Most teams reach for Redis pub/sub or a message broker before their actual traffic warrants it. Postgres has had pub/sub built in since 2010. Here is the working pattern, the limits to watch, and how to migrate to a real broker once you outgrow it.
Server-sent events are a 1.5KB drop-in for streaming notifications, progress, and live updates — and they work through every proxy and CDN that already handles HTTP. WebSockets are the right tool for genuine bidirectional, low-latency traffic. Here is the decision tree and the implementation patterns for each.