1. ECI-M-811 Distributed Systems and Internetworking
Coordinator: Dr. Zhanfang Zhao
Office: T409
Tel:
Fax:
URL:
Blackboard:

2. Textbooks Core reading: Background reading:
‘Distributed Systems: Principles and Paradigms Tanenbaum AS. Prentice Hall 2002.
Background reading:
‘Distributed Systems: Concept and Design’ Coulouris G etc, Addison Wesley, 2001.

3. Topics Introduction to DS Communication in DS Process in DS
Naming System in DS
Synchronization in DS
New technologies

4. Outcomes (Distributed System Part)
At the end of this unit the student will be able to:
Familiar with the concepts of distributed systems hardware and distributed system facilities.
Able to appreciate the capabilities and limitations of applications software running over local and wide area networks.
Able to understand, design and implement simple client-server systems.
Able to know how the World Wide Web system works

5. Introduction to Distributed System
Contents of this lesson:
Definition of the Distributed System
The Goals when Building a Distributed System
Basic Software Concepts
The Middleware
System Architecture & Client-Server Model

6. Definition of a Distributed System (1)
A distributed system is:
A collection of independent computers that appears to its users as a single coherent system. --Tanenbaum
Two aspects of this definition:
Hardware: The machines are autonomous.
Software: The user think they are dealing with a single system.
Important characteristics of DS:
Differences between various computers and the ways in which they communicate are hidden from users
Users and applications can interact with a DS in a consistent and uniform way
Ds should also be easy to extend or scale
To support heterogeneous computers and networks while offering a single system view, DS is often organized in certain layers called middleware

7. Definition of a Distributed System (2)
A distributed system is a collection of autonomous hosts that that are connected through a computer network. Each host executes components and operates a distribution middleware, which enables the components to coordinate their activities in such a way that users perceive the system as a single, integrated computing facility.
Wolfgang Emmerich, 2000
We employ the (adapted) definition of Colouris, Dollimore and Kindberg of a distributed system.
It requires autonomous computers to be interconnected through a network.
Each computer has to be equipped with distributed operating system software, which enables the computers to coordinate activities and to share resources in a controlled way
We also require transparency of distribution for the computer users. They shall not have to be aware of the fact that the system is distributed.

8. Definition of a Distributed System (3)
Component1
Componentn
Hostn-1
Hostn
Host2
Host1
Middleware
Network Operating System
Hardware
Network

9. Middleware Examples Transaction-oriented Procedural Object-oriented
IBM CICS
BEA Tuxedo
IBM Encina
Microsoft Transaction Server
Message-oriented
Microsoft Message Queue
Sun Tooltalk
Procedural
Sun ONC
Linux RPCs
OSF DCE
Object-oriented
OMG CORBA
Sun Java/RMI
Microsoft COM
.net Remote
Sun Enterprise Java Beans

10. Centralised System Characteristics
One component with non-autonomous parts
Component shared by users all the time
All resources accessible
Software runs in a single process
Single Point of control
Single Point of failure

11. Distributed System Characteristics
Multiple autonomous components
Components are not shared by all users
Resources may not be accessible
Software runs in concurrent processes on different processors
Multiple Points of control
Multiple Points of failure

12. Goals
Four important goals should be met to make it worth to build a distributed systems:
Connecting users and resources
Transparency
Openness
Scalability

13. Transparency in a Distributed System
Description
Access
Hide differences in data representation and how a resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another location while in use
Replication
Hide that a resource may be replicated
Concurrency
Hide that a resource may be shared by several competitive users
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or on disk
Different forms of transparency in a distributed system.
Definition: A DS that is able to present itself to users and applications as if it were only a single computer system is said to be transparent

14. Transparency
Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components.
Transparency has different dimensions
These represent various properties that distributed systems should have.

15. Distribution Transparency
Scalability
Transparency
Performance
Transparency
Failure
Transparency
Migration
Transparency
Replication
Transparency
Concurrency
Transparency
Access
Transparency
Location
Transparency

16. Access Transparency
Enables local and remote information objects to be accessed using identical operations.
Example: File system operations in NFS.
Example: Navigation in the Web.
Example: SQL Queries

17. Location Transparency
Enables information objects to be accessed without knowledge of their location.
Example: File system operations in NFS
Example: Pages in the Web
Example: Tables in distributed databases

18. Concurrency Transparency
Enables several processes to operate concurrently using shared information objects without interference between them.
Example: Automatic teller machine network
Example: Database management system

19. Replication Transparency
Enables multiple instances of information objects to be used to increase reliability and performance without knowledge of the replicas by users or application programs
Example: Distributed DBMS
Example: Mirroring Web Pages.

20. Failure Transparency Enables the concealment of faults
Allows users and applications to complete their tasks despite the failure of other components.
Example: Database Management System

21. Migration Transparency
Allows the movement of information objects within a system without affecting the operations of users or application programs
Example: NFS
Example: Web Pages

22. Performance Transparency
Allows the system to be reconfigured to improve performance as loads vary.
Example: Distributed make.

