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Communication Services: Messages, Queuing, Multiplexing, and More

This chapter explores various communication services, including messages, queuing, multiplexing, message with reply, remote method invocation, and conversation. It also discusses examples and objectives of these services.

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Communication Services: Messages, Queuing, Multiplexing, and More

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  1. Chapter 12 by David G. Messerschmitt

  2. Communication services by David G. Messerschmitt

  3. Example communication services • Messages • Queuing and multiplexing • Message with reply • Remote method invocation • Conversation • Broadcast

  4. Some objectives • Allow modules to interact in the same way across different hosts as on a single host • Make it easier to develop applications by capturing common generic needs • Support a range of applications with distinct needs

  5. Message • The smallest unit of information meaningful to the sender and recipient • Recipient must be ready and listening • Directs or informs the recipient • Pure push model • Employs a send-receive protocol

  6. Supporting deferred applications • Queuing • Queuing allows a message to be sent even though the recipient is not waiting • The message sits in a queue somewhere in the infrastructure until it is actively retrieved by the recipient • Combines send-receive and request-reply protocols

  7. Participants in message with queuing Sender Recipient Send/ receive Queue Request/ response Sender can send message whenever it wants Recipient can access message whenever it wants Infrastructure

  8. Multiplexing • Multiplexing • Allows recipient to receive messages from two or more senders • How are the senders identified? • Presumes queuing: what if two messages are sent simultaneously? • Supported by Message Oriented Middleware (MOM)

  9. Multiplexing and queuing Sender1 Recipient Mux & queue Sender2 Recipient deals with one message at a time even if coming from many senders Infrastructure

  10. Example application: workflow Receiving Administrative assistants Purchasing Workers

  11. What message is not • Delivery may not be guaranteed • No shared context of messages from same sender to recipient • Application is free to create that context on its own • Messages may not be delivered in the same order as sent

  12. Message with reply • Service couples two messages: • Request and coupled response message • Sender freed of burden of associating response • Requests information or a service • Pure pull model • Immediate • Recipient presumes sender is waiting for reply and responds as quickly as possible

  13. RMI method: parameters return_values An action object-oriented programming is called a method Method invocation method: parameters return_values Remote method invocation (RMI)

  14. Sender is blocked waiting for response method: parameters -> return_values Client object Server object method,parameters Blocked: can’t do anything else Computes method() returns Time = Blocked waiting for response = Working on request

  15. Comparison Sender Recipient Sender Queue Recipient Send/receive Send/receive Message waiting Request/ response Request/ response = Work on message or request = Blocked waiting for response

  16. Conversation • Two-way exchange of messages • Messages in one direction are influenced by messages in the other • To maintain conversation • Messages should be delivered reliably • Messages should be delivered in the same order as sent • No coupling of replies with messages

  17. Session protocol A session typically provides guaranteed ordered delivery to maintain proper context Establish Conversation Disestablish

  18. Some advantages of sessions • Aids application by managing a shared context for a conversation • Identification, ordering, reliable delivery • Reduces overhead • Example: only one name server query • Security (Chapter 8) • Quality of service (Chapter 11)

  19. Examples • What are examples of applications that would naturally leverage: • Messages? • Queuing and multiplexing? • Message with reply? • Conversation?

  20. Multimedia record-store-playback Audio coder Audio decoder Record Playback One (big) message Storage Storage Video coder Video decoder

  21. Streaming multimedia Audio coder Audio decoder Video coder Video decoder

  22. A session supports streaming multimedia Accumulate audio in message Send message Time Receive message Playback audio in message Delay

  23. Delay and latency Message latency = time from sending of message until it is received in its entirety Time Delay = time elapsing between an event (or series of events) and its (their) representation

  24. Relationship of delay and latency Latency Time Delay Delay = latency (of network) + message accumulation time If delay is important, keep the messages short!

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