[System Design] Uber RAMEN: How Ride-Hailing Apps Push Real-Time Notifications to Millions of Devices

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 offer to exactly John Doe's phone (out of millions of connected phones).

Deliver it in milliseconds (not seconds).