diem/diem · error · Error

Cannot subscribe to zero event keys!

Error message

Cannot subscribe to zero event keys!

What it means

`Error::CannotSubscribeToZeroEventKeys` is thrown by the event-notifications service when a caller attempts to create a subscription with an empty set of event keys. The service requires at least one event key per subscription, so zero keys is rejected immediately as a programming/API misuse error.

Source

Thrown at state-sync/inter-component/event-notifications/src/lib.rs:47

        Arc,
    },
    task::{Context, Poll},
};
use storage_interface::DbReaderWriter;
use thiserror::Error;

#[cfg(test)]
mod tests;

// Maximum channel sizes for each notification subscriber. If messages are not
// consumed, they will be dropped (oldest messages first). The remaining messages
// will be retrieved using FIFO ordering.
const EVENT_NOTIFICATION_CHANNEL_SIZE: usize = 100;
const RECONFIG_NOTIFICATION_CHANNEL_SIZE: usize = 1;

#[derive(Clone, Debug, Deserialize, Error, PartialEq, Serialize)]
pub enum Error {
    #[error("Cannot subscribe to zero event keys!")]
    CannotSubscribeToZeroEventKeys,
    #[error("Missing event subscription! Subscription ID: {0}")]
    MissingEventSubscription(u64),
    #[error("Unable to send event notification! Error: {0}")]
    UnableToSendEventNotification(String),
    #[error("Unexpected error encountered: {0}")]
    UnexpectedErrorEncountered(String),
}

impl From<SendError> for Error {
    fn from(error: SendError) -> Self {
        Error::UnableToSendEventNotification(error.to_string())
    }
}

/// The interface between state sync and the subscription notification service,
/// allowing state sync to notify the subscription service of new events.
#[async_trait]

View on GitHub (pinned to fc4714a8ea)

Solutions

1. Ensure the event key list passed to subscribe is non-empty before calling; short-circuit or default-fill empty lists.
2. Fix the configuration/filtering logic that produced an empty key set.
3. Add an upfront assert/validation on config that at least one event key is configured.

Example fix

// before
subscriptions.subscribe(&event_keys).await?;
// after
if event_keys.is_empty() {
    return Err(anyhow!("refusing to subscribe: no event keys configured"));
}
subscriptions.subscribe(&event_keys).await?;

fn validate_subscription_keys(keys: &[EventKey]) -> Result<(), String> {
    if keys.is_empty() {
        Err("cannot subscribe to zero event keys; provide at least one key".to_string())
    } else {
        Ok(())
    }
}
// before subscribe: validate_subscription_keys(&keys)?;

fn has_keys(keys: &[EventKey]) -> bool {
    !keys.is_empty()
}

match subscriptions.subscribe(&keys).await {
    Err(Error::CannotSubscribeToZeroEventKeys) => {
        log::error!("subscription request had no event keys; check config/filter");
    }
    other => other?,
}

When it happens

Trigger: Calling the subscribe API with an empty events/keys collection (e.g. `&[]`, empty Vec, or a filtered list that ended up empty).

Common situations: Building the subscription list dynamically from config and all keys being filtered out; typos in key names causing an empty intersection; initialization order where the configured keys have not been loaded yet.

Related errors

• Missing event subscription! Subscription ID: {0}
• Unable to send event notification! Error: {0}
• Unexpected error encountered: {0}
• Specified output file path is a directory
• Unable to serialize keys

AI-assisted analysis of diem/diem@fc4714a8ea (2026-09-04). Data as JSON: /api/errors/7794be5ce8368469. Report an issue: GitHub.