Using an external PubSub Engine
graphql also supports any external PubSub system that implements the
graphql-subscriptions exports an in-memory (
EventEmitter) event system to re-run subscriptions.
This is not suitable for running in a serious production app, because there is no way to share subscriptions and publishes across many running servers.
graphql-subscriptions - PubSub Implementations lists existing external PubSub implementations.