Infographic 02 · three.js + D3

One activity log, three independent readers

A Cloudflare Worker cannot open a TCP socket, so the event is written to a Postgres outbox inside the post’s own transaction and a host-resident relay moves it onto Kafka.

Six partitions along one axis, three consumer groups along the otherdrag to orbit

event redelivered duplicate · drag to orbit

Partition and consumer group are independent dimensions, which a flat diagram has to fake with crossing arrows. Watch the accent-coloured events: roughly one in twelve is a redelivery after a relay crash. There is no exactly-once here, only three sinks that each absorb repeats a different way.

The cost of at-least-once, paid at every sink

Consumer group	Sink	How it absorbs a duplicate
Feed materializer	→ Cassandra	upsert is idempotent
Notification worker	→ Postgres	unique on event id
Realtime fanout	→ WebSocket	Redis dedupe, TTL

The full write-up, with the code paths and the commit, is the Billboard Activity Graph.