1. Outline Review of Classical Operating Systems - continued
Distributed Systems
4/7/2019
COP5611

2. Announcement The class email list Materials to make up
I created a class list using ACNS’ service
If you do not receive an from me this morning, you need to send me an to add your address to the list
Materials to make up
In general, the technical issues in distributed operating systems are different to the ones in classical operating systems
For this class, the following undergraduate topics may be helpful
Mutual exclusion (Process synchronization)
Deadlock detection
File systems
Memory management
Scheduling
4/7/2019
COP5611

3. Operating System
An operating system is a layer of software on a bare machine that performs two basic functions
Resource management
To manage resources so that they are used in an efficient and fair manner
User friendliness
4/7/2019
COP5611

4. Distributed Systems
A distributed system is a collection of independent computers that appears to its users as a single coherent system
Independent computers mean that they do not share memory or clock
The computers communicate with each other by exchanging messages over a communication network
4/7/2019
COP5611

5. Distributed Systems – cont.
4/7/2019
COP5611

6. Distributed Systems – cont.
Is each of the following systems a distributed system according to our definition?
Workstations at the Computer Science department
World wide web
4/7/2019
COP5611

7. Distributed Systems – cont.
Motivations
The availability of powerful microprocessors
Significant advances in communication technology
A group of people working together need to share data and expensive resources
4/7/2019
COP5611

8. Distributed Systems – cont.
Advantages
The computing power of a group of cheap workstations can be enormous
Decisive price/performance advantage over traditional time-sharing systems
Resource sharing
Enhanced performance
Improved reliability and availability
Modular expandability
4/7/2019
COP5611

9. Distributed System Architecture
The minicomputer model
Consists of several minicomputers, each supports multiple users and provides access to remote resources
The workstation model
Consists of a number of workstations, each supports a single user in general
The processor model
The ratio of the number of processors to the number of users is greater than one
4/7/2019
COP5611

10. Distributed System Architecture – cont.
The classification given above does not consider the underlying hardware
Distributed systems are often classified based on the hardware
Multiprocessor systems
Homogenous multi-computer systems
Heterogeneous multi-computer systems
4/7/2019
COP5611

11. Basic Distributed Systems
1.6
4/7/2019
COP5611

12. Multiprocessor Systems – cont.
Multiprocessor systems are often divided into two categories
In tightly coupled systems, all processors share the same memory address space and all processors can directly access a global main memory
In loosely coupled systems, not only is the main memory partitioned and attached to processors, but each processor has its own address space
A processor cannot directly access memory attached to other processors
4/7/2019
COP5611

13. Multiprocessor Systems – cont.
Based on the vicinity and accessibility of the main memory to the processors, there are three main types of multiprocessor system architectures
UMA – Uniform memory access
NUMA – Non-uniform memory access
NORMA – no remote memory access
4/7/2019
COP5611

14. Multiprocessor Systems – cont.
A bus-based multiprocessor system
4/7/2019
COP5611

15. Multiprocessors Systems – cont.
4/7/2019
Multiprocessors Systems – cont.
A crossbar switch
An omega switching network
4/7/2019
COP5611
COP5611

16. Homogeneous Multicomputer Systems
Compared to multiprocessor systems, building multicomputer systems is relatively easy
Each CPU has a direct connection to its own local memory
The problem is how the CPUs communicate with each other
There are kinds of multi-computer systems
Bus-based / switch-based
4/7/2019
COP5611

17. Bus-Based Multicomputer Systems
4/7/2019
COP5611

18. Homogeneous Multicomputer Systems – cont.
Grid
Hypercube
1-9
4/7/2019
COP5611

19. Heterogeneous Multicomputer Systems
Most of the distributed systems are built on top of a heterogeneous multicomputer systems
Computers can vary widely
Some of them can be even multiprocessor or homogeneous multicomputer systems
The interconnection network may be highly heterogeneous as well
4/7/2019
COP5611

20. Distributed Operating Systems
Hardware for distributed systems is important, but the software largely determines what a distributed system looks like to a user
Distributed operating systems are much like the traditional operating systems
Resource management
User friendliness
The key concept is transparency
4/7/2019
COP5611

21. Distributed Operating Systems – cont.
In a truly distributed operating system, the user views the system as a virtual uniprocessor system even though physically it consists of multiple computers
In other words, the use of multiple computers and accessing remote data and resources should be invisible to the user
4/7/2019
COP5611

22. Distributed Operating Systems – cont.
Transparency
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
4/7/2019
COP5611

23. Distributed Operating Systems – cont.
Degree of transparency and performance
There is a trade-off between a high degree of transparency and the performance of the system
Three categories
Distributed Operating System (DOS)
Network Operating System (NOS)
Middleware
4/7/2019
COP5611

24. Overview of Different Kinds of Distributed Systems
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
4/7/2019
COP5611

25. Uniprocessor Operating Systems
Separating applications from operating system code through a microkernel.
1.11
4/7/2019
COP5611

26. Multicomputer Operating Systems
General structure of a multicomputer operating system
4/7/2019
COP5611

27. Distributed Shared Memory Systems
Pages of address space distributed among four machines
Situation after CPU 1 references page 10
Situation if page 10 is read only and replication is used
4/7/2019
COP5611

