Designing a Polling Pipeline for Rate-Limited APIs Without Losing Data

Every system that ingests data from someone else's API eventually writes a poller. It starts as fifteen lines in a cron job: hit the endpoint, compare against the last timestamp, insert what's new. It works immediately, which is the problem — it keeps appearing to work long after it has started losing records.

I maintain an ingestion layer that polls public discussion APIs continuously, and the gap between "the poller is running" and "the poller is correct" took considerably longer to close than I expected. What follows are the four failure modes that caused real data loss, and the patterns that fixed them.