The Problem: Pushing Instant Notifications to Millions of Devices Answer-first: Uber's RAMEN system maintains persistent gRPC bidirectional streams to every active driver app. When a match is made, the ride offer travels from the matching engine to the driver's phone in under 100ms — without polling. This is how millions of connections are held open simultaneously without crashing the backend. When DISCO decides to match you with Driver John Doe, the system must: Send the ride offe...