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How a Solo SaaS Developer Built Idempotent Email Delivery for a Cron-Based Product

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A developer behind Upwork Scout, a small SaaS that matches freelancers to job listings via email, has detailed the engineering challenges of building a reliable scheduled email system. The core problem is that unlike database writes, sent emails cannot be undone or retried safely, making duplicate sends a real user-facing failure. To prevent duplicate emails, the developer uses deterministic Firestore document IDs derived from user and job data, turning idempotency checks into simple lookups rather than complex queries. A distributed lock prevents multiple concurrent scan runs from duplicating work and inflating scraping costs. The developer also intentionally chose at-least-once delivery for daily digest emails, reasoning that silently losing a digest is worse than occasionally sending a rare duplicate.

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