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The @withSubscription directive enables subscription operation on a GraphQL type. A subscription notifies your client with changes to back-end data using the WebSocket protocol. Subscriptions are useful to get low-latency, real-time updates. To enable subscriptions on any type add the @withSubscription directive to the schema as part of the type definition, as in the following example: 
type Todo @withSubscription {
  id: ID!
  title: String!
  description: String!
  completed: Boolean!
}
Refer to GraphQL Subscriptions to learn how to use subscriptions in you client app.