Distributed Systems

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Distributed Systems

• Topic 3 Communication
• Dr. Michael R. Lyu
• Computer Science Engineering Department
• The Chinese University of Hong Kong

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Outline

• 1 Communication Primitives
• 2 Client/Server Communication
• 3 Group Communication
• 4 CORBA Event Service
• 5 Summary

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1 Communication Primitives

• The ISO/OSI
• Reference Model

HTTP, FTP, Telnet CORBA IIOP
Application
XDR, CORBA Data Secure Sockets (SSL)
Presentation
for connection- oriented comm.
Session
Transport
message TCP, UDP
Network
packetIP ATM VC
Data link
error-free trans. PPP, CSMA/CD
Physical
ISDN, baseband signaling
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1.1 ISO/OSI Transport Layer

• Level 4 of ISO/OSI reference model.
• Concerned with the transport of information
  through a network.
• Two facets in UNIX networks
• TCP
• UDP

Application
Presentation
Session
Transport
Network
Data link

• connection oriented
• virtual connection
• w sequencing
• acknowledgement

Physical

• connectionless
• up to 64k bytes datagram
• no seqs and acks

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1.1 ISO/OSI Transport Layer (TCP)

• Transmission Control Protocol (TCP) provides
  bi-directional stream of bytes between two
  distributed components.
• UNIX rsh, rcp and rlogin are based on TCP.
• Reliable but slow protocol.
• Buffering at both sides decouples computation
  speeds.

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1.1 ISO/OSI Transport Layer (UDP)

• User Datagram Protocol (UDP) enables a component
  to pass a message containing a sequence of bytes
  to another component.
• Other component is identified within message.
• Unreliable but very fast protocol.
• Restricted message length.
• Queuing at receiver.
• UNIX rwho command is UDP based.

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1.2 ISO/OSI Presentation Layer

• At application layer complex data types
• How to transmit complex values through transport
  layer?
• Presentation layer issues
• Complex data structures and
• Heterogeneity.

Application
Presentation
Session
Transport
Network
Data link
Physical
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1.2 Complex Data Structures

• Marshalling Disassemble data structures into a
  transmittable form
• Unmarshaling Re-assemble the complex data
  structure.

class Person private int dob char
name public char marshal() char
msg msgnew charstrlen(name)10
sprintf(msg,d,d,s, dob,
strlen(name),name) return(msg)

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1.2 Heterogeneity

• Heterogeneous data representation on different
  hardware platforms.
• Approach 1 (Example XDR)
• Define a shared representation,
• For each different platform, provide mapping
  between common and specific representation.
• Approach 2 (Example ASN)

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1.3 Communication Patterns

• Basic operations send and receive messages (as
  in UDP).
• Message delivery
• Synchronous or
• Asynchronous
• Messages are used to model
• Notification and
• Request.

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1.3 Synchronous Communication
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1.3 Communication Deadlocks
P1 send() to P2 receive() from P2
P2 send() to P1 receive() from P1

• Components are mutually waiting for each other.
• To avoid deadlocks Waits-for relation has to be
  acyclic!

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1.3 Asynchronous Communication
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1.3 Notification

• Uni-directional communication
• Message contains marshaled notification
  parameters.

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1.3 Request

• Bi-directional communication.
• Request message contains marshaled parameters.
• Requester receives reply message.
• Reply message contains marshaled results.

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1.3 Reliability Issues

• Unreliable message refers to message transmission
  without acknowledgement or retries (e.g., UDP).
• A reliable delivery service may be constructed
  from an unreliable one by the use of ack.
• Positive ack. for client-server communication and
  negative ack. for group multicast.
• Reliable communication involves overheads.
• Each message should have a unique identifier.

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2 Client/Server Communication

• Qualities of service.
• Request protocol (R).
• Request reply protocol (RR).
• Request reply acknowledgement protocol (RRA).

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2.1 Qualities of service

• Exactly once,
• At most once,
• At least once and
• Maybe?

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2.2 Request Protocol

• If service
• does not have out or inout parameters and
• does not have a return type
• client may not want to wait for server to finish.

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2.3 Request/Reply Protocol

• To be applied if client expects result from
  server.
• Client requests service execution from server
  through request message.
• Delivery of service result in reply message.

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2.4 RRA Protocol

• In addition to RR protocol, client sends
  acknowledgement after it received reply.
• Acknowledgement sent asynchronously.

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3 Group Communication

• Client/server requests
• There is no other party involved.
• Client has to identify server.
• Sometimes other properties are required
• Communication between multiple components.
• Anonymous communication.

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3.1 Concepts

• Broadcast Send msg to a group.

• Multicast Send msg to subgroup only.

• Useful applications
• Fault tolerance
• Object location
• Better performance
• Multiple update

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3.2 Qualities of Service

• Ideal Immediate and reliable.

• Optimal Simultaneous and reliable.

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3.2 Qualities of Service

• In reality not simultaneous ...

... and not reliable
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3.2 Qualities of Service

• Problem To achieve reliable broadcast/multicast
  is very expensive.
• Degrees of reliability
• Best effort,
• K-reliability,
• totally ordered,
• Atomicity.
• Choose the degree of reliability needed and be
  prepared to pay the price.

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3.3 CORBA Event Management

• CORBA event management service defines interfaces
  for different group communication models.
• Events are created by suppliers (producers) and
  communicated through an event channel to multiple
  consumers.
• Service does not define a quality of service
  (left to implementers).

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3.3.1 Push Model

• Consumers register with those event channel
  through which events they are interested in are
  communicated.
• Event producers create a new event by invoking a
  push operation from an event channel.
• Event channel notifies all registered consumers
  by invoking their push operations.

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3.3.1 Push Model (Example)
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3.3.2 The Pull Model

• Event producer registers its capability of
  producing events with event channel.
• Consumer obtains event by invoking pull operation
  from event channel.
• Event channel asks producer to produce event and
  delivers it to the consumer.

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3.3.2 Pull Model (Example)
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3.3.2 Event Channel

• Supported combinations
• push suppliers, push consumers
• push suppliers, pull consumers
• pull suppliers, push consumers
• pull suppliers, pull consumers

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3.3.2 Event Channel (with proxies)
Proxy push consumer
Proxy push supplier
Push consumer
Push supplier
Event Channel
Direction of event transfer
Proxy pull consumer
Proxy pull supplier
Pull supplier
Pull consumer
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4 Summary

• What communication primitives do we use?
• How are differences between application and
  communication layer resolved?
• What quality of service do the client/server
  protocols achieve that we discussed?
• What quality of services are involved in group
  communication?
• CORBA Event Service for group communication.
• Read Textbook Chapter 3 through Chapter 4.

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Homework 1

• 1.7 Total 10 questions, 10 points each
• 1.11
• 1.13 Due 7/10/2003 (Tuesday) in class
• 2.9
• 2.13
• 3.1
• 3.7
• 3.9
• 4.7
• 4.21