Four Race Conditions in Serverless Postgres Billing — and How to Fix Them

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A developer running an AI image-generation SaaS encountered four critical race conditions while building a credit billing system on a serverless Postgres driver that does not support multi-statement transactions. Because each query is a separate round trip with no BEGIN/COMMIT support, every billing operation had to be made correct within a single atomic statement. Issues included double-spending from concurrent requests, duplicate credit grants caused by Stripe's webhook retry behavior, and a modeling bug arising from managing two separate credit buckets with different expiry rules. The developer resolved the idempotency problem using a data-modifying CTE that treats the audit ledger insert as a deduplication gate, backed by a partial unique index scoped to Stripe-driven event types. The fixes highlight how Postgres statement atomicity and careful WHERE-clause design can substitute for transactions in constrained serverless environments.

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