risingwavelabs/risingwave · error

no actors to assign; assignment is meaningless

Error message

no actors to assign; assignment is meaningless

What it means

Same guard family as the worker check: `ensure!` in the inner `assign_hierarchical` (assignment.rs:753) refuses to run when the actor list is empty, because a scheduling decision over zero actors has no meaning.

Source

Thrown at src/meta/src/stream/stream_graph/assignment.rs:753

    }

    /// Hierarchical assignment: Actors → Workers → `VNodes` → Actors.
    pub fn assign_hierarchical<W, A, V>(
        &self,
        workers: &BTreeMap<W, NonZeroUsize>,
        actors: &[A],
        vnodes: &[V],
    ) -> Result<BTreeMap<W, BTreeMap<A, Vec<V>>>>
    where
        W: Ord + Hash + Eq + Clone + Debug,
        A: Ord + Hash + Eq + Copy + Debug,
        V: Hash + Eq + Copy + Debug,
    {
        ensure!(
            !workers.is_empty(),
            "no workers to assign; assignment is meaningless"
        );
        ensure!(
            !actors.is_empty(),
            "no actors to assign; assignment is meaningless"
        );
        ensure!(
            !vnodes.is_empty(),
            "no vnodes to assign; assignment is meaningless"
        );
        ensure!(
            vnodes.len() >= actors.len(),
            "not enough vnodes ({}) for actors ({}); each actor needs at least one vnode",
            vnodes.len(),
            actors.len()
        );

        let chunk_size = match self.vnode_chunking_strategy {
            VnodeChunkingStrategy::NoChunking => {
                return assign_hierarchical(
                    workers,

View on GitHub (pinned to 6469eb736d)

Solutions

1. Ensure actors are generated (parallelism >= 1) for every fragment before assignment.
2. Add an earlier caller-side check with a fragment-id-bearing error message.
3. Investigate plan generation if a valid fragment ends up with zero actors.

Example fix

// before
let plan = assign_hierarchical(&workers, &[], &vnodes, strategy)?;
// after
if actors.is_empty() {
    return Err(anyhow!("fragment {} generated no actors", fragment_id));
}
let plan = assign_hierarchical(&workers, &actors, &vnodes, strategy)?;

// Rust, before scheduling
if actors.is_empty() {
    anyhow::bail!("fragment produced no actors");
}

fn non_empty_actors<'a>(a: &'a [ActorId]) -> Option<&'a [ActorId]> {
    (!a.is_empty()).then_some(a)
}

match assign_hierarchical(&workers, &actors, &vnodes, strategy) {
    Ok(m) => m,
    Err(e) if e.to_string().contains("no actors") => {
        Err(anyhow!("plan generation bug: zero actors: {e:#}"))
    }
    Err(e) => Err(e),
}

When it happens

Trigger: Calling the inner assignment routine with `actors.is_empty()`; mirrors the public entry's earlier check but in the ensure! chain.

Common situations: Fragments that produced no actors due to plan generation bugs; direct API/test usage with empty actor vectors.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• no actors to assign
• no vnodes to assign
• actor count ({}) exceeds vnode count ({})
• no workers to assign; assignment is meaningless
• no vnodes to assign; assignment is meaningless

AI-assisted analysis of risingwavelabs/risingwave@6469eb736d (2026-09-11). Data as JSON: /api/errors/50482ede5dd68743. Report an issue: GitHub.