1. Distributed Systems
Definition

2. Definition of a Distributed System
A distributed system is a collection of independent computers that appears to its users as a single system.

3. A definition (Coulouris, et al)
System of networked computers that
communicate and coordinate their actions only by passing messages
concurrent execution of programs
components fail independently of one another

4. A definition (Lamport)
“You know you have a distributed system when the crash of a computer you’ve never heard of stops you from getting any work done.”
inter-dependencies
shared state

5. Distributed System

6. Operating System types
Centralized Systems
Process management
Network System
Share resources
Remote access
Telnet / FTP
No direct control from machine to another
Distributed system
Global view of files system
Global name
Global time….

7. Distributed system properties
Connect users and resources
Transparency
Scalability
Openness

8. Transparency
To hide the fact that machines are physically distributed and how much the distributed system appears as single system

9. 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 shared by several competitive users
Concurrency
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.

10. Types of transparency
Location Transparency: users cannot tell where hardware and software resources such as CPUs, printers, files, data bases are located.
Migration Transparency: resources must be free to move from one location to another without their names changed. E.g., /usr/lee, /central/usr/lee
Replication Transparency: OS can make additional copies of files and resources without users noticing.
Concurrency Transparency: The users are not aware of the existence of other users. Need to allow multiple users to concurrently access the same resource. Lock and unlock for mutual exclusion.
Parallelism Transparency: Automatic use of parallelism without having to program explicitly. The holy grail for distributed and parallel system designers.
Users do not always want complete transparency: a fancy printer 1000 miles away

11. Scalability It is to able to send anything to anyone anywhere
System scalability to size means easily add more users & resources
Scalability of geographically

12. Scalability
A distributed system is scalable if it remains effective as the number of users and/or resources increase
Challenges:
Controlling resource costs
Controlling performance loss
Preventing resources from running out
Avoiding performance bottlenecks

13. Scalability Problems
No machine has complete information about the system state.
Machines make decisions based only on local information.
Failure of one machine does not ruin the algorithm.
There is no implicit assumption that a global clock exists.

14. Openness / Flexibility
The distributed system has a clear rules controls its services
Open distributed system is flexible means easy to configure system for different developers

15. Openness
When protocols are known to developers extensibility and maintainability becomes possible
Openness allows re-implementation of different components of the system
Important factors:
Specification
Documentation
Published interfaces (often bypassing standards organizations)

16. Reliability
Distributed system should be more reliable than single system. Example: 3 machines with .95 probability of being up **3 probability of being up.
Availability: fraction of time the system is usable. Redundancy improves it.
Need to maintain consistency
Need to be secure
Fault tolerance: need to mask failures, recover from errors.

17. Security Three components:
Confidentiality (protection against disclosure to unauthorized individuals)
Integrity (protection against alteration or corruption)
Availability (protection against interference with the means of accessing the resources)
The challenge: sending sensitive information in a network message in a secure manner efficiently

18. Security
Scenario 1: Accessing exam information via a network file system
Authentication: how do we know for sure that the user is a teacher who should have access to the data?
Scenario 2: Sending a credit card number over the Internet
Confidentiality: no other than the recipient should be able to read the data

19. Hardware concept of DS Distributed system may be
multi-processor or multi-computers

20. Hardware Concepts Tightly Coupled versus Loosely Coupled
Tightly coupled systems (multiprocessors)
shared memory
intermachine delay short, data rate high
Loosely coupled systems (multicomputers)
private memory
intermachine delay long, data rate low

21. Multicomputers Bus-Based Multicomputers Switched Multicomputers
easy to build
communication volume much smaller
relatively slow speed LAN ( MIPS, compared to 300 MIPS and up for a backplane bus)
Switched Multicomputers
interconnection networks: E.g., grid, hypercube
hypercube: n-dimensional cube
11/14/00

22. Multi-processor Multi-computers
Hardware Concepts
1.6
Multi-processor Multi-computers

23. Bus-based Multiprocessors
cache memory
hit rate
cache coherence
write-through cache: propagate write immediately
snoopy cache: monitor when its entry becomes obsolete
11/14/00

24. Multiprocessors
1.7

25. Switched Multiprocessors
for connecting large number (over 64) of processors
crossbar switch: n**2 switch points
omega network: 2x2 switches for n CPUs and n memories, log n switching stages, each with n/2 switches,
total (n log n)/2 switches
building a large, tightly-coupled, shared memory multiprocessor is possible, but is difficult and expensive
11/14/00

26. Multiprocessors (2)
1.8

27. Multicomputer Systems
Grid Hypercube
1-9

28. Software Concepts DOS (Distributed Operating Systems)
Description
Main Goal
DOS
Tightly-coupled operating system for multi-processors and multicomputers
Tries to maintain global view of resources
NOS
Loosely-coupled operating system for multicomputers (LAN and WAN)
Manages collection of machines with local services and OS
Middleware
Additional layer atop of NOS implementing general-purpose services
Provide distribution transparency
DOS (Distributed Operating Systems)
NOS (Network Operating Systems)
Middleware

29. Software Concepts Software more important for users Three types:
Network Operating Systems
(True) Distributed Systems
Multiprocessor Time Sharing
11/14/00

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

31. Network Operating Systems
loosely-coupled software on loosely-coupled hardware
A network of workstations connected by LAN
each machine has a high degree of autonomy
Files servers: client and server model
Clients mount directories on file servers
Best known network OS:
Sun’s NFS (network file servers) for shared file systems

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

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

34. (True) Distributed Systems
tightly-coupled software on loosely-coupled hardware
provide a single-system image or a virtual uniprocessor
a single, global interprocess communication mechanism, process management, file system; the same system call interface everywhere
Ideal definition:
“ A distributed system runs on a collection of computers that do not have shared memory, yet looks like a single computer to its users.”

35. Comparison between Systems
Item
Distributed OS
Network OS
Middleware-based OS
Multiproc.
Multicomp.
Degree of transparency
Very High
High
Low
Same OS on all nodes
Yes No
Number of copies of OS 1 N
Basis for communication
Shared memory
Messages
Files
Model specific
Resource management
Global, central
Global, distributed
Per node
Scalability
Moderately
Varies
Openness
Closed
Open
A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.

36. Distributed object architecture

37. A data mining system

38. Data mining system
The logical model of the system is not one of service provision where there are distinguished data management services
It allows the number of databases that are accessed to be increased without disrupting the system
It allows new types of relationship to be mined by adding new integrator objects