Total shard count:{shard_count} must be greater than sum of

Error message

Total shard count:{shard_count} must be greater than sum of all shard counts:{typeshard_count} for inidividual types

What it means

This is a panic from print_schema's shard-printing logic. When splitting printed schema output across N shards, shards are allocated for the 'leftover' directives only if the total shard count exceeds the sum of per-type shard counts; if typeshard_count >= shard_count, the arithmetic `shard_count - typeshard_count` would underflow (usize), so the function panics instead. It is a caller precondition violation, not a recoverable runtime error.

Source

Thrown at compiler/crates/schema-print/src/print_schema.rs:88

/// Prints shards in sequence. No parallelism is used.
///
/// # Arguments
///
/// * `schema` - GraphQL SDLSchema
///
/// * `shard_count` - Total shard count. Returned vec will have this size.
///
/// * `type_shard_count` - To further shard a single type, provide this.
///   For e.g you might want to shard Query type because its huge.
///   Sum of all the shard counts provided here must be less than shard_count param.
pub fn print_types_directives_as_shards(
    schema: &SDLSchema,
    shard_count: usize,
    type_shard_count: FnvHashMap<StringKey, usize>,
) -> Vec<String> {
    let typeshard_count: usize = type_shard_count.values().sum();
    if typeshard_count >= shard_count {
        panic!(
            "Total shard count:{shard_count} must be greater than sum of all shard counts:{typeshard_count} for inidividual types",
        );
    }
    let mut shards: Vec<String> = vec![String::new(); shard_count - typeshard_count];

    // Print directives to first shard
    shards
        .first_mut()
        .unwrap()
        .push_str(&print_directives(schema));

    let mut type_shards: FnvHashMap<StringKey, Vec<String>> = type_shard_count
        .iter()
        .map(|(type_name, count)| (*type_name, vec![String::new(); *count]))
        .collect();
    write_types_as_shards(
        schema,
        &mut shards,

View on GitHub (pinned to 668b1b85e0)

Solutions

Increase the shard_count argument so it is strictly greater than the sum of all type_shard_count values.
Reduce the per-type shard counts in type_shard_count so their sum is below shard_count.
Pre-validate before calling: assert type_shard_count.values().sum::<usize>() < shard_count and produce a clear config error instead of a panic.
Fix the panic message typo ('inidividual' -> 'individual') and message wording in a patch if you maintain this code.

Example fix

// before
print_types_directives_as_shards(&schema, 4, type_shard_count); // sum=5 -> panic
// after
let typeshard_sum: usize = type_shard_count.values().sum();
assert!(typeshard_sum < 8);
print_types_directives_as_shards(&schema, 8, type_shard_count);

Defensive patterns

Strategy: validation

Validate before calling

let typeshard_sum: usize = type_shard_count.values().sum();
if typeshard_sum >= shard_count {
    return Err(anyhow!(
        "shard_count ({shard_count}) must exceed sum of per-type shards ({typeshard_sum})"
    ));
}

Try / catch

// Rust: avoid the panic entirely via pre-validation; if wrapping, use catch_unwind only in fixture tools
let result = std::panic::catch_unwind(|| print_types_directives_as_shards(&schema, n, map));

Prevention

Always compute shard_count as typeshard_sum + desired_leftover_shards.
Add a debug_assert in callers that sum < shard_count.
Convert the panic into a returned Result upstream.

When it happens

Trigger: Calling print_types_directives_as_shards (directly or via transform_fixture) with a shard_count that is less than or equal to the sum of all values in the type_shard_count map, e.g. shard_count=4 with per-type counts {A:2,B:3} (sum 5).

Common situations: Test fixture generators (transform_fixture) configured with too few total shards while enumerating many types; scripts that auto-compute per-type shard counts but fix the total shard count too low; typos mixing up 'count' vs 'sum' arguments.

Related errors

AI-assisted analysis of facebook/relay@668b1b85e0 (2026-09-02). Data as JSON: /api/errors/9b6d87cc59f86d16. Report an issue: GitHub.