Rows are grouped per data source and sent as one NDJSON request to the Events API after 2s (plus up to 1s of jitter) or 5 MB. SendEvent still returns once its row is recorded, so a Pub/Sub ack keeps meaning the result is in Tinybird. Tinybird allows 100 appends/s per workspace, whatever the row count. Under Pub/Sub every checker wakes at cronTimestamp, so one POST per check sent thousands of appends in the first seconds of each minute. The 429s nacked the message and re-ran the probe, which fed the storm. A batch is retried on 429 (honoring Retry-After), 5xx and network errors for up to 30s before its rows get the error. One request per data source is in flight at a time. Each flush logs a sampled 'tinybird flush' wide event, and pending batches are flushed on shutdown. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>