23. Scaling Transparency
Allows the system and applications to expand in scale without change to the system structure or the application algorithms.
Example: World-Wide-Web
Example: Distributed Database

24. Openness
An open distributed system is a system that offers services according to standard rules that describe the syntax and semantics of those services.
Openness means that the system can easily be extended and modified.
To facilitate the openness, the system should have a well defined and well-documented interfaces. These interface must declare the services that a component offers. And these interfaces are often described in an Interface Define Language (IDL)
A service is an operation that a component performs on behalf of a user or another component.
The interface definition should be complete and neutral.
Complete means that everything that is necessary to make an implementation has indeed been specified.

25. Scalability Adaption of distributed systems to
accomodate more users
respond faster (this is the hard one)
Usually done by adding more and/or faster processors.
Components should not need to be changed when scale of a system increases.
Design components to be scalable!

26. Examples of scalability limitations.
Scalability Problems
Concept
Example
Centralized services
A single server for all users
Centralized data
A single on-line telephone book
Centralized algorithms
Doing routing based on complete information
Examples of scalability limitations.

27. Software Concepts An overview between
System
Description
Main Goal
DOS
Tightly-coupled operating system for multi-processors and homogeneous multicomputers
Hide and manage hardware resources
NOS
Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)
Offer local services to remote clients
Middleware
Additional layer atop of NOS implementing general-purpose services
Provide distribution transparency
An overview between
DOS (Distributed Operating Systems)
NOS (Network Operating Systems)
Middleware

28. Uniprocessor Operating Systems
Separating applications from operating system code through a microkernel.
1.11

29. Multiprocessor Operating Systems (1)
An important extension of MOS is that it supports multiple processors having access to a shared memory. These data in the shared memory must be protected against the concurrent access to guarantee consistency.
Two synchronized primitives are used, one is semaphore, another is monitor
monitor Counter {
private:
int count = 0;
public:
int value() { return count;}
void incr () { count = count + 1;}
void decr() { count = count – 1;} }
A monitor to protect an integer against concurrent access.

30. Multiprocessor Operating Systems (2)
monitor Counter {
private:
int count = 0;
int blocked_procs = 0;
condition unblocked;
public:
int value () { return count;}
void incr () {
if (blocked_procs == 0)
count = count + 1;
else
signal (unblocked); }
void decr() {
if (count ==0) {
blocked_procs = blocked_procs + 1;
wait (unblocked);
blocked_procs = blocked_procs – 1; }
else
count = count – 1;
A monitor to protect an integer against concurrent access, but blocking a process.

31. Multicomputer Operating Systems (1)
1.14
General structure of a multicomputer operating system

32. Multicomputer Operating Systems (2)
Alternatives for blocking and buffering in message passing.
1.15

33. Network Operating System (1)
General structure of a network operating system.
1-19

34. Network Operating System (2)
Two clients and a server in a network operating system.
File systems is supported by one or more machines called file servers
1-20

35. Positioning Middleware
General structure of a distributed system as middleware.
1-22

36. Middleware and Openness
1.23
In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.

37. System Architecture & Client Server Model
Important styles of architecture for distributed systems
Layered architectures
Object-based architectures
Data-centered architectures
Event-based architectures

38. Architectural Styles (1)
The layered architectural style and …

39. Architectural Styles (2)
The object-based architectural style.

40. Architectural Styles (3)
The event-based architectural style ( Publish/Subscribe System)

41. Architectural Styles (4)
The shared data-space architectural style.

42. Clients and Servers
1.25
General interaction between a client and a server.
In the basic client server model, the process in a computing system are divided into two groups. A server is a process implementing a specific service, for example database service. A client is a process that requests a service from a server by sending it a request and subsequently waiting or the server’s reply

43. An Example Client and Server (1)
Used for parameters
The header.h file used by the client and server.

44. An Example Client and Server (2)
A sample server.

45. An Example Client and Server (3)
1-27 b
A client using the server to copy a file.

46. Application Layering
For the web applications, the CS applications generally been organized into three levels:
1-28

47. Multitiered Architectures (1)
Alternative client-server organizations (a) – (e).
1-29

48. Multitiered Architectures (2)
An example of a server acting as a client.
1-30

49. Modern Architectures
An example of horizontal distribution of a Web service.
1-31

50. Distributed System Paradigms
World Wide Web Architecture Model:
Traditional Web-based Systems
Overall organization
Web Documents
Programming Languages: HTML, XML & MIME (Multipurpose Internet Mail Exchange)
Multi-tiered Architectures
Web Services

51. Traditional Web-Based Systems
Figure The overall organization of a traditional Web site.

52. Web Documents
Six top-level MIME types and some common subtypes.

53. Multitiered Architectures
The principle of using server-side CGI programs.

54. Web Services Fundamentals
The principle of a Web service:
WSDL (Web Services Definition Language) is a formal language very much the same as the interface definition languages of RPC
SOAP ( Simple Object Access Protocol) is essentially a framework in which much of the communication between two process can be standardized
UDDI ( Universal Description, Discovery and Integration)