What are the challenges of scaling Subscriptions in a distributed architecture?

Subscriptions require maintaining long-lived, stateful connections (WebSockets), which are fundamentally difficult to scale horizontally.

When a load balancer routes WebSocket connections across 10 API instances, a mutation hitting Instance A must somehow trigger subscriptions held on Instances B and C. This requires an external PubSub event broker (like Redis or Kafka) to broadcast events across the entire cluster.