A feed that everybody reads and almost nobody writes
You serve a public activity feed. Reads outnumber writes a hundred to one, every read hits the same database, and the database is the most expensive thing you own.
The store is a single relational instance. You can put things in front of it and you can change how it is asked, but you cannot make it faster.
Sixty reads a second against sixty distinct feeds. Half a second at the tail, and under twenty cents.
Before anything runs: which promise gives way first, and what is the mechanism?
The slowest read in a hundred is under 0.5s
Every number appears here, on every change.