Near-Real-Time Activity System with Infinite Scroll
An activity feed that got slower the more you used it. Power users were being punished for showing up.
Here’s the part that annoyed me about the old system: it looked everything up live, every single time. Someone opens their activity feed, and the system goes digging through years of history on the spot, stitching it together on the fly. That’s fine for a new user with three actions in their history. It’s a disaster for someone with three years of them.
So I stopped making the system do that work live, and made it do the work as things happened instead.
Every meaningful action in the app now feeds into a pipeline the moment it fires. That pipeline is smart enough to know a single action often isn’t one event. It’s a chain. A purchase isn’t “purchase happened,” it’s “started → processing → completed,” and those pieces can show up in any order, at any time, because that’s just how distributed systems behave. I built it to wait for the pieces to make sense before calling an activity finished, rather than trusting whatever arrives first.
Once something’s actually complete, it drops into a search index built for exactly this: fast, paginated, no live computation required. Scrolling the feed isn’t “go dig through history” anymore. It’s “ask the index for the next page.” That’s the whole trick.