User Connectivity: Making the System Scale with Event Hub Partitions, ACA, and KEDA

Part 3 of the User Connectivity Architecture series. Introduction The first post in this series described the pattern: a heartbeat on a timer, an Event Hub, a worker writing sessions into Redis, and Redis key expiration driving facility online/offline status. One detail matters later. The heartbeat interval is not hard-coded in the client. The API tells the client when to call next, and the default is 30 seconds. The second post covered two years of running that in production. This post is about the month it stopped working. In January 2026 our heartbeat traffic went from boring to terrifying and stayed there for about four weeks. This is the story of what broke, why the original design had a ceiling we never noticed, and the changes that fixed it: more Event Hub partitions, Azure Container Apps, and KEDA . The Storm A normal day looked like this: 51,000-58,000 heartbeats per hour , hour after hour Roughly 15-16 events per second at idle Flat, predictable, forgettable On January 5, around 7:00 AM PST , it stopped being flat. Time (PST) Heartbeats/hour Baseline ~57,000 12:00 PM 80,005 1:00 PM 216,351 5:00 PM 343,480 9:00 PM 466,760 That is eight times normal event volume in a single hour, and it was still climbing. Events were only half the story. SignalR connection counts told the other half. At the worst of it we were holding roughly eleven times the connections we normally maintain, and every one of those was a browser session we had to track, keep alive, and report status for. It did not spike and recover. It stayed elevated for weeks while we hunted for the cause. When we finally found it, the answer was almost funny: 507 zombie sessions that never ended, running months-old cached client code, and a single user account responsible for 33% of all our token API traffic . One account. Eight times the load. Four weeks. What Eight Times Load Actually Did Here is the part that matters, and it has nothing to do with the number itself. Our Event Hub had one partition. I