Distributed Software Engineering




1
To explain the advantages and disadvantages of
different distributed systems architectures
To discuss client-server and distributed object
architectures
To describe object request brokers and the principles
underlying the CORBA standards
To introduce peer-to-peer and service-oriented
architectures as new models of distributed computing.
Topics covered




2
Multiprocessor architectures
Client-server architectures
Distributed object architectures
Inter-organisational computing
Distributed systems



3
Virtually all large computer-based systems are
now distributed systems.
Information processing is distributed over
several computers rather than confined to a
single machine.
Distributed software engineering is therefore
very important for enterprise computing
systems.
System types
4

Personal systems that are not distributed and that are
designed to run on a personal computer or workstation.

Embedded systems that run on a single processor or
on an integrated group of processors.

Distributed systems where the system software runs on
a loosely integrated group of cooperating processors
linked by a network.
Distributed system characteristics

Resource sharing
–

Openness
–

Increased throughput by adding new resources.
Fault tolerance
–
5
Concurrent processing to enhance performance.
Scalability
–

Use of equipment and software from different vendors.
Concurrency
–

Sharing of hardware and software resources.
The ability to continue in operation after a fault has occurred.
Distributed system disadvantages

Complexity
–

Security
–

More effort required for system management.
Unpredictability
–
6
More susceptible to external attack.
Manageability
–

Typically, distributed systems are more complex than
centralised systems.
Unpredictable responses depending on the system
organisation and network load.
Distributed systems architectures

Client-server architectures
–

Distributed object architectures
–
7
Distributed services which are called on by clients.
Servers that provide services are treated differently
from clients that use services.
No distinction between clients and servers. Any
object on the system may provide and use services
from other objects.
Middleware



Software that manages and supports the different
components of a distributed system. In essence, it sits
in the middle of the system.
Middleware is usually off-the-shelf rather than specially
written software.
Examples
–
–
–
8
Transaction processing monitors;
Data converters;
Communication controllers.
Multiprocessor architectures




9
Simplest distributed system model.
System composed of multiple processes which
may (but need not) execute on different
processors.
Architectural model of many large real-time
systems.
Distribution of process to processor may be
pre-ordered or may be under the control of a
dispatcher.
A multiprocessor traffic control system
Sensor
processor
Sensor
contr ol
process
Tr aff ic flow
processor
Display
process
Tr aff ic light cont r ol
processor
Light
contr ol
process
Tr aff ic light s
Trafficflowsensorsand
cam er as
10
Opera tor c onsoles
Client-server architectures




11
The application is modelled as a set of services
that are provided by servers and a set of
clients that use these services.
Clients know of servers but servers need not
know of clients.
Clients and servers are logical processes
The mapping of processors to processes is not
necessarily 1 : 1.
A client-server system
c3
c2
c4
c1 2
c1 1
Ser v er p ro ce ss
s4
s1
c1
c1 0
c5
Clien t pr o cess
s2
c6
c7
12
s3
c9
c8
Computers in a C/S network
c1
CC1
c2
CC2
c3 , c4
CC3
Net wo rk
s1, s2
s3, s4
SC2
Ser v er
co m pu ter
SC1
Clien t
co m pu ter
c5 , c6 , c 7
c8 , c9
CC4
13
CC5
c1 0 , c1 1 , c1 2
CC6
Layered application architecture

Presentation layer
–

Application processing layer
–

Concerned with providing application specific functionality e.g.,
in a banking system, banking functions such as open account,
close account, etc.
Data management layer
–
14
Concerned with presenting the results of a computation to
system users and with collecting user inputs.
Concerned with managing the system databases.
Application layers
15
Thin and fat clients

Thin-client model
–

Fat-client model
–
16
In a thin-client model, all of the application
processing and data management is carried out on
the server. The client is simply responsible for
running the presentation software.
In this model, the server is only responsible for data
management. The software on the client
implements the application logic and the interactions
with the system user.
Thin and fat clients
17
Thin client model

Used when legacy systems are migrated to
client server architectures.
–

18
The legacy system acts as a server in its own right
with a graphical interface implemented on a client.
A major disadvantage is that it places a heavy
processing load on both the server and the
network.
Fat client model



