nautechsystems/nautilus_trader · error

Cannot request aggregated bars: one of the aggregators in…

Error message

Cannot request aggregated bars: one of the aggregators in `bar_types` is already running

What it means

Before starting bar aggregators for a new request, the engine checks can_start_request_bar_aggregators; if any aggregator for one of the requested bar_types is already running for another request, it refuses to avoid duplicate aggregation. This guards against conflicting concurrent bar aggregation for the same bar type.

Solutions

1. Cancel or let the earlier bar request complete before issuing an identical aggregation request
2. Reuse the existing bar subscription/request instead of requesting the same bar_types aggregation again
3. Use a distinct bar_type (different aggregation source or interval) if both requests are genuinely needed
4. If aggregators are stale, reset/restart the engine or deregister the prior request's aggregation state

Example fix

// before
engine.request_bars(bar_req_1min);
engine.request_bars(bar_req_1min); // duplicate aggregation
// after
if !engine.has_bars(bar_req_1min.bar_type()) {
    engine.request_bars(bar_req_1min);
} else {
    // reuse existing subscription
}

// skip duplicate aggregation requests
if engine.has_bars(&bar_type) {
    log::info!("aggregation already active for {bar_type}");
    return Ok(());
}
engine.request_bars(req);

match engine.request_bars(req) {
    Err(e) if e.to_string().contains("already running") => {
        log::warn!("reusing existing aggregators for requested bar_types");
    }
    other => other?,
}

When it happens

Trigger: Calling DataEngine.request_bars (or a second bars request with the same bar_types aggregation) while a previous request's aggregators for those bar_types are still active — e.g. requesting 1-MINUTE bars from 1-SECOND trades while an identical aggregation is already registered.

Common situations: Re-issuing a bars request without canceling/deregistering the previous one; two strategies independently requesting the same aggregated bar type from the same source; restarting a backtest segment without cleaning up prior aggregation state.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

• Cannot setup bar aggregator: no aggregator found for
• Cannot start bar aggregation for
• Cannot start bar aggregation: no instrument found for
• Cannot stop bar aggregator: no aggregator to stop for
• No live aggregator for continuous future subscription

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/03362ccd25311e9b. Report an issue: GitHub.

Appendix: source

Thrown at crates/data/src/engine/mod.rs:1521

        target.activation_ns = UnixNanos::default();
        target.expiration_ns = UnixNanos::default();

        if let Err(e) = self
            .cache
            .borrow_mut()
            .add_instrument(InstrumentAny::FuturesContract(target))
        {
            log_error_on_cache_insert(&e);
        }
    }

    fn prepare_request_bar_aggregators_from_state(
        &mut self,
        request_id: UUID4,
        state: &RequestBarAggregation,
    ) -> anyhow::Result<()> {
        if !self.can_start_request_bar_aggregators(request_id, state) {
            anyhow::bail!(
                "Cannot request aggregated bars: one of the aggregators in `bar_types` is already running"
            );
        }

        self.request_bar_aggregations
            .insert(request_id, state.clone());

        if let Err(e) = self.init_request_bar_aggregators(request_id, state) {
            self.cleanup_request_bar_aggregators(&request_id);
            return Err(e);
        }

        Ok(())
    }

    fn dispatch_next_continuous_future_segment(&mut self, request_id: UUID4) -> anyhow::Result<()> {
        let Some(state) = self.continuous_future_requests.get(&request_id).cloned() else {
            anyhow::bail!("No active continuous future request for {request_id}");

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