02 Witting Msg Stream

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Message - orientedvs.

Stream - orientedCommunication

Witting Jrgen [email protected]

Seminar Applied Computer Science Problems

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Message oriented communication

persistent vs. transient, synchronous vs. asynchronous

forms of communication

examples

Stream oriented communication

discrete vs. continuous media

asynchronous, synchronous, isochronous transmission mode

QoS, setting up a stream, synchronization

Content

Message - oriented vs. Stream - oriented communication


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Message - orientedCommunication

vs.Stream - orientedCommunication

Outline

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Message - oriented communication

RPC and RMI - hide communication, access transparency, synchronous

client asks a service and waits until it gets a reply

server waits for requests, performs it and returns to waiting

requires that the other party is up

In large distributed systems, all parties are not constantly running

Messaging is the way

Why messaging?


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General organization of a communication system

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Synchronous vs. asynchronous communication

synchronous communication

the sender is blocked until the message is stored in a local buffer at the

receiving host or is delivered to the receiver

asynchronous communication

the sender continues after it has submitted the message for transmission


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Persistent vs. transient communication

persistent communication

a message is stored by the communication system as long as it takes todeliver it never lost or thrown away

transient communication

a message is stored by the communication system only as long as thesending and receiving application are executing

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Example - Persistent communication of letters


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Forms of communication

(a) persistent asynchronous communication

(b) persistent synchronous communication

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(c) transient asynchronous communication

(d) receipt-based transient synchronous communication


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(e) delivery-based transient synchronous communication

(f) response-based transient synchronous communication

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Examples

Message-oriented transient communication

Berkeley Sockets

Message Passing Interface (MPI)

Message-oriented persistent communication

Message Queuing Model


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Message - oriented transient communication

Berkeley Sockets

Socket: communication endpoint to write and read data

interface to transport layer

port number & IP number used to bind to a socket

UNIX OSs are the first in providing sockets

Socket primitives for TCP/IP:

socked, bind, listen, accept, connect, send, receive, close

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Berkeley Sockets - socket primitives for TCP/IP

Release the connectionClose

Receive some data over the connectionReceive

Send some data over the connectionSend

Actively attempt to establish a connectionConnect

Block caller until a connection request arrivesAccept

Announce willingness to accept connections. Byallocating enough memory

Listen

Attach a local address to a socketBind

Create a new communication endpointSocket

MeaningPrimitive


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Berkeley Sockets - Connection-oriented communication pattern

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Message Passing Interface (MPI)

designed for efficient parallel applications

higher level interface for transient async & sync communication

incurs minimal overhead

communication in a group of processes (group_ID, process_ID)

Message passing primitives of MPI:

MPI_bsend, MPI_send, MPI_ssend, MPI_sendrecv, MPI_isend,

MPI_issend, MPI_recv, MPI_irecv


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Some of the most intuitive message-passing primitives of MPI

Check if there is an incoming message, but do not blockMPI_irecv

Receive a message; block if there are noneMPI_recv

Pass reference to outgoing message, and wait until receipt startsMPI_issend

Pass reference to outgoing message, and continueMPI_isend

Send a message and wait for replyMPI_sendrecv

Send a message and wait until receipt startsMPI_ssend

Send a message and wait until copied to local or remote bufferMPI_send

Append outgoing message to a local send bufferMPI_bsend

MeaningPrimitive

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Message - oriented persistent communication

Message Queuing Systems

Message Oriented Middleware MOM

messages are forwarded over series communication servers

provides queues to sender and receiver

senders and receivers do not need to be active at the same time

for non time critical transmission

guarantee for delivering, no guarantee for the delivering moment

queues are managed by queue managers


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Message Queing Model

Four combinations for loosely-coupled communications using queues

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Basic interface to a queue in a message-queuing system

Install a handler to be called when a message is put into the specifiedqueue.

Notify

Check a specified queue for messages, and remove the first. Never block.Poll

Block until the specified queue is nonempty, and remove the first messageGet

Append a message to a specified queuePut

MeaningPrimitive


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General architecture of a message-queuing system

Queues are managed by queue managers.Static mapping is easier, dynamic mapping is more complex.

