Systems Software
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Topics
• Operating System
• Distributed systems
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Objectives
• Become familiar with the functions
provided by operating systems
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Introduction
• The biggest and fastest computer in the world
is of no use if it cannot efficiently provide
beneficial services to its users.
• Users see the computer through their
application programs. These programs are
ultimately executed by computer hardware.
• System software-- in the form of operating
systems and middleware-- is the glue that
holds everything together.
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Operating Systems
• The evolution of operating systems has paralleled
the evolution of computer hardware.
– As hardware became more powerful, operating systems
allowed people to more easily manage the power of the
machine.
• In the days when main memory was measured in
kilobytes, and tape drives were the only form of
magnetic storage, operating systems were simple
resident monitor programs.
– The resident monitor could only load, execute, and
terminate programs.
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Operating Systems
• In the 1960s, hardware has become powerful
enough to accommodate multiprogramming, the
concurrent execution of more than one task.
• Multiprogramming is achieved by allocating each
process a given portion of CPU time (a timeslice).
• Interactive multiprogramming systems were
called timesharing systems.
– When a process is taken from the CPU and replaced by
another, we say that a context switch has occurred.
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Operating Systems
• Today, multiprocessor systems have become
commonplace.
– They present an array of challenges to the operating
system designer, including the manner in which the
processors will be synchronized, and how to keep their
activities from interfering with each other.
• Tightly coupled multiprocessor systems share a
common memory and the same set of I/O devices.
– Symmetric multiprocessor systems are tightly coupled and
load balanced.
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Operating Systems
• Loosely coupled multiprocessor systems have
physically separate memory.
– These are often called distributed systems.
– Another type of distributed system is a networked system,
which consists of a collection of interconnected,
collaborating workstations.
• Real time operating systems control computers that
respond to their environment.
– Hard real time systems have tight timing constraints, soft
real time systems do not.
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Operating Systems
• Personal computer operating systems are designed
for ease of use rather than high performance.
• The idea that revolutionized small computer
operating systems was the BIOS (basic input-output
operating system) chip that permitted a single
operating system to function on different types of
small systems.
– The BIOS takes care of the details involved in addressing
divergent peripheral device designs and protocols.
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Operating Systems
• Two operating system components are crucial:
The kernel and the system programs.
• As the core of the operating system, the kernel
performs scheduling, synchronization, memory
management, interrupt handling and it provides
security and protection.
• Microkernel systems provide minimal functionality,
with most services carried out by external programs.
• Monolithic systems provide most of their services
within a single operating system program.
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Operating Systems
• Microkernel systems provide better security,
easier maintenance, and portability at the expense
of execution speed.
– Examples are Windows 2000, Mach, and QNX.
– Symmetric multiprocessor computers are ideal platforms
for microkernel operating systems.
• Monolithic systems give faster execution speed,
but are difficult to port from one architecture to
another.
– Examples are Linux, MacOS, and DOS.
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Operating Systems
• Process management lies at the heart of operating
system services.
– The operating system creates processes, schedules their
access to resources, deletes processes, and deallocates
resources that were allocated during process execution.
• The operating system monitors the activities of each
process to avoid synchronization problems that can
occur when processes use shared resources.
• If processes need to communicate with one another,
the operating system provides the services.
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Operating Systems
• The operating system schedules process execution.
• First, the operating system determines which process
shall be granted access to the CPU.
– This is long-term scheduling.
• After a number of processes have been admitted, the
operating system determines which one will have
access to the CPU at any particular moment.
– This is short-term scheduling.
• Context switches occur when a process is taken from
the CPU and replaced by another process.
– Information relating to the state of the process is preserved
during a context switch.
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Operating Systems
• Short-term scheduling can be nonpreemtive or
premptive.
• In nonpreemptive scheduling, a process has use of
the CPU until either it terminates, or must wait for
resources that are temporarily unavailable.
• In preemptive scheduling, each process is allocated
a timeslice. When the timeslice expires, a context
switch occurs.
• A context switch can also occur when a higher-
priority process needs the CPU.
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Operating Systems
• Four approaches to CPU scheduling are:
– First-come, first-served where jobs are serviced in arrival
sequence and run to completion if they have all of the resources
they need.
– Shortest job first where the smallest jobs get scheduled first.
(The trouble is in knowing which jobs are shortest!)
– Round robin scheduling where each job is allotted a certain
amount of CPU time. A context switch occurs when the time
expires.
– Priority scheduling preempts a job with a lower priority when a
higher-priority job needs the CPU.
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Intro to Distributed Software Systems
Architectures
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Objectives
• 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
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Topics covered
• Multiprocessor architectures
• Client-server architectures
• Distributed object architectures
• Inter-organisational computing
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Distributed systems
• 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.
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System types
• 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.
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Distributed system characteristics
• Resource sharing
– Sharing of hardware and software resources.
• Openness
– Use of equipment and software from different
vendors.
• Concurrency
– Concurrent processing to enhance performance.
• Scalability
– Increased throughput by adding new resources.
• Fault tolerance
– The ability to continue in operation after a fault has 21
Distributed system disadvantages
• Complexity
– Typically, distributed systems are more complex
than centralised systems.
• Security
– More susceptible to external attack.
• Manageability
– More effort required for system management.
• Unpredictability
– Unpredictable responses depending on the system
organisation and network load. 22
Distributed systems architectures
• Client-server architectures
– Distributed services which are called on by
clients. Servers that provide services are
treated differently from clients that use
services.
• Distributed object architectures
– No distinction between clients and servers.
Any object on the system may provide and
use services from other objects.
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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
– Transaction processing monitors;
– Data converters;
– Communication controllers.
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Multiprocessor architectures
• 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.
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A multiprocessor traffic control system
Sens or T lo
raffic f w Traffic ligh t con trol
pro cess or pro cess or pro cess or
Sens or Ligh t
control Disp lay control
pro cess pro cess pro cess
Traffic ligh ts
f low nd
Trafic f sensors a
cameras Op erator co ns oles
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Client-server architectures
• 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.
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A client-server system
c2 c3 c4
c12
c11
Server pro cess
c1 s1 s4
c10
c5
Clien t pro cess
s2 s3 c9
c6
c7 c8
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Computers in a C/S network
c1 c2 c3, c4
CC1 CC2 CC3
s 1, s2 Netw ork Server
s 3, s4
comp uter
SC2 SC1
Clien t
comp uter
c5, c6 , c7 c8, c9 c10, c1 1, c1 2
CC4 CC5 CC6
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Layered application architecture
• Presentation layer
– Concerned with presenting the results of a
computation to system users and with collecting user
inputs.
• 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
– Concerned with managing the system databases.
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Application layers
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Thin and fat clients
• Thin-client model
– 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.
• Fat-client model
– In this model, the server is only responsible
for data management. The software on the
client implements the application logic and 32
the interactions with the system user.
Thin and fat clients
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Thin client model
• Used when legacy systems are migrated to
client server architectures.
– 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.
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Fat client model
• 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. 35
A client-server ATM system
ATM
ATM Acco un t s erver
T ele- Cu stomer
pro cess in g accoun t
mo nito r datab as e
ATM
ATM
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Three-tier architectures
• 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 fat-client approach.
• A more scalable architecture - as demands
increase, extra servers can be added.
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A 3-tier C/S architecture
38
An internet banking system
Clien t HTTP in teraction
Web serv er Database server
Clien t
SQL q uery
Acco un t s ervice Cu stomer
SQL accoun t
pro vision
datab as e
Clien t
Clien t
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Use of C/S architectures
Architecture Applications
Two-tier C/S Legacy system applications where separating application processing and
architecture with data management is impractical.
thin clients Computationally-intensive applications such as compilers with little or
no data management.
Data-intensive applications (browsing and querying) with little or no
application processing.
Two-tier C/S Applications where application processing is provided by off-the-shelf
architecture with software (e.g. Microsoft Excel) on the client.
fat clients Applications where computationally-intensive processing of data (e.g.
data visualis ation) is required.
Applications with relatively stable end-user functionalit y used in an
environment with well-established system management.
Three-tier or Large scale applications with hundreds or thousands of clients
multi-tier C/S Applications where both the data and the application are volatile.
architecture Applications where data from multiple sources are integrated.
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Distributed object architectures
• 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.
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Distributed object architecture
o1 o2 o3 o4
S (o1 ) S (o2 ) S (o3 ) S (o4 )
Ob ject requ es t b ro ker
o5 o6
S (o5 ) S (o6 )
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Advantages of distributed object architecture
• 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.
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Uses of distributed object architecture
• 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
client-server 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 44
communication framework.
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:
– 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. 45
CORBA application structure
Ap plication Do main Ho rizo ntal CORBA
ob jects facilities facilities
Ob ject requ es t b ro ker
CORBA s ervices
46
Application structure
• 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.
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CORBA standards
• An object model for application objects
– 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 48
CORBA objects
• 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.
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Object request broker (ORB)
• 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.
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ORB-based object communications
o1 o2
S (o1 ) S (o2 )
IDL IDL
s tu b s keleto n
Ob ject Reques t Brok er
51
Inter-ORB communications
• 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. 52
Inter-ORB communications
o1 o2 o3 o4
S (o1 ) S (o2 ) S (o3 ) S (o4 )
IDL IDL IDL IDL
s tu b s keleto n s tu b s keleto n
Ob ject Requ es t Brok er Ob ject Requ es t Brok er
Netw ork
53
CORBA services
• Naming and trading services
– These allow objects to discover and refer to
other objects on the network.
• Notification services
– These allow objects to notify other objects
that an event has occurred.
• Transaction services
– These support atomic transactions and
rollback on failure. 54
Distributed systems: Key points
• Distributed systems support resource sharing,
openness, concurrency, scalability, fault tolerance
and transparency.
• Client-server architectures involve services being
delivered by servers to programs operating on
clients.
• User interface software always runs on the client
and data management on the server. Application
functionality may be on the client or the server.
• In a distributed object architecture, there is no
distinction between clients and servers. 55
Distributed systems: Key points
• Distributed object systems require middleware to
handle object communications and to add and
remove system objects.
• The CORBA standards are a set of middleware
standards that support distributed object
architectures.
• Peer to peer architectures are decentralised
architectures where there is no distinction
between clients and servers.
• Service-oriented systems are created by linking
software services provided by different service 56
suppliers.
End of Chapter 8
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