28. Network Operating System
1-19
4/7/2019
COP5611

29. Network Operating System – cont.
Two clients and a server in a network operating system.
4/7/2019
COP5611

30. Network Operating System – cont.
Different clients may mount the servers in different places.
4/7/2019
COP5611

31. Positioning Middleware
General structure of a distributed system as middleware.
4/7/2019
COP5611

32. 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.
4/7/2019
COP5611

33. The Global Architecture of CORBA
4/7/2019
COP5611

34. Object Model in CORBA The general organization of a CORBA system.
4/7/2019
COP5611

35. CORBA Services 4/7/2019 COP5611 Service Description Collection
Facilities for grouping objects into lists, queue, sets, etc.
Query
Facilities for querying collections of objects in a declarative manner
Concurrency
Facilities to allow concurrent access to shared objects
Transaction
Flat and nested transactions on method calls over multiple objects
Event
Facilities for asynchronous communication through events
Notification
Advanced facilities for event-based asynchronous communication
Externalization
Facilities for marshaling and unmarshaling of objects
Life cycle
Facilities for creation, deletion, copying, and moving of objects
Licensing
Facilities for attaching a license to an object
Naming
Facilities for systemwide name of objects
Property
Facilities for associating (attribute, value) pairs with objects
Trading
Facilities to publish and find the services on object has to offer
Persistence
Facilities for persistently storing objects
Relationship
Facilities for expressing relationships between objects
Security
Mechanisms for secure channels, authorization, and auditing
Time
Provides the current time within specified error margins
4/7/2019
COP5611

36. 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
4/7/2019
COP5611

37. Issues in Distributed Operating Systems
Absence of global knowledge
In a distributed system, due to the unavailability of a global memory and a global clock and due to unpredictable message delays, it is practically impossible to for a computer to collect up-to-date information about the global state of the distributed system
Therefore a fundamental problem is to develop efficient techniques to implement a decentralized system wide control
Another problem is how to order all the events
4/7/2019
COP5611

38. Issues in Distributed Operating Systems – cont.
Naming
Plays an important role in achieving location transparency
A name service maps a logical name into a physical address by making use of a table lookup, an algorithm, or a combination of both
In distributed systems, the tables may be replicated and stored at many places
Consider naming in a distributed file system
4/7/2019
COP5611

39. Issues in Distributed Operating Systems – cont.
Scalability
Systems generally grow with time, especially distributed systems
Scalability requires that the growth should not result in system unavailability or degraded performance
This puts additional constraints on design approaches
4/7/2019
COP5611

40. Scalability – cont.
Consider the scalability of centralized design approaches
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
4/7/2019
COP5611

41. Scaling Techniques The difference between letting: a server or
a client check forms as they are being filled
4/7/2019
COP5611

42. An example of dividing the DNS name space into zones.
Scalability – cont.
An example of dividing the DNS name space into zones.
4/7/2019
COP5611

43. Issues in Distributed Operating Systems – cont.
Compatibility
Refers to the interoperability among the resources in a system
Three different levels
Binary level
All processors execute the same binary instruction repertoire
Virtual binary level
Execution level
Same source code can be compiled and executed properly
Protocol level
4/7/2019
COP5611

44. Issues in Distributed Operating Systems – cont.
Process synchronization
The synchronization of processes in distributed systems is difficult because of the unavailability of shared memory
It needs to synchronize processes running on different computers when they try to concurrently access a shared resource
This is the mutual exclusion problem as in classical operating systems
4/7/2019
COP5611

45. Issues in Distributed Operating Systems – cont.
Resource management
Resource management needs to make both local and remote resources available to uses in an effective manner
Data migration
Distributed file system
Distributed shared memory
Computation migration
Remote procedure call
Distributed scheduling
4/7/2019
COP5611

46. Issues in Distributed Operating Systems – cont.
Security and protection
The same two issues as in classical operating systems need to be considered
Authentication
Authorization
4/7/2019
COP5611

47. Issues in Distributed Operating Systems – cont.
Structuring
The distributed operating system requires some additional constraints on the structure of the underlying operating system
The collective kernel structure
An operating system is structured as a collection of processes that are largely independent of each other
Object-oriented operating system
The operating system’s services are implemented as objects
4/7/2019
COP5611

48. The Client-Server Model
The client-server model provides one organization for a distributed system
The processes are organized into clients and servers
Clients request services from servers which provide services
A server is a process implementing a specific service
A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server’s reply
4/7/2019
COP5611

49. Clients and Servers General interaction between a client and a server.
4/7/2019
COP5611

50. An Example Client and Server (1)
4/7/2019
COP5611

51. An Example Client and Server (2)
A sample server.
4/7/2019
COP5611

52. An Example Client and Server (3)
A client using the server to copy a file.
4/7/2019
COP5611

53. Application Layering
A main issue in the client-server model is there is no clear distinction between a client and a server
Especially in the context of databases, the entire system is divided into three levels
The user-interface level
The processing level
The data level
4/7/2019
COP5611

54. Processing Level
The general organization of an Internet search engine into three different layers
1-28
4/7/2019
COP5611

55. Multitiered Architectures
Plausible client-server organizations
4/7/2019
COP5611

56. Multitiered Architectures – cont.
An example of a server acting as a client.
4/7/2019
COP5611

57. Modern Architectures
An example of horizontal distribution of a Web service.
4/7/2019
COP5611

58. Summary Next time Definition of a distributed system Motivations
Distributed system architectures
Distributed operating systems
DOS / NOS / Middleware
Technical issues in a distributed operating system
The client-server model
Next time
Communication in distributed systems
4/7/2019
COP5611