Fan-out on read vs fan-out on write
Fan-out on read builds each feed when a user asks for it. Fan-out on write prepares the feed when its data changes. With 2,000 feed reads/s and six lookups per feed, building on read pushes p99 to seconds and a prepared feed keeps it near 26 ms.
Tables · a read touches 3 places in 0.5 ms · a shared change writes 1 place
Calls per page: 20 · 18% of pages wait 2.0 s
Case 1
Users read feeds 2,000 times/s. Each feed needs six indexed lookups, and 1 percent of requests change a feed. A change must show within 1 s.
| Measure | Goal | Fan-out on read | Fan-out on write |
|---|---|---|---|
| p99 latency | under 150 ms | 6.8 s Fail | 26 ms Pass |
| Data age | under 1 s | 0 ms Pass | 0 ms Pass |
| Monthly cost | 16 USD/mo | 16 USD/mo | |
| Error rate | 36.5% | 0% |