Introduction to
Distributed systems
2.2 Communication Concepts
CDS
Wt
Chapter 2
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Wt
Chapter 2
2.2 Communication Concepts
CDS
2.1 What are distributed systems?
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– What are systems?
– Different kind of systems
– How to distribute systems?
2.2 Communication concepts
–
–
–
–
Client-Server Approach
Producer-consumer approach
Message oriented
Stream oriented
CDS
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Need of communication
Exchange of information between computers
or processes
• communication takes time
2.2 Communication Concepts
– limits due to application, number of bytes
– synchronization
• communication needs „protocols“
– What kind of information will be exchanged
– How information will be exchanged
• communication may cause errors
– data integrity
– error-free data
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CDS
Wt
Communication models
• Multiprocessors or Processes:
– exchange using shared memory
– Synchronization using semaphores and monitors
2.2 Communication Concepts
• Multicomputer:
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– exchange using message passing
– Synchronization with blocking in message passing
– or clock based solutions
Focus:
communication models for multicomputer
systems
CDS
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Client-Server (1)
Client
Server
2.2 Communication Concepts
request
indication
response
confirmation
Confirmed service
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Wt
Client-Server (2)
CDS
service
request
2.2 Communication Concepts
Client
requests
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service
confirmation
service
indication
communication
system
service
response
Server
realise
service
client
server
CDS
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Example: World Wide Web
2.2 Communication Concepts
Browser
Web Server
2. Indication
fetch document
OS
OS
4. confirmation
1. Request: Get Document
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2.2 Communication Concepts
CDS
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Multitired Architectures (1)
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3. Response,
gives document
2.2 Communication Concepts
CDS
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Multitired Architectures (2)
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2.2 Communication Concepts
CDS
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Client and Server Stubs
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• Remote Procedure Calls (RPC)
• Principle of RPC between a client and server
program.
• The interconnection between client and
server in a traditional RPC
• The interaction using asynchronous RPC
2.2 Communication Concepts
CDS
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Asynchronous RPC (1)
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• A client and server interacting through two
asynchronous RPCs
2.2 Communication Concepts
CDS
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Asynchronous RPC (2)
Deferred synchronous RPC
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2.2 Communication Concepts
CDS
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Remember!
• Client / Server or producer/consumer are a
logical models and can be realized with
every communication system
• Client / Server models can be used at
different interfaces, even for internal
software-interfaces
• The WWW is the most popular client-server
system
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CDS
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Producer/Consumer
producer
push-model
consumer(s)
service.request
2.2 Communication Concepts
Service indication
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Unconfirmed service
publisher
subscriber
CDS
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Producer/Consumer
consumer
producer
pull-model
request(s)
2.2 Communication Concepts
indication
response
confirmation
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CDS
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Principles of communication
• Address based
– Source- and destination address
– Broadcast addressing
– Addresses must be available
2.2 Communication Concepts
– Like sending a letter
• Information based
– Based on identifier, tagging the message
– destination of message is unknown
– Communication controller decides whether to take the
message or not.
– Like Advertising
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2.2 Communication Concepts
CDS
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When takes Communication place
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• Event driven
Communication takes only place in case of
an event
–
–
–
–
–
aim: minimize communication
delay time (Event-communication)
undefined workload (statistic)
flexible system
ensure that system is working
• Cyclic
there is a periodic exchange of information
between subsystems
–
–
–
–
cycle time (defined by the process)
defined workload
Timeout condition can be checked
less flexibility
CDS
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Communication types
• persistent communication
– stored in the system, even if sending or receiving
application are not running
• transient communication
2.2 Communication Concepts
– stored in system only, as long as sending and
receiving application are running
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• asynchronous communication
– sender continues immediately after message
submission
• synchronous communication
– sender is blocked until message is stored at local host
at the receiver
– worst: until message is processed at the receiver
• Persistent communication of letters back in
the days of the Pony Express.
2.2 Communication Concepts
CDS
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Persistence Communication (1)
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CDS
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Persistence Communication (2)
• General organization of a communication
system in which hosts are connected
through a network
2.2 Communication Concepts
2-20
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CDS
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Persistence and Synchronicity in
Communication (1)
a) persistent asynchronous communication
b) persistent synchronous communication
2.2 Communication Concepts
2-22.1
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2.2 Communication Concepts
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Persistence and Synchronicity in
Communication (2)
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c) Transient asynchronous communication
d) Receipt-based transient synchronous
communication
e) Delivery-based transient synchronous
communication at message delivery
f) Response-based transient synchronous
communication
2.2 Communication Concepts
CDS
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Persistence and Synchronicity in
Communication (3)
RPC and RMI
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CDS
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Message oriented communication
• transient synchronous communication
– RPC, RMI
2.2 Communication Concepts
• transient asynchronous communication
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• Message oriented persistent communication
• Message oriented persistent communication
– message queuing systems
– min instead of ms
– message oriented middelware ( MOM)
2.2 Communication Concepts
CDS
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Message oriented persistent
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CDS
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Message-Queuing Model (1)
• Four combinations for loosely-coupled
communications using queues.
2.2 Communication Concepts
2-26
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Message-Queuing Model (2)
2.2 Communication Concepts
CDS
• Basic interface to a queue in a messagequeuing system.
Primitive
Meaning
Put
Append a message to a specified queue
Get
Block until the specified queue is nonempty, and
remove the first message
Poll
Check a specified queue for messages, and
remove the first. Never block.
Notify
Install a handler to be called when a message is
put into the specified queue.
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2.2 Communication Concepts
CDS
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General Architecture of a
Message-Queuing System (1)
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• The relationship between queue-level
addressing and network-level addressing.
• The general organization of a messagequeuing system with routers.
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2.2 Communication Concepts
CDS
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General Architecture of a
Message-Queuing System (2)
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CDS
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Message Brokers
• The general organization of a message broker in a
message-queuing system.
2.2 Communication Concepts
2-30
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Message Broker: change formats, similar to Gateway
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Channels
2.2 Communication Concepts
CDS
• Some attributes associated with message
channel agents.
Attribute
Description
Transport
type
Determines the transport protocol to be used
FIFO delivery
Indicates that messages are to be delivered in the order
they are sent
Message
length
Maximum length of a single message
Setup retry
count
Specifies maximum number of retries to start up the
remote MCA
Delivery
retries
Maximum times MCA will try to put received message into
queue
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CDS
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Data Stream (1)
• Setting up a stream directly between two
devices.
2.2 Communication Concepts
2-35.2
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• An example of multicasting a stream to
several receivers.
2.2 Communication Concepts
CDS
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Data Stream (2)
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• A flow specification.
2.2 Communication Concepts
CDS
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Specifying QoS (1)
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Characteristics of the Input
Service Required
• maximum data unit size
(bytes)
• Token bucket rate (bytes/sec)
• Token bucket size (bytes)
• Maximum transmission rate
(bytes/sec)
• Loss sensitivity (bytes)
• Loss interval (μsec)
• Burst loss sensitivity
(data units)
• Minimum delay noticed (μsec)
• Maximum delay variation (μsec)
• Quality of guarantee
Wt
Specifying QoS (2)
2.2 Communication Concepts
CDS
• The principle of a token bucket algorithm.
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