Constant INDEX_WALK_RETRY_AFTER_SECS 

const INDEX_WALK_RETRY_AFTER_SECS: u64 = 5;

Retry-After handed to a caller whose removal was shed because another walk was in flight. Removal is idempotent and retry-safe, so shedding costs a legitimate client one retry, where queueing would let a caller park unbounded requests on work that starves cache writes.