Spot instances are 60-90% cheaper than on-demand and exactly the right shape for video transcoding (mostly idempotent, retry-tolerant, storage-out-of-worker). These pieces cover the architecture that makes spot pools production-safe: fleet diversification across instance types and AZs, two-minute interruption handling, atomic upload semantics, chunked encoding for long jobs, and the on-demand baseline that catches anything spot can't reliably finish.