If you've ever built a backend with stream-processing, then you're familiar with the kind of systems we'll be exploring. If you're not — no worries! We'll step through it. The classic, happy Web architecture is the “one big SQL database” behind our app server. The app talks to the database and handles requests from the frontend. As our application grows, we hit some performance limits so we toss some caches into the stack. Then let's say we scale our database horizontally through sharding and re...