapshot, kind), content_type(kind)).await?;
+        artifacts.push((attachment_name(kind), artifact));
+    }
+
+    let mut payload = Map::new();
+    if json.len() <= HEADER_INLINE_MAX {
+        let value: Value = serde_json::from_slice(&json).expect("a serialized header re-parses");
+        payload.insert("header".into(), value);
+    } else {
+        artifacts.push((
+            HEADER_ARTIFACT,
+            seal(client, &json, "application/json").await?,
+        ));
+    }
+    let attachments: Vec<(&str, &Artifact)> = artifacts
+        .iter()
+        .map(|(name, artifact)| (*name, artifact))
+        .collect();
+    client.publish(TOPIC, payload, &attachments).await
+}
+
+/// Rebuild the snapshot one publication carries. The message's delivery is released when the
+/// caller drops it; every byte has been copied out by then.
+pub async fn receive(message: &Message) -> Result<Snapshot, BusError> {
+    let invalid = |what: String| BusError::new(ErrorCode::InvalidEnvelope, what);
+    let header: FeedHeader = match message.payload().get("header") {
+        Some(value) => serde_json::from_value(value.clone())
+            .map_err(|error| invalid(format!("feed header: {error}")))?,
+        None => {
+            let bytes = message.artifact(HEADER_ARTIFACT)?.read_all().await?;
+            serde_json::from_slice(&bytes)
+                .map_err(|error| invalid(format!("feed header artifact: {error}")))?
+        }
+    };
+    let mut snapshot = Snapshot {
+        header,
+        frame: Arc::new(Vec::new()),
+        audio: Arc::new(Vec::new()),
+        spikes: Arc::new(Vec::new()),
+    };
+    let kinds = snapshot.header.attachments.clone();
+    for kind in kinds {
+        let bytes = Arc::new(message.artifact(attachment_name(kind))?.read_all().await?);
+        match kind {
+            AttachmentKind::Frame => snapshot.frame = bytes,
+            AttachmentKind::Audio => snapshot.audio = bytes,
+            AttachmentKind::Spikes => snapshot.spikes = bytes,
+        }
+    }
+    Ok(snapshot)
+}
+
+/// How often a failing publisher repeats its warning.
+const WARN_EVERY: Duration = Duration::from_secs(10);
+
+/// Publish every snapshot the sim puts in its watch slot until the sim is gone.
+///
+/// Connects in process as [`PUBLISHER`], declares [`TOPIC`] with `latest` retention and
+/// publishes the current snapshot first, so an edge that connects at once still sees the boot
+/// state. A refused publication (a full store, say) is counted and skipped; a lost connection
+/// is re-made after a second. Borrows of the watch slot end before any await, exactly as in
+/// [`crate::feed`]: a held borrow is a lock the sim thread's next publish would wait on.
+pub async fn run_publisher(
+    router: Router,
+    mut snapshots: watch::Receiver<Arc<Snapshot>>,
+    metrics: Arc<Metrics>,
+) {
+    let store_root = router.store_root().to_path_buf();
+    let mut last_warning: Option<Instant> = None;
+    let mut warn = |error: &BusError, what: &str| {
+        if last_warning.is_none_or(|at| at.elapsed() >= WARN_EVERY) {
+            tracing::warn!(%error, "feed bus: {what}");
+            last_warning = Some(Instant::now());
+        }
+    };
+    loop {
+        let transport = router.connect_in_memory_as(PUBLISHER);
+        let client =
+            match Client::connect(transport, ClientConfig::new(PUBLISHER, &store_root)).await {
+                Ok(client) => client,
+                Err(error) => {
+                    warn(&error, "the publisher could not connect");
+                    tokio::time::sleep(Duration::from_secs(1)).await;
+                    continue;
+                }
+            };
+        if let Err(error) = client.declare_topic(TOPIC, Retained::Latest).await {
+            warn(&error, "the feed topic could not be declared");
+            tokio::time::sleep(Duration::from_secs(1)).await;
+            continue;
+        }
+        let mut current = snapshots.borrow_and_update().clone();
+        loop {
+            match publish(&client, &current).await {
+                Ok(_) => Metrics::incr(&metrics.bus_published),
+                Err(error) => {
+                    Metrics::incr(&metrics.bus_publish_failures);
+                    warn(&error, "a snapshot was not published");
+                    if client.closed().is_some() {
+                        break;
+                    }
+                }
+            }
+            if snapshots.changed().await.is_err() {
+                // The sim thread is gone; so is the service.
+                client.close().await;
+                return;
+            }
+            current = snapshots.borrow_and_update().clone();
+        }
+        tokio::time::sleep(Duration::from_secs(1)).await;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_limits_validate_and_hold_the_worst_case_with_room() {
+        let limits = limits();
+        limits.validate().unwrap();
+        // A full snapshot on the live fly (see `limits`).
+        let snapshot_bytes = crate::snapshot::FRAME_BYTES + 139_255usize.div_ceil(8) + 12_800;
+        assert_eq!(snapshot_bytes, 122_367);
+        let subscribers = limits.max_clients as u64 - 1;
+        let pinned = subscribers * (1 + limits.max_latest_in_flight) + 1 + 2;
+        assert!(
+            pinned * snapshot_bytes as u64 * 10 <= limits.max_store_bytes,
+            "{pinned}"
+        );
+        assert!(limits.max_artifact_bytes >= crate::snapshot::FRAME_BYTES as u64 * 40);
+    }
+}

services/flysim/crates/flysim/src/lib.rs

@@ -8,7 +8,8 @@
 //!
 //! ```text
 //!                      +-- watch<Snapshot> --> feed  :7400/feed   (axum + ws)
-//!  sim thread ---------+
+//!  sim thread ---------+                   \-> feedbus -> flybus -> fly-edge :7400/feed
+//!                      |                        (FLY_FEED_VIA=bus instead of the line above)
 //!   agent              +-- Shared ------------> api   :7401        (axum)
 //!   emulator           |                          /status /stimulate /reward /checkpoint
 //!   adapter            |                          /pause /resume /events /healthz /metrics
@@ -25,6 +26,7 @@ pub mod chat;
 pub mod config;
 pub mod eventlog;
 pub mod feed;
+pub mod feedbus;
 pub mod macros;
 pub mod metrics;
 pub mod pacing;
@@ -41,7 +43,7 @@ use std::sync::Arc;
 use anyhow::{Context, Result};
 use tokio::sync::{mpsc, watch};
 
-use crate::config::Config;
+use crate::config::{Config, FeedVia};
 use crate::eventlog::{EventRing, now_wall_ms};
 use crate::simloop::{COMMAND_QUEUE, Command, Shared, Sim, booting_snapshot};
 use crate::snapshot::Snapshot;
@@ -95,10 +97,17 @@ pub fn run(config: Config) -> Result<()> {
     let feed_addr = config.feed.bind;
     let control_addr = config.control.bind;
     let metrics_addr = config.control.metrics_bind;
+    let via = config.feed.via;
     let listeners = runtime.block_on(async {
-        let feed = tokio::net::TcpListener::bind(feed_addr)
+        // In bus mode the feed port belongs to `fly-edge`; binding it here would take it away.
-            .await
+        let feed = match via {
-            .with_context(|| format!("binding the feed listener on {feed_addr}"))?;
+            FeedVia::Direct => Some(
+                tokio::net::TcpListener::bind(feed_addr)
+                    .await
+                    .with_context(|| format!("binding the feed listener on {feed_addr}"))?,
+            ),
+            FeedVia::Bus => None,
+        };
         let control = tokio::net::TcpListener::bind(control_addr)
             .await
             .with_context(|| format!("binding the control listener on {control_addr}"))?;
@@ -113,9 +122,15 @@ pub fn run(config: Config) -> Result<()> {
         Ok::<_, anyhow::Error>((feed, control, metrics))
     })?;
     let (feed_listener, control_listener, metrics_listener) = listeners;
-    tracing::info!(feed = %feed_addr, control = %control_addr, metrics = ?metrics_addr, "listening");
+    tracing::info!(
+        feed = %feed_addr,
+        feed_via = via.as_str(),
+        control = %control_addr,
+        metrics = ?metrics_addr,
+        "listening"
+    );
 
-    {
+    if let Some(feed_listener) = feed_listener {
         let state = state.feed();
         runtime.spawn(async move {
             if let Err(error) = axum::serve(feed_listener, feed::router(state)).await {
@@ -123,6 +138,26 @@ pub fn run(config: Config) -> Result<()> {
             }
         });
     }
+    // The bus gets a runtime of its own, so neither its router nor the artifact copies can take
+    // a worker from the control API; and it is fed from the watch slot, never from the sim thread.
+    let bus_runtime = match via {
+        FeedVia::Direct => None,
+        FeedVia::Bus => {
+            let bus_runtime = tokio::runtime::Builder::new_multi_thread()
+                .worker_threads(2)
+                .thread_name("flysim-bus")
+                .enable_all()
+                .build()
+                .context("building the bus runtime")?;
+            let bus = bus_runtime.block_on(feedbus::start_router(&config.feed.bus_dir))?;
+            bus_runtime.spawn(feedbus::run_publisher(
+                bus.router.clone(),
+                state.snapshots.clone(),
+                Arc::clone(&state.shared.metrics),
+            ));
+            Some((bus_runtime, bus))
+        }
+    };
     {
         let state = state.clone();
         runtime.spawn(async move {
@@ -148,6 +183,11 @@ pub fn run(config: Config) -> Result<()> {
     let result = sim.run(&notifier);
     notifier.notify("STOPPING=1\n");
     drop(sim);
+    if let Some((bus_runtime, bus)) = bus_runtime {
+        bus.router.shutdown();
+        drop(bus);
+        bus_runtime.shutdown_timeout(std::time::Duration::from_secs(1));
+    }
     runtime.shutdown_timeout(std::time::Duration::from_secs(2));
     result
 }

services/flysim/crates/flysim/src/metrics.rs

@@ -43,6 +43,10 @@ pub struct Metrics {
     pub lag_ms: AtomicU64,
     /// 1 when the restore fell back past the newest candidate.
     pub restore_fallback: AtomicU64,
+    /// Snapshots published on the feed bus (`FLY_FEED_VIA=bus`); 0 in direct mode.
+    pub bus_published: AtomicU64,
+    /// Snapshots the feed bus refused or could not take; each one is skipped, never retried.
+    pub bus_publish_failures: AtomicU64,
 }
 
 impl Metrics {
@@ -114,6 +118,20 @@ pub fn render(metrics: &Metrics, snapshot: &Snapshot, now_wall_ms: u64) -> Strin
         "Snapshots superseded before a slow client could be sent them.",
         Metrics::get(&metrics.feed_dropped),
     );
+    metric(
+        &mut out,
+        "fly_bus_published_total",
+        "counter",
+        "Snapshots published on the feed bus (FLY_FEED_VIA=bus).",
+        Metrics::get(&metrics.bus_published),
+    );
+    metric(
+        &mut out,
+        "fly_bus_publish_failures_total",
+        "counter",
+        "Snapshots the feed bus did not take; skipped, like any superseded snapshot.",
+        Metrics::get(&metrics.bus_publish_failures),
+    );
     metric(
         &mut out,
         "fly_snapshots_published_total",