# Package net.corda.core.messaging

- Interface Summary
  Interface | Description
  --- | ---
  AllPossibleRecipients | A special base class for the set of all possible recipients, without having to identify who they all are.
  CordaRPCOps | RPC operations that the node exposes to clients using the Java client library. These can be called from client apps and are implemented by the node in the CordaRPCOpsImpl class.
  Message | A message is defined, at this level, to be a (topic, timestamp, byte arrays) triple, where the topic is a string in Java-style reverse dns form, with "platform." being a prefix reserved by the platform for its own use. Vendor specific messages can be defined, but use your domain name as the prefix e.g. "uk.co.bigbank.messages.SomeMessage".
  MessageHandlerRegistration |
  MessageRecipientGroup | A base class for a set of recipients specifically identified by the sender.
  MessageRecipients | The interface for a group of message recipients (which may contain only one recipient)
  MessagingService | A `interface MessagingService` sits at the boundary between a message routing / networking layer and the core platform code.
  RPCOps | Base interface that all RPC servers must implement. Note: in Corda there's only one RPC interface. This base interface is here in case we split the RPC system out into a separate library one day.
  RPCReturnsObservables | If an RPC is tagged with this annotation it may return one or more observables anywhere in its response graph. Calling such a method comes with consequences: it's slower, and consumes server side resources as observations will buffer up on the server until they're consumed by the client.
  ReceivedMessage |
  SingleMessageRecipient | A base class for the case of point-to-point messages

- Class Summary
  Class | Description
  --- | ---
  Ack | A general Ack message that conveys no content other than it's presence for use when you want an acknowledgement from a recipient. Using Unit can be ambiguous as it is similar to Void and so could mean no response.
  CordaRPCOpsKt |
  FlowHandle<A> | `class FlowHandle` is a serialisable handle for the started flow, parameterised by the type of the flow's return value.
  MessagingKt |
  StateMachineInfo |
  StateMachineUpdate |
  TopicSession | An identifier for the endpoint `interface MessagingService` message handlers listen at.
  TopicStringValidator | A singleton that's useful for validating topic strings