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Message-Queuing System with routers

Routers know about network, queue manager know the nearest router Only routers need to be updated when queues are added or removed


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Message Brokers

converts a message to the format of the destination application contains a database of conversion rules

Message - orientedCommunication

vs.Stream - oriented

Communication

Outline


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Stream - oriented communication

Communication as discussed so far:

- independent and complete units of information- moment of receiving is not important for correctness

Stream-oriented communication is used for audio, video, sensor data ...-> moment of receiving and correct representation is essential!

Streams are uni-directional

Streams are virtual connections between source and sink

Streams are simple or complex (e.g. sound and video)

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So far focus on transmitting discrete data -> time independent data

Discrete (representation) media

the temporal relationships between data items is not fundamental(text, still images, exe - files,..)

Continuous (representation) media

the temporal relationships between data items is fundamental tocorrectly interpreting what the data means(audio, video, animation, sensor data)

Support for Continuous Media


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The following timing requirements can be used:

Asynchronous transmission mode:no timing constraints - data items are transmitted one after the other(discrete data streams - file)

Synchronous transmission mode:maximum end-to-end delay for each unit in a data stream (sensor data)

Isochronous transmission mode:- max and min end-to-end delay for each data unit -> bounded jitter- interesting for distributed multimedia systems (audio, video)

Data Stream - Transmission Modes

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Data Streams - Multicasting a stream to several receivers

Multiple sinks:

the data stream is multicasted (and filtered) to several receivers

Problem - different requirements with respect to the quality of the stream


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Quality of Service

Time-dependent and non-functional requirements are called QoS req.

describes what is needed from the underlying distributed system andnetwork

Specifying QoS requirements:- flow specification (bandwidth, transmission rate, delays, ) -> complex- stream classification (defaults for detailed flow specification)

Example: audio and video streams may be available between high,medium or low specification

difficult to map QoS parameters to resources

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Setting up a Stream

Once a data stream has been described, a distributed systems is in aposition to allocate resources to set up a stream that satisfies the QoS req.

RSVP (ReSource reserVation Protocol)

Application

RSVP process

Packet

Classifier

Packet

Scheduler

AdmissionControl

PolicyControl

data

Routing

process

RSVP process

Packet

Classifier

Packet

Scheduler

PolicyControl

RSVP

AdmissionControl

data data

Host Router


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Stream Synchronization

Two types of synchronization:

discrete and a continuous (slide show & audio) two continuous data streams (video & audio, stereo)

Synchronization of streams:

based on transmission units

process simply executes read operations on simple substreams

based on high-level middleware-interface

to control streams and devices by an application

-> the receiving side is responsible for synchronization

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Stream Synchronization

Synchronization by multiplexing

multiplexing the different simple streams into a single stream on the

sender side

-> MPEG streams

- several standards

- collection of algorithms for compressing

- multiplexed into a program stream

- receiving side demultiplexes the stream


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Message - oriented vs. stream - oriented communication

Summary

Message oriented communication

Message-oriented communication systems are mostly used for persistentasynchronous communication (E-Mail-System).

Stream oriented communication

Streams have a temporal relationship (sequence of representation) betweentwo successive messages.

In continuous data streams a maximum and minimum end-to-end delay isspecified for each message -> jitter.

Other non-functional requirements (bandwidth, transmission rate, ) can bedefined with QoS.

Extra Slides

Example: IBM WebSphere(IBM MQSeries)

Outline


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Example: IBM WebSphere (IBM MQSeries)

The basic architecture of WebSphere consists of Message Queue

Queues are managed by queue managers.

Messages are passed through message channels.They are uni-directional, reliable connections (TCP)

At both ends of a message channel is a Message Channel Agent MCAthat checks the send queue for a message, wraps it, and sends it, or

respectively listening to a channel for incoming message, unwrappingit, and storing it in a queue

WebSphere uses unique addresses for naming

RPC is used for synchronous communication between Client and QM

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Example: IBM WebSphere (IBM MQSeries)


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Primitives available in IBM WebSphere

Get a message from a (local) queueMQget

Put a message into an opened queueMQput

Close a queueMQclose

Open a (possibly remote) queueMQopen

DescriptionPrimitive

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02 Witting Msg Stream