Definition of a Distributed System (1)
A distributed system is:
A collection of independent
computers that appears to its
users as a single coherent
Definition of a Distributed System (2)
A distributed system organized as middleware.
Note that the middleware layer extends over multiple machines.
Examples of DS
1. The Internet
1. Vast interconnected collection of computer network of
many different types
1. Portion of Internet that is separately administrated and has a
boundary that can be configured to enforce local security
3. Mobile and ubiquitous computing
1. Small and portable computing device.
2. Laptops computers.
3. PDA, mobiles phone, pagers, video cameras and digital
4. Wearable device, such as smart watches
5. Devices embedded in appliances- hi-fi+washing machine
using one “universal remote control”.
Transparency in a Distributed System
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
Hide that a resource may be moved to another
location while in use
Replication Hide that a resource is replicated
Hide that a resource may be shared by several
Failure Hide the failure and recovery of a resource
Hide whether a (software) resource is in memory or
Different forms of transparency in a distributed system.
A system is described scalable
if remain effective when there is a
significant increase in the number
of resources and users
Date Computers Web servers
1979, Dec. 188 0
1989, July 130,000 0
1999, July 56,218,000 5,560,866
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.
Scaling Techniques (1)
The difference between letting:
a) a server or
b) a client check forms as they are being filled 8
Scaling Techniques (2)
An example of dividing the DNS name space into zones.
Homogenous Multicomputer System
Uniprocessor Operating Systems
Multiprocessor Operating System
Multicomputer Operating System
Distributed Shared Memory
Different basic organizations and memories in distributed computer
A bus-based multiprocessor.
b) Incoherent – while using the cache
a) A crossbar switch
b) An omega switching network
Homogeneous Multicomputer Systems
1. DS as resource manager
2. Provide Virtual Machine to hide the
heterogeneous nature of underlying hardware
System Description Main Goal
Tightly-coupled operating system for multi- Hide and manage
DOS processors and homogeneous hardware
Loosely-coupled operating system for Offer local
NOS heterogeneous multicomputers (LAN and services to remote
Additional layer atop of NOS implementing
An overview of
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
DOS (Distributed Operating Systems)
• Multiprocessor Operating System
– Manage the resource of multiprocessor
• Multicomputer Operating System
– Develop for homogenous multicomputer
Uniprocessor Operating Systems
• The goal- user and application sharing resources
• User mode & kernel mode- Controlled by OS.
• Separating applications from operating system code
through a microkernel-code for device register, switching
between process etc.
Multicomputer Operating Systems (1)
General structure of a multicomputer operating system
Multicomputer Operating Systems (2)
• Alternatives for blocking and buffering in message passing.
• Leading to 4 possible synchronization point.
Multicomputer Operating Systems (3)
Synchronization point Send buffer
Block sender until buffer not full (s1) Yes Not necessary
Block sender until message sent (s2) No Not necessary
Block sender until message received (s3) No Necessary
Block sender until message delivered (s4) No Necessary
Relation between blocking, buffering, and reliable
Distributed Shared Memory Systems (1)
a) Pages of address
b) Situation after CPU
1 references page
c) Situation if page 10
is read only and
replication is used
Distributed Shared Memory Systems (2)
False sharing of a page between two independent processes.
Network Operating System(1)
– Variety and difference in terms of:
• Computer hardware
• Operating system
• Programming language
• Implementation by different developers
• Constructed from a collection of uniprocessor
system using own OS.
• Connected to each other using computer
Network Operating System (2)
General structure of a network operating system.
Network Operating System (3)
Two clients and a server in a network operating system.
Generally maintain hierarchical file system.
Client can import/mount these file system.
Network Operating System (3)
Different clients may mount the servers in different places.
#showmount –e <server_name>
#mount /mygames <server_name>/private/games 27
– Programs written by different developers cannot communicate unless
use common standard
– Middleware applied to a software masking the heterogeneity of
hardware & software
General structure of a distributed system as middleware. 28
1. Communication facilities – message passing
eg: RPC & Distributed Object Invocation.
2. Naming- URL still works even the server has
3. Persistence – special facilities for storage.
4. Distributed Transactions- multiple read/write
5. Security – cannot rely on OS only.
Middleware and Openness
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.
Comparison between Systems
OS based OS
Degree of transparency Very High High Low High
Same OS on all nodes Yes Yes No No
Number of copies of OS 1 N N N
Basis for communication Messages Files Model specific
Resource management Per node Per node
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open
A comparison between multiprocessor operating systems,
multicomputer operating systems, network operating systems, and
middleware based distributed systems.
Clients and Servers
– General interaction between a client and a server.
– Request and reply behavior.
– Connectionless vs connection oriented protocol.
An Example Client and Server (1)
The header.h file used by the client and server.
An Example Client and Server (2)
A sample server.
An Example Client and Server (3)
A client using the server to copy a file. 35
The general organization of an Internet
search engine into three different layers
Multitiered Architectures (1)
• 2 types of machines –two-tiered architecture
» client just a dumb terminal(user interface)
» Server is handle everything
• Alternative client-server organizations (a) – (e).
Multitiered Architectures (2)
An example of a server acting as a client.
eg: transaction processing – next chap
• Multitier Client Server Architecture-
– Divide application into 3 components
– vertical distribution
• Modern Architecture is more consent about the distribution of
client and the servers.
– An example of horizontal distribution of a Web service.
• DS: autonomous computer+single coherent
• Hardware Concept
• Software Concept
• Client Server model-application layering
» Vertical distribution vs horizontal distribution
Do the problems on page 54 and 55.
Ask the student in a group to find a journal
about the distributed system. Work into group,
(5- 6 people) each group. Make a synopsis and
give a critique.