nautechsystems/nautilus_trader · error

Failed to start AX execution session generation: {e}

Error message

Failed to start AX execution session generation: {e}

What it means

During AX execution client connect, the adapter must (re)start its background task generations for pending-task and session-task groups. This error wraps a failure from `session_tasks.start_generation()` (the tokio task-group generation starter), raised after `teardown_partial_connect()` ran because a prior group was not open. It indicates the async runtime task set for the execution session could not be spawned.

Source

Thrown at crates/adapters/architect_ax/src/execution.rs:525

    }

    fn get_account(&self) -> Option<AccountAny> {
        self.core.cache().account_owned(&self.core.account_id)
    }

    async fn connect(&mut self) -> anyhow::Result<()> {
        if self.core.is_connected() && self.pending_tasks.is_open() && self.session_tasks.is_open()
        {
            return Ok(());
        }

        if !self.pending_tasks.is_open() || !self.session_tasks.is_open() {
            self.teardown_partial_connect().await?;
            self.pending_tasks.start_generation().map_err(|e| {
                anyhow::anyhow!("Failed to start AX execution task generation: {e}")
            })?;
            self.session_tasks.start_generation().map_err(|e| {
                anyhow::anyhow!("Failed to start AX execution session generation: {e}")
            })?;
        }
        let http_client = self.http_client.clone();
        let ws_orders = self.ws_orders.clone();
        let setup_guard =
            TaskGroupGuard::new(&[&self.session_tasks, &self.pending_tasks], move || {
                http_client.cancel_all_requests();
                ws_orders.begin_shutdown();
            });

        // Reset so requests work after a previous disconnect
        self.http_client.reset_cancellation_token();

        let credential = Credential::resolve(
            self.config.api_key.clone().map(|value| value.into_inner()),
            self.config
                .api_secret
                .clone()

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Ensure `connect` is not called after the client has been shut down; create a fresh client instance instead of reconnecting a torn-down one.
  2. Avoid concurrent/duplicate `connect` calls; serialize connection attempts behind a lock or once-guard.
  3. Check the wrapped source error (`: {e}`) from `start_generation` — it usually names the actual task-group failure (channel closed, runtime gone).
  4. Verify the client lives inside an active tokio runtime for the whole session, including reconnects.

Example fix

// before
let client = AxExecutionClient::new(...);
client.connect().await?; // after a prior shutdown this can fail

// after
let client = if client.is_connected() {
    client
} else {
    AxExecutionClient::new(...)? // fresh client for a fresh connect
};
client.connect().await?;
Defensive patterns

Strategy: try-catch

Validate before calling

// before connect
if client_was_previously_shutdown { /* build a new client instead */ }
assert!(connect_once_flag.compare_exchange(false, true).is_ok(), "connect already in progress");

Try / catch

match client.connect().await {
    Ok(()) => {},
    Err(e) if e.to_string().contains("Failed to start AX execution") => {
        log::error("task-group generation failed: {e:#}; rebuild client");
        client = rebuild_client()?;
        client.connect().await?;
    }
    Err(e) => return Err(e),
}

Prevention

When it happens

Trigger: Calling `connect` on the AX execution client when `pending_tasks` or `session_tasks` is not open, and the subsequent `start_generation()` call on the session task group fails (e.g. the task group's internal channel/generation handle is already shut down or the runtime cannot spawn the task).

Common situations: Reconnecting after a disconnect where the task groups were torn down mid-flight; calling connect concurrently from two places; a shutdown runtime or reactor context when the client is dropped while connecting; a prior connect failure leaving the group in a closed state.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/ef83331d5147532b. Report an issue: GitHub.