CP3397Network Design and Security

Lecture 1

An Introduction to Distributed Information Systems

Contents

The Driving ForcesElements of Distributed ISDistributed IS InfrastructureTransparencyData Content & Presentation

Changes in Computing Technology

Decline in centralised mainframe systemsGrowth of “desktop computing”Growth of Local & Wide Area NetworksGrowth of Client/Server SystemsRelative decline of hardware companies Increasing dominance of software companies especially Microsoft (PC & Server software)

Multimedia information requirementsIncreasingly interactive user interfacesEver cheaper componentsDevelopment of middlewareInternet 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 structuredStructure directly reflