Event bus dropped messages in context engine poll

Error message

Event bus dropped {result.dropped} messages in context engine poll

What it means

The context engine capsule polls the IPC event bus via a typed PollResult. The bus reports how many messages it dropped (e.g. due to buffer overflow or lag); when result.dropped > 0, dispatch_poll_result logs a warning so the developer knows some events never reached the dispatcher. It is a diagnostics warning, not a hard failure — processing continues with the messages that survived.

Solutions

Increase the event-bus subscription capacity/buffer size for the context engine so bursts fit.
Profile and speed up the dispatch loop; ensure run() polls frequently and never blocks inside message handling.
Instrument which topics drop most and throttle publishers emitting into those topics.
Treat the warning as data loss signal: re-derive lost context (e.g. re-run compaction) after observing drops.

Example fix

// before: ignore polling cadence, large gaps let the queue overflow
loop {
    sleep(LONG_INTERVAL);
    let result = bus.poll();
    dispatch_poll_result(&result, &config);
}
// after: poll promptly and drain fully each cycle
loop {
    let result = bus.poll();
    dispatch_poll_result(&result, &config);
    if result.dropped > 0 {
        rebuild_missed_context(&result, &config);
    }
    sleep(SHORT_INTERVAL);
}

Defensive patterns

Strategy: validation

Validate before calling

// after each poll, check for drops before trusting the dispatch
if result.dropped > 0 {
    eprintln!("context engine dropped {} events; refresh derived state", result.dropped);
}

Prevention

Poll the bus frequently; never sleep long between polls
Size subscription buffers for peak burst traffic
Alert on dropped counters rather than ignoring them

When it happens

Trigger: Any call to dispatch_poll_result (via the capsule run loop) where the PollResult returned from the event bus poll carries dropped > 0, meaning the bus had to discard messages before delivery.

Common situations: Under heavy event traffic the subscription buffer overflows; a slow consumer (long compaction hooks, blocked dispatch) causes the bus to drop queued messages; bursts of tool.v1 events during parallel tool execution.

Related errors

AI-assisted analysis of unicity-aos/aos-ce@f6f22024fb (2026-09-13). Data as JSON: /api/errors/16b751c4f007ee44. Report an issue: GitHub.

Appendix: source

Thrown at capsules/capsule-context-engine/src/lib.rs:328

            let _ = after_sub.poll();
        }
    }
}

// ── Envelope dispatch ───────────────────────────────────────────────

/// Returns `true` if the topic should be dispatched (not a self-echo).
fn should_dispatch_topic(topic: &str) -> bool {
    !topic.starts_with("context_engine.v1.response.")
        && !topic.starts_with("context_engine.v1.hook_response.")
        && topic != "context_engine.v1.hook.before_compaction"
        && topic != "context_engine.v1.hook.after_compaction"
}

/// Dispatch messages from a typed `PollResult`.
fn dispatch_poll_result(result: &ipc::PollResult, config: &Config) {
    if result.dropped > 0 {
        log::warn(format!(
            "Event bus dropped {} messages in context engine poll",
            result.dropped
        ));
    }

    for msg in &result.messages {
        if !should_dispatch_topic(&msg.topic) {
            continue;
        }

        let payload: serde_json::Value = match serde_json::from_str(&msg.payload) {
            Ok(v) => v,
            Err(e) => {
                log::warn(format!("failed to deserialize IPC message payload: {e}"));
                continue;
            }
        };

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