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Hardcoded cron means a redeploy every time a schedule changes. Store schedules in the database and have a daemon react to edits with change streams. Here is the design and its failure handling.
skip(N) reads and discards every document it steps over, and a per-read database charges you for all of them. Here is the arithmetic, the query that replaces it, and the tie-breaker that keeps order stable.
When a database forbids automatic write retries, every write must be idempotent on its own. A content-derived id makes that free. Here is the derivation and the one invariant that keeps it safe.
Firestore speaks the MongoDB wire protocol, but rejects retryable writes. If your driver defaults to on, writes fail in a way that looks random. Here is why, and the fix.