seq-file is a small Node.js utility that safely persists the ever‑increasing sequence numbers used when tracking CouchDB _changes feeds. It writes the current sequence to a file atomically, throttles writes (via a configurable frequency), and guarantees that only one write occurs at a time so crashes or concurrent saves won’t corrupt the stored value. It’s aimed at developers building CouchDB replication or change‑feed consumers who need reliable checkpointing of their progress.