Constant MAX_INFLIGHT_REVALIDATIONS 

pub const MAX_INFLIGHT_REVALIDATIONS: usize = 32;

Upper bound on concurrently in-flight background revalidations (DW-037): the stale-while-revalidate path serves stale immediately and refreshes in the background; beyond this many DISTINCT keys the gateway skips spawning (the next request past the stale window revalidates synchronously) so a cache-wide expiry cannot stampede.