What are GraphQL subscriptions?
Subscriptions are a GraphQL feature that allows a server to send data to its clients when a specific event happens. Subscriptions are usually implemented with WebSockets. In that setup, the server maintains a steady connection to its subscribed client. This also breaks the “Request-Response-Cycle” that were used for all previous interactions with the API.
Instead, the client initially opens up a long-lived connection to the server by sending a subscription query that specifies which event it is interested in. Every time this particular event happens, the server uses the connection to push the event data to the subscribed client(s).