All Known Subinterfaces:

ActorFlightRecorder, ClusterSharding, ReplicatedShardingExtension, ShardedDaemonProcess

All Known Implementing Classes:

ActorRefResolver, Cluster, ClusterSingleton, DistributedData, DistributedData, NoOpActorFlightRecorder$, Receptionist
```
public interface Extension
```
Marker trait/interface for extensions. An extension can be registered in the ActorSystem and is guaranteed to only have one instance per ActorSystem instance per ExtensionId. The extension internals must be thread safe. For mutable state it should be preferred to use an Actor rather than extensions as first choice.

See Also:

ExtensionId

Interface Extension