NewHandler creates a new subscription handler.

ExecutorV2Pool - provides reusable executors.

Handler is the actual subscription handler which will keep track on how to handle messages coming from the client.

Message defines the actual subscription message wich will be passed from client to server and vice versa.

client provides an interface which can be implemented by any possible subscription client like websockets, mqtt, etc.

Executor is an abstraction for executing a GraphQL engine.

ExecutorPool is an abstraction for creating executors.