2. 1.1 Introduction and Definition
Definition of a Distributed System
Characteristics of Distributed System
Organization and Goals of A Distributed Systems
Hardware Concept and Software Concept
The Client-Server model
2
3. 1.1 Introduction and Definition
a distributed system is:
a collection of independent computers that appears to its
users as a single coherent system - computer
(Tanenbaum & Van Steen)
this definition has two aspects:
1. hardware: autonomous machines
2. software: a single system view for the users
Distributed system involves a collection of autonomous
computers, they are independent systems that posses
their own memory and CPU
Distributed system consists of multiple software
Components that are on multiple computers, but runs as a
single system
3
4. 1.1 Introduction and Definition
A distributed system contains multiple nodes that are
physically separated but linked together using network
The computers that are in distributed system can be
physically close together and connected by LAN
Or they can be geographically distant and connected by a
WAN
Distributed computing has become increasingly common
due advances that have made machines and networks
cheaper and faster
Examples of distributed
Distributed database
World wide web
Email
4
5. 1.1 Introduction and Definition
Distributed system example
Think about a large bank system with hundreds
of branch offices all over the country. Each office
has a master computer to store local accounts
and handle local transactions .In addition, each
computer has the ability to talk all other branch
computers and with central computer at
headquarter. If transactions can be done without
regarding to where costumer and account is
5
6. Characteristics of Distributed Systems
differences between the computers and the ways they
communicate are hidden from users
users and applications can interact with a distributed
system in a consistent and uniform way regardless of
location
distributed systems should be easy to expand and scale
a distributed system is normally continuously available,
even if there may be partial failures
6
7. 1.2 Organization and Goals of a Distributed System
to support heterogeneous computers and networks and
to provide a single-system view, a distributed system is
often organized by means of a layer of software called
middleware that extends over multiple machines
7
8. Goals of a distributed system: a distributed system
should
easily connect users with resources (printers, computers,
storage facilities, data, files, Web pages, ...)
reasons: economics, to collaborate and exchange
information
be transparent: hide the fact that the resources and
processes are distributed across multiple computers
be open
be scalable
Transparency in a Distributed System
a distributed system that is able to present itself to users
and applications as if it were only a single computer
system is said to be transparent
8
9. users and applications see the DS as a single coherent
system
different forms of transparency in a distributed system
Transparency Description
Access Hide differences in data representation
and how a resource is accessed
Location Hide where a resource is physically 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
9
10. Openness in a Distributed System
a distributed system should be open
So that different open systems would be able to interact and use
services from each other
interoperability
components of different origin can communicate
Support portability
components work on different platforms
We need well-defined interfaces
such services are often specified through interfaces often described using an
Interface Definition Language (IDL):
specify only syntax: the names of the functions, types of parameters, return
values
Distributed system should be independent from heterogeneity of the
underlying environment
Hardware, Software Platforms, and Languages
an Open Distributed System is a system that offers services according to
standard rules that describe the syntax and semantics of those services;
e.g., protocols in networks 10
11. Scalability in Distributed Systems
a distributed system should be scalable: there are three
dimensions
size: adding more users and resources to the system
geographically: users and resources may be far apart
administratively: should be easy to manage even if it
spans many administrative organizations
A distributed system is scalable if it will remain effective
when the number of resources and users is significantly
increased
but a scalable system may exhibit performance problems
11
12. scalability problems: performance problems caused by
limited capacity of servers and networks
Solution :Simply improving their capacity (e.g., by
increasing memory, upgrading CPUs, or replacing network
modules) is often a solution
Scaling Techniques
how to solve scaling problems for geographical scalability
three possible solutions: hiding communication latencies,
distribution, and replication
12
13. a. Hide Communication Latencies
try to avoid waiting for responses to remote service
requests
let the requester do other useful job
i.e., construct requesting applications that use only
asynchronous communication instead of synchronous
communication; when a reply arrives the application is
interrupted
good for batch processing and parallel applications but
not for interactive applications
for interactive applications, move part of the job to the
client to reduce communication; e.g. filling a form and
checking the entries
13
14. e.g., shipping code is now supported in Web applications using Java
Applets
(a) a server checking the correctness of field entries
(b) a client doing the job
14
15. b. Distribution
Taking a component, splitting into smaller parts, and
subsequently spreading them across the system. (E.g.
Domain Name System)
There are multiple name servers that map symbolic
name(hostname) to IP
In a URL, the part between the // and the following / is the
hostname of the server to which the client is going to send
the request
for details, see later in Chapter 4 - Naming
an example of dividing the DNS name space into zones
15
16. c. Replication
replicate components across a distributed system to
increase availability and for load balancing, leading
to better performance
that makes multiple copies of the same services or
data available at different machines
By placing a replica close to the place where it is
accessed, also reduces communication latency
caching (a special form of replication)
but, caching and replication may lead to consistency
problems (see Chapter 6 - Consistency and
Replication)
16
17. 1.3 Hardware and Software Concepts
Hardware Concepts
different classification schemes exist
multiprocessors - with shared memory
multicomputer - that do not share memory
can be homogeneous or heterogeneous
17
19. a bus-based multiprocessor
Multiprocessors - Shared Memory
the shared memory has to be coherent - the same value
written by one processor must be read by another
processor
performance problem for bus-based organization since the
bus will be overloaded as the number of processors
increases
the solution is to add a high-speed cache memory between
the processors and the bus to hold the most recently
accessed words; may result in incoherent memory
bus-based multiprocessors are difficult to scale even with
caches
two possible solutions: crossbar switch and omega
network
19
20. Crossbar switch
divide memory into modules and connect them to the
processors with a crossbar switch
at every intersection, a crosspoint switch is opened and
closed to establish connection
problem: expensive; with n CPUs and n memories, n2
switches are required
20
21. Omega network
use switches with multiple input and output lines
drawback: high latency because of several switching
stages between the CPU and memory
21
22. Homogeneous Multicomputer Systems
also referred to as System Area Networks (SANs)
could be bus-based or switch-based
bus-based
shared multi access network such as Fast Ethernet can
be used and messages are broadcasted
performance drops highly with more than 25-100 nodes
22
24. Heterogeneous Multicomputer Systems
most distributed systems are built on heterogeneous
multicomputer systems
the computers could be different in processor type,
memory size, architecture, power, operating system, etc.