19
More processing is delegated to the client as
the application processing is locally executed.
Most suitable for new C/S systems where the
capabilities of the client system are known in
advance.
More complex than a thin client model
especially for management. New versions of
the application have to be installed on all
clients.
A client-server ATM system
AT M
AT M
Acco un t ser ve r
Telep ro cessin g
m on it or
AT M
AT M
20
Cu st om er
acco un t
dat abase
Three-tier architectures



21
In a three-tier architecture, each of the
application architecture layers may execute on
a separate processor.
Allows for better performance than a thin-client
approach and is simpler to manage than a fatclient approach.
A more scalable architecture - as demands
increase, extra servers can be added.
A 3-tier C/S architecture
22
An internet banking system
Client
Client
HT T P int erac tion
S QL query
Account ser vice
provision
Client
Client
23
Dat abase ser ver
W eb server
S QL
Cust om er
account
dat abase
Use of C/S architectures
24
Arc hit ec tu re
A ppli cat io ns
T wo- ti er C/ S
arch it ec tu re w ith
th in c li en ts
L egacy sy stem app li ca ti on s whe re sep a ra ti ng app li ca ti on p roce ssing and
da ta m anage m en t i s im p rac ti ca l.
Co m pu tati ona ll y -in tens ive app li ca ti ons su c h as co m p il ers w it h littl e o r
no da ta m an a ge m en t.
Da ta-in tens iv e app li ca ti on s (bro w sing a nd que ry ing) w it h littl e or no
app li ca ti on proc e ssing .
T wo- ti er C/ S
arch it ec tu re w ith
fat c li en ts
App li ca ti on s whe re app li ca ti on p roce s sing i s p rov ided by o ff -the -sh e lf
so ft wa re (e .g. M ic ro sof t Exc e l) on the cli en t.
App li ca ti on s whe re co m pu tati ona ll y- in ten si ve p roce ssing of da ta (e .g.
da ta v isua lis ati on) i s requ ir ed .
App li ca ti on s w it h relati ve ly stab le end -use r func ti ona lit y u sed in an
env ir on m en t w it h we ll -e stab li sh e d sy st em m an a ge m en t.
T hre e -ti er o r
m u lti -ti er C/ S
arch it ec tu re
L arg e sca le app lic ati ons w ith hund reds or t housand s o f cli en ts
App li ca ti on s whe re bo th the da ta a nd the app li ca ti on a re vo latil e.
App li ca ti on s whe re da ta fr o m m u lti p le sour c es a re in teg rated .
Distributed object architectures




25
There is no distinction in a distributed object
architectures between clients and servers.
Each distributable entity is an object that provides
services to other objects and receives services from
other objects.
Object communication is through a middleware system
called an object request broker.
However, distributed object architectures are more
complex to design than C/S systems.
Distributed object architecture
o1
o2
S ( o1 )
o3
S ( o2 )
o4
S ( o3 )
S ( o4 )
Object req uest br ok er
o5
S ( o5 )
26
o6
S ( o6 )
Advantages of distributed object architecture




27
It allows the system designer to delay decisions on
where and how services should be provided.
It is a very open system architecture that allows new
resources to be added to it as required.
The system is flexible and scaleable.
It is possible to reconfigure the system dynamically
with objects migrating across the network as required.
Uses of distributed object architecture


28
As a logical model that allows you to structure and
organise the system. In this case, you think about how
to provide application functionality solely in terms of
services and combinations of services.
As a flexible approach to the implementation of clientserver systems. The logical model of the system is a
client-server model but both clients and servers are
realised as distributed objects communicating through
a common communication framework.
A data mining system
Dat abase 1
Re po r t g en.
I nt eg r ato r 1
Dat abase 2
Visu aliser
I nt eg r ato r 2
Dat abase 3
Disp lay
29
Data mining system



30
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.
CORBA


CORBA is an international standard for an Object
Request Broker - middleware to manage
communications between distributed objects.
Middleware for distributed computing is required at 2
levels:
–
–
31
At the logical communication level, the middleware allows
objects on different computers to exchange data and control
information;
At the component level, the middleware provides a basis for
developing compatible components. CORBA component
standards have been defined.
CORBA application structure
Applic ation
object s
Dom ain
f acilities
Object request br oker
COR BA ser vice s
32
Horizonta l C OR BA
f acilities
Application structure




33
Application objects.
Standard objects, defined by the OMG, for a
specific domain e.g. insurance.
Fundamental CORBA services such as
directories and security management.
Horizontal (i.e. cutting across applications)
facilities such as user interface facilities.
CORBA standards

