1. Chapter 4: Communication*
Message-Oriented Communication
&
Stream-Oriented Communication
*Referred to slides by Manhyung Han at Kyung Hee University, Hitesh Ballani at Cornell University, and Mary Ellen Weisskopf at University of Alabama in Huntsville

2. Review In a distributed system, processes
run on different machines
exchange information through message passing
Successful distributed systems depend on communication models that hide or simplify message passing

3. Middleware Communication Techniques
Remote Procedure Call
Message-Oriented Communication
Stream-Oriented Communication
Multicast Communication

4. Types of Communication
Persistent versus transient
Synchronous versus asynchronous
Discrete versus streaming

5. Persistent versus Transient Communication
Persistent: messages are held by the middleware comm. service until they can be delivered (e.g., )
Sender can terminate after executing send
Receiver will get message next time it runs
Transient: messages exist only while the sender and receiver are running
Communication errors or inactive receiver cause the message to be discarded
Transport-level communication is transient
RPC?
If the router can’t forward the message it will drop it.

6. Asynchronous v Synchronous Communication
Asynchronous: (non-blocking) sender resumes execution as soon as the message is passed to the communication/middleware software
Synchronous: sender is blocked until
The OS or middleware notifies acceptance of the message, or
The message has been delivered to the receiver, or
The receiver processes it & returns a response

7. Evaluation
Fully synchronous primitives may slow processes down, but program behavior is easier to understand
In multithreaded processes, blocking is not as big a problem because a special thread can be created to wait for messages

8. Discrete versus Streaming Communication
Discrete: communicating parties exchange discrete messages
Streaming: one-way communication; a “session” consists of multiple messages from the sender that are related either by send order (TCP streams), temporal proximity (multimedia streams), etc.

9. Message Oriented Communication
RPC supports access transparency, but is not always appropriate
Message-oriented communication is more flexible
RPC is usually synchronous, and is transient.

10. Message Passing Interface (MPI)
Designed for parallel applications using transient communication
MPI is
a standardized and portable message-passing system designed by a group of researchers from academia and industry
used in many environments, e.g., clusters
platform independent

11. Message Primitives Asynchronous: e.g. MPI_bsend
Synchronous: e.g. MPI_send, MPI_ssend, MPI_sendrecv:

12. MPI Apps versus C/S
Processes in an MPI-based parallel system act more like peers (or peer slaves to a master processor)
Communication may involve message exchange in multiple directions
C/S communication is more structured

13. Message-Oriented Middleware (MOM) - Persistent
Processes communicate through message queues
Queues are maintained by the message-queuing system
Sender appends to queue, receiver removes from queue
Neither the sender nor receiver needs to be on-line when the message is transmitted

14. 4.4 Stream-Oriented Communication
RPC and message-oriented communication are based on the exchange of discrete messages
Timing might affect performance, but not correctness
In stream-oriented communication the message content (multimedia streams) must be delivered at a certain rate, as well as correctly
e.g., music or video

15. Discrete and Continuous Media
Media: means by which information is conveyed
Types of media
Discrete media
No temporal dependence between data items
ex) text, still images, object code or executable files
Continuous media
Temporal dependence between data items
ex) Motion - series of images

16. Data Streams Data stream = sequence of data items
Can apply to discrete, as well as continuous media
e.g. UNIX pipes or TCP/IP connections which are both byte oriented (discrete) streams
Messages are related by send order
Audio and video require continuous time-based data streams

17. Data Streams
Asynchronous transmission mode: the order is important, and data is transmitted one after the other, no restriction to when data is to be delivered
Synchronous transmission mode defines a maximum end-to-end delay for individual data packets
Isochronous transmission mode has a maximum and minimum end-to-end delay requirement (jitter is bounded)
Not too slow, but not too fast either

21. Distributed System Support
Data compression, particularly for video
Quality of the transmission
Synchronization

22. An example of multicasting a stream to several receivers
Filter to reduce the data rate of the media content
Figure. 2-36
An example of multicasting a stream to several receivers

23. The Internet only provides best-effort services and has no guarantees on the QoS for multimedia data transmission.

24. The Internet only provides best-effort services and has no guarantees on the QoS for multimedia data transmission.
So, distributed system support is needed, for instance,

26. Figure The effect of packet loss in (a) non interleaved transmission and (b) interleaved transmission.

27. Lip synchronization of audio and video streams
Stereo audio with CD quality (two sequences of 16 bit samples)
Sampling rate 44.1 KHz -> synchronize 22.6 micro sec
NTSC: short for National Television System Committee
NTSC is the video system or standard used in North America and most of South America. In NTSC, 30 frames are transmitted each second. Each frame is made up of 525 individual scan lines.
Lip synchronization of audio and video streams
Video stream: NTSC standard of 30Hz (a frame every ms),
Audio stream: CD Quality sound
Synchronized every 1470 sound samples

28. Synchronization Mechanisms(1)
Figure. 2-40
The principle of explicit synchronization on the data units
Synchronization Mechanisms(1)
read&write data units of several simple streams
adhere to specific timing and synchronization constraints

29. For example, a movie composed of
A video stream of low-quality images of 320x240 pixels, i.e., 76,800 bytes video data units;
A audio stream: audio samples group into units of 11,760 bytes, each corresponding to 33 ms of audio;
If the input process can handle 2.5 MB/sec, lip synchronization is achieved by alternating between reading an image and reading a block of audio samples every 33 ms.

30. Synchronization Mechanisms(2)
Figure. 2-41
The principle of synchronization as supported by high-level interfaces
Synchronization Mechanisms(2)
Synchronization achieved by middleware according to application instructions, e.g., desired image display rate

31. The principle of synchronization as supported by high-level interfaces
Figure. 2-41
The principle of synchronization as supported by high-level interfaces
MPEG: moving picture expert group
MPEG is a standard for lossy compression of video and audio.
MPEG transport stream (MPEG-TS, MTS or TS) is a standard format for transmission and storage of audio, video,