and the interconnection network may be highly
heterogeneous as well
the distributed system provides a software layer to hide
the heterogeneity at the hardware level; i.e., provides
transparency
24
25. Software Concepts
OSs in relation to distributed systems
distributed OSs (DOS)
network OSs (NOS)
Middleware
25
26. Distributed Operating Systems
OS essentially tries to maintain a single, global view of the
resources it manages (Tightly-coupled OS)
used for multiprocessors and homogeneous multi
computers
Full transparency: users feel they are interacting with a big
system and are not aware of the existence of multiple
machines
26
general structure of a multicomputer operating system
27. Network Operating Systems(loosely coupled OS)
a collection of computers each running its own OS; they work together to
make their services and resources available to others via network
possibly heterogeneous underlying hardware
No transparency: users are aware of the multiplicity of the machines
explicitly login into remote machines, or copy files from other
machines
Access to remote services similar to local resources
general structure of a network operating system
27
28. Middleware
Most modern distributed systems are designed to provide a
level of transparency through a software layer on top of local
OSs
This software layer is called Middleware
28
general structure of a distributed system as middleware
29. Middleware
Middleware hides the differences between various
computers and the ways in which they communicate
It provides a single-system view
As a result, middleware facilitates the
integration and interaction of various
networked applications in a consistent and
uniform manner
29
30. different middleware models exist
through Remote Procedure Calls (RPCs) - calling a procedure
on a remote machine
distributed object invocation
Message-oriented middleware
(details later in Chapter 2 - Communication)
middleware services
access transparency: by hiding the low-level message
passing(calling a procedure or invoking an object
remotely)
Naming : such as a URL in the WWW
Distributed transactions: by allowing multiple read and
write operations to occur atomically(TPM)
Security: middleware authenticate access to data and
services 30
31. general interaction between a client and a server
1.4 The Client-Server Model
how are processes organized in distributed system
thinking in terms of clients requesting services from
servers
A server is a process implementing a specific service(file,
database server
A Client is a process that requests a service from server
and subsequently waiting for the server’s reply
31
32. 1.4 The Client-Server Model
Client-Server Architectures
how to physically distribute a client-server application
across several machines:
Two-Tired architecture
Physically distribute a client‐server application
across two machines:
1. A client machine containing only the programs
implementing (part of) the user-interface level
2. A server machine containing the rest, that is the
programs implementing the processing and data
level
Everything is handled by the server while the client is
essentially no more than dump terminal, possibly with a
pretty graphical interface 32
33. Two-tiered architecture: alternative client-server organizations
a) Place only terminal-dependent part of the user interface on
the client machine
b) place the entire user-interface software on the client side
c) move part of the application to the client, e.g. checking
correctness in filling forms
d) and e) are for powerful client machines
33
34. three tiered Architectures
Many client‐server applications are organized into three
layers
the user-interface level: consists of the program that allows end
users to interact with application; usually through GUIs, but not
necessarily
the processing level: contains the core functionality of the
application
the data level: contains the actual data that a client wants to
manipulate through the application
34
three tiered architecture: an example of a server acting as a client
35. the general organization of an Internet search engine into three
different layers
35
browser acts as an entry point to a site, passing requests to an
application server where the actual processing takes place, this
application server, in tum, interacts with a database server
36. an example of horizontal distribution of a Web service
Modern Architectures
vertical distribution: placing logically different components
on different machines
Dividing applications into a user-interface , processing
component and data and distribute across multiple
machines
horizontal distribution: physically split up the client or the
server into logically equivalent parts. e.g. Web server
36
37. Distributed Computing Systems: Cluster
Many distributed systems are configured for High-
Performance Computing
Cluster computing: a group of high-end systems
connected through a LAN
• Homogeneous: same OS, near-identical hardware
• Single managing node
37
38. Distributed Computing Systems: Grid
Grid computing: lots of nodes from everywhere
• Heterogeneous
•Dispersed across several organizations
•Can easily span a wide-area network
To allow for collaborations, grids generally use virtual
organizations.
• A group of users that will allow for authorization on
resource allocation
38
39. Distributed Computing Systems: Cloud
Cloud computing: make a distinction between four layers.
Hardware: processors, routers, power and cooling
systems. Customers normally never get to see these.
Infrastructure: deploys virtualization techniques. Evolves
around allocating and managing virtual storage devices
and virtual servers. (IaaS)
Platform: provides higher-level abstractions for storage
and such. (PaaS)
Application: actual applications, such as office suites, e.g.,
text processors, spreadsheet applications. (SaaS)
39