An object model for application objects
–



34
A CORBA object is an encapsulation of state with a
well-defined, language-neutral interface defined in
an IDL (interface definition language).
An object request broker that manages
requests for object services.
A set of general object services of use to many
distributed applications.
A set of common components built on top of
these services.
CORBA objects




35
CORBA objects are comparable, in principle, to objects
in C++ and Java.
They MUST have a separate interface definition that is
expressed using a common language (IDL) similar to
C++.
There is a mapping from this IDL to programming
languages (C++, Java, etc.).
Therefore, objects written in different languages can
communicate with each other.
Object request broker (ORB)



36
The ORB handles object communications. It knows of
all objects in the system and their interfaces.
Using an ORB, the calling object binds an IDL stub that
defines the interface of the called object.
Calling this stub results in calls to the ORB which then
calls the required object through a published IDL
skeleton that links the interface to the service
implementation.
ORB-based object communications
o2
o1
S ( o1 )
S ( o2 )
IDL
stub
IDL
skelet o n
Object Requ est Br ok er
37
Inter-ORB communications




38
ORBs are not usually separate programs but are a set
of objects in a library that are linked with an application
when it is developed.
ORBs handle communications between objects
executing on the sane machine.
Several ORBS may be available and each computer in
a distributed system will have its own ORB.
Inter-ORB communications are used for distributed
object calls.
Inter-ORB communications
o1
o2
o3
S ( o1 )
S ( o2 )
S ( o3 )
S ( o4 )
IDL
stub
I DL
skele to n
IDL
stub
IDL
skele to n
Object Req uest Br ok er
Object Req uest Br ok er
Net wo rk
39
o4
CORBA services

Naming and trading services
–

Notification services
–

These allow objects to notify other objects that an
event has occurred.
Transaction services
–
40
These allow objects to discover and refer to other
objects on the network.
These support atomic transactions and rollback on
failure.
Inter-organisational computing



41
For security and inter-operability reasons, most
distributed computing has been implemented
at the enterprise level.
Local standards, management and operational
processes apply.
Newer models of distributed computing have
been designed to support inter-organisational
computing where different nodes are located in
different organisations.
Peer-to-peer architectures



42
Peer to peer (p2p) systems are decentralised systems
where computations may be carried out by any node in
the network.
The overall system is designed to take advantage of
the computational power and storage of a large
number of networked computers.
Most p2p systems have been personal systems but
there is increasing business use of this technology.
P2p architectural models

The logical network architecture
–
–

Application architecture
–

43
Decentralised architectures;
Semi-centralised architectures.
The generic organisation of components making up
a p2p application.
Focus here on network architectures.
Decentralised p2p architecture
n4
n6
n8
n7
n2
n1 3
n1 2
n3
n1 3
n9
n1
44
n5
n1 0
n1 1
Semi-centralised p2p architecture
Disc over y
server
n4
n1
n3
n6
n5
n2
45
Service-oriented architectures


Based around the notion of externally provided
services (web services).
A web service is a standard approach to
making a reusable component available and
accessible across the web
–
46
A tax filing service could provide support for users to
fill in their tax forms and submit these to the tax
authorities.
A generic service


47
An act or performance offered by one party to
another. Although the process may be tied to a
physical product, the performance is
essentially intangible and does not normally
result in ownership of any of the factors of
production.
Service provision is therefore independent of
the application using the service.
Web services
48
Services and distributed objects







49
Provider independence.
Public advertising of service availability.
Potentially, run-time service binding.
Opportunistic construction of new services through
composition.
Pay for use of services.
Smaller, more compact applications.
Reactive and adaptive applications.
Services standards


Services are based on agreed, XML-based
standards so can be provided on any platform
and written in any programming language.
Key standards
–
–
–
50
SOAP - Simple Object Access Protocol;
WSDL - Web Services Description Language;
UDDI - Universal Description, Discovery and
Integration.
Services scenario


51
An in-car information system provides drivers with
information on weather, road traffic conditions, local
information etc. This is linked to car radio so that
information is delivered as a signal on a specific radio
channel.
The car is equipped with GPS receiver to discover its
position and, based on that position, the system
accesses a range of information services. Information
may be delivered in the driver’s specified language.
Automotive system
52
Descargar

INTRODCUTION - University of Texas at Arlington