1. Distributed Systems CS Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520)

2. Introduction
Chapter 1

3. Local Area Network: LAN
Computer Networks
A collection of computers communicating through an underlying network is called a computer network (in contrast to a single-processor system)
Local Area Network: LAN
10 to 1000 Mb/sec
Wide Area Network: WAN
64 kbps to gigabits/sec

4. 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.
A. Tanenbaum

5. Definition of a Distributed System (2)
“You know you have one when the crash of a computer you never heard of stops you from getting any work done.”
L. Lamport

6. Organization of a Distributed System
1.1
A distributed system organized as middleware. Note that the middleware layer extends over multiple machines.

7. Characteristics of a distributed System
 Differences between computers are hidden
 Communication between computers is hidden
 Internal organization of the system is hidden
 Single system image: interaction with the system is independent from location (and time)
 Ease of extension
 High availability

8. Examples of distributed Systems
Network of workstations + Pool of processors:
- Single file system (uniform naming scheme)
- Processors are allocated dynamically when needed (load sharing)
- System acts like a single-processor system
2. Workflow systems
- Orders arrive dynamically (e.g. via laptops, cellular phones)
- System assigns orders to the corresponding departments and initiates the needed business processes and users are unaware of the internal flow of orders
- System acts like a centralized database
WWW
- No need to know where documents are stored (at least in theory)
- Accessing remote documents is like accessing local ones

9. Goals of distributed Systems
Connecting users and resources:
 consequence: communication and collaboration (e.g. joint editing)
 issue: security
2. Openness
- Services should obey to standard rules specifying syntax and semantics
- Services are specified in general as interfaces in an interface definition language
- IDL includes rather syntax and no semantics
- Specification of interface should be completed and neutral (w.r.t. implementation)
- Openness promotes interoperability and portability
Transparency (see next slides)
Scalability (see next slides)

10. 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 is 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.
Degree of transparency: More transparency means less performance
In reality: Systems are transparent only to certain degree

11. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Network Provider

12. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Company A
Company B
Network Provider

13. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Company A
Company B
Network Provider

14. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Company A
Company B
Network Provider

15. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Company A
Company B
Network Provider

16. Network
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Applications
Operating System & Hardware
Network Provider

17. Network (e.g. Telecom) Clients Software Developers e.g. Company
Applications
AEM: Availability Enhancing Middleware
Operating System & Hardware
Network Provider

18. Scalability Problems
A system is scalable when it is easy to extend without loss of performance.
Concept
Example
Centralized services
A single server for all users (e.g. a single DNS server)
Centralized data
A single on-line telephone book (e.g. single table in a frequently used database)
Centralized algorithms
Doing routing based on complete information
Examples of scalability limitations.
Centralized solutions tend to be non-scalable
Decentralized solutions promote scalability

19. Decentralized Algorithms
No machine has complete information about the system state
Machines make decisions based only on local information
Failures of one machine does not ruin the whole algorithm
No assumption on the existence of a global time

20. Scaling Techniques (1) Ways to solve scalability problems:
Hide communication latencies (geographical scalability)
Use of distribution (in order to avoid bottlenecks)
Use of replication
Ad a)
Batch processing: Asynchronous communication instead of synchronous communication
Interactive applications: Reduce overall communication
Example: Move part of computation from server to client (e.g. Java applet)

21. Scaling Techniques (2) 1.4 The difference between letting: a server or
a client check forms as they are being filled

22. Scaling Techniques (3) Ad b) Use of distribution 1.5
An example of dividing the DNS name space into zones.
Clientserver Z1: nl.vu.cs.fluit
Server Z1  Client: address of Z2
Clientserver Z2: vu.cs.fluit …

23. Scaling Techniques (4) Ad c) Use of replication Replication raises:
Availability: in general the primary goal
Performance:
By balancing the load among replicas
By locating replicas close to users/clients
Example: Caching (e.g. browser cache for used WWW pages)
Main issue in connection with replication: consistency

24. Different basic organizations and memories in distributed computer
Hardware Concepts
1.6
Different basic organizations and memories in distributed computer
systems

25. A bus-based multiprocessor.
Multiprocessors (1)
1.7
A bus-based multiprocessor.
Issues:
Scalability
Cache consistency

26. Multiprocessors (2) A crossbar switch: n2 switches!
1.8
A crossbar switch: n2 switches!
An omega switching network: less switches

27. Homogeneous Multicomputer Systems
1-9
Grid
Hypercube

28. Software Concepts An overview of DOS (Distributed Operating 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
An overview of
DOS (Distributed Operating Systems)
NOS (Network Operating Systems)
Middleware

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

30. Multiprocessor Operating Systems (1)
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.

31. 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.

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

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

34. Synchronization point Reliable comm. guaranteed?
Multicomputer Operating Systems (3)
Synchronization point
Send buffer
Reliable comm. guaranteed?
Block sender until buffer not full
Yes
Not necessary
Block sender until message sent No
Block sender until message received
Necessary
Block sender until message delivered
Relation between blocking, buffering, and reliable communications.

35. Distributed Shared Memory Systems (1)
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
Replicating all pages:
 Coherence protocols:
a) strong (transparent)
b) weak (not transparent)

36. Distributed Shared Memory Systems (2)
Page size: small  more communication overhead
large  less communication, but false sharing may occur
1.18
False sharing of a page between two independent processes.
Problem with DSM: not as efficient as expected

37. Network Operating System - NOS (1)
1-19
General structure of a network operating system.
Main features: Independent operating systems
Services for accessing remote resources

38. Network Operating System (2)
1-20
Two clients and a server in a network operating system.

39. Network Operating System (3)
1.21
Different clients may mount the servers in different places.

40. NOS vs DOS (3) Distributed operating system: Fully transparent
For homogeneous systems  computers are not independent More secure, but less scalable and less open
Network operating system:
Not transparent
For heterogeneous system  computers are independent
Easier to extend (e.g. adding a new node in the Internet)
Both do not really qualify as a distributed system!!!
 Solution: Middleware atop of a NOS hiding heterogeneity and improving transparency

41. Positioning Middleware
1-22
General structure of a distributed system as middleware.
Middleware Services: rather a functionally complete set of services in order to hide heterogeneity, direct access to NOS is discouraged.

42. 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.
 implementations of the middleware should not use the NOS-provided protocols

43. 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.

44. General interaction between a client and a server.
Clients and Servers
1.25
General interaction between a client and a server.

45. An Example Client and Server (1)
The header.h file used by the client and server.

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

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

48. Processing Level
1-28
The general organization of an Internet search engine into three different layers

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

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

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