Transcript CP3397 Design of Information Networks Lecture 1 An

CP3397
Network Design and Security
Lecture 1
An Introduction to
Distributed Information Systems
Contents
The Driving Forces
Elements of Distributed IS
Distributed IS Infrastructure
Transparency
Data Content & Presentation
Changes in Computing
Technology
Decline in centralised mainframe systems
Growth of “desktop computing”
Growth of Local & Wide Area Networks
Growth of Client/Server Systems
Relative decline of hardware companies
Increasing dominance of software companies
 especially Microsoft (PC & Server software)
Multimedia information requirements
Increasingly interactive user interfaces
Ever cheaper components
Development of middleware
Internet and Web technology
Changes in Business
Environments
Business structures can be characterised in terms
of 3 components

Complexity
 the degree of differentiation of activities

Formalisation
 the extent to which processes and jobs are standardised and
structured

Centralisation
 the degree to which decision-making is concentrated at single
points
Centralised Organisations
Highly structured
Structure directly reflects function
Multi-level hierarchy
Multiple tiers of management
Centralised authority
Top-down control
Decentralised Organisations
Flatter structure

fewer tiers of “middle management”
Less formalised structures
More local autonomy and accountability
Customer/market oriented
More responsive to changing business
environment

more like bottom-up
Advantages of Distributed
Systems
Improved Flexibility
Local Autonomy
Increased Reliability & Availability
Improved Performance
Isolation of Security Problems
Improved Flexibility
Resources can be located within the organisation
where they will be most effective utilised
Resources can be relocated as required
Components can be added and upgraded
independently and incrementally
Provides Scalability

the ability of an infrastructure to grow to meet
increasing demand with minimal disruption
Local Autonomy
Allows domains of control to be established
A domain of control covers purchasing,
ownership, IT budgets, operating priorities,
IS development, IT management, etc
Domains allow decisions about IT resources
to be made where they are utilised
Autonomy is recognition of the distributed
nature of many organisational activities
Increased Reliability & Availability
Component failure in a centralised system can
mean total systems failure
In a distributed system, component failure is
limited to that component

usually means only limited service outage for limited
group of users
Component replication can provide fault tolerance

failure modes can be planned for
Improved Performance
Centralised systems can be performance
bottlenecks
Increased transaction processing or
volumes of data result in performance
degradation
Distributed systems allow partitioned
services
Isolation of Security Problems
A single centralised system provides a focus for
security breaches


Any breach potentially compromises the whole system
“Denial of service” attacks have maximum effect
In a distributed system security can be addressed
via domains of control

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Limits impact of attacks
Each security domain can have varying degrees of
authentication, access control and auditing
Disadvantages of Distributed
Systems
More difficult to manage and secure
Reduced reliability and availability
Shortage of skilled support and
development staff
More difficult to manage and
secure
Centralised systems are inherently easier to
manage

because there’s only one of them
Centralised systems are inherently easier to
secure

Only have to have to worry about one point of
vulnerability
Distributed systems introduce complexity

require more resources and cost more to run
Reduced reliability and
availability
Centralised systems now benefit from years
of experience and development


in terms of physical, operational and
environmental conditions
usually single vendor systems
Distributed systems, are inherently more
complex



more to go wrong
usually heterogeneous systems
unpredictable interoperability
Staff Shortages
Distributed systems suffer from a loss of
economies of scale


require more staff to achieve same support
require higher “skill-mix” in staff
Vendor support not yet comparable to centralised
systems

support from many vendors required
 no one vendor has “big picture”


systems integrator support also necessary
problems often arise at interfaces between subsystems
Elements of a Distributed IS
An Information System (IS) captures, stores,
processes and communicates data
Information Technology (IT) combines computing
and communications technology to facilitate IS
Three main elements of Distributed IS

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Distributed IT Infrastructure (Processing)
Data
Presentation
Distributed IT Infrastructure
The components that make up the
“physical” system

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Primarily concerns processing devices (e.g., PCs,
servers) but includes
storage (on and off-line, RAID)
Communication & networks
Operating systems (usually NOS)
DBMSs (e.g., relational, object)
Services and utilities
Assumes a model for process interaction
 e.g., master/slave or client/server
Data
Structures required for data storage
Organisational data model
Different levels of representation

Data can be represented as objects
 data and associated processes
 an object could be an RDBMS, an audio file, etc

Database model of data is useful
 Physical - storage and retrievability issues
 Logical - data model - entities/objects, attributes,
relationships
 User View - what each user needs to manipulate
Presentation
The way in which data is made visible to the user
and interactions are handled
User Interface


“look-and-feel”
behaviour (consistency and predictability)
Presentation Management

User Interface Management
 Display Services - e.g., interface devices
 Dialogue Control - e.g., event handling
 API - e.g., interface libraries
Distributed Support Services
Much functionality in a DIS can be provided as
distributed services that are an integral part of
the IT infrastructure:
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Distributed Presentation
Distributed Processing
Remote Data Access
Remote File Access
Distributed Data Management
Distributed Object Management
Distributed Presentation Services
Distributed Presentation enables the presentation
component of an application to be either:
wholly located on the client

e.g., Microsoft Windows
split between the client and the server


requires a presentation protocol to enable
communication between client and server
e.g., X-Windows
Server
Client
PROC
DATA
PRES
Presentation Protocol (e.g. X-Windows)
Server
Client
PRES
PROC
DATA
PRES
Network
Distributed
Presentation Services
Distributed Processing Services
Co-operating processing objects are distributed
across both client and server elements
At the client side a user agent conceals the
complexity of client/server interactions

e.g. identifying the required server object and routing
requests
At the server side a server wrapper receives client
requests and passes the request to the
appropriate server object
Remote Inter-Process
Communication Protocol
Client
Server
PRES
PROC
PROC
DATA
Network
Distributed Processing
Services
Client A
PRES
PROC
Server
PROC
Client B
PRES
DATA
A Distributed
Processing System
PROC
Network
Remote Data Access Services
Presentation and processing are client side
Data components reside on server(s)

managed by a DBMS
A data manipulation language is used to retrieve
data (e.g., SQL)


client submit request
server receives it and returns results set
Data access is at the record level
Minimises network traffic

only processed data is communicated
Remote Data
Access Service
Server
DBMS
Records
DATA
Request
Client
Record Set
Data
Base
PRES
PROC
Network
Remote File Access Services
Presentation and processing are client side
Data objects reside on server(s)

server implements some form of file service
Data access is at the file level
When a client requests an object:
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a user agent (redirector) resolves the location (i.e.,
local/remote)
if local, request is passed to local OS
If remote, initiates communications and issues
appropriate request to server
 Server wrapper handles request
Remote File
Access Service
Server
DBMS
File Block
DATA
Client
Request
File Block
File
System
PROC
DATA
Network
Distributed Data Management
Services
Remote Data Access (RDA) services provide
access to remote databases
When multiple RDA servers exist each may only
manage part of the overall data set

Procssing Objects would need to know the location of
all data objects and the server(s) they are on - this is
problematic
A Distributed Data Management Service
“conceals” this information and provides
transparent access

data appears to be in one centralised database
Logically Centralised
but
Physically Distributed
Client
DBMS
PROC
DATA
Server
DATA
DBMS
DATA
Distributed Data
Management
Data
Base
Network
Distributed Object Management
Services
An object (in this context) is an entity with a
clearly defined interface and services invoked by
sending messages to it
Objects closely couple data and operations
Objects act as both clients and servers
DISs can use objects as a unit of distribution
A Distributed Object Management Service gives
the illusion that distributed objects occupy a
single “object space”
CORBA is an example DOM service
Developing Distributed IS
Reusable, generic services simplify DIS
development

Consistent with RAD, ODP, OO approaches
Service functionality is available via well-defined
standard APIs

Developers can concentrate on interfacing with existing
services
Services can be selected to meet user
requirements

e.g., cost, performance, availability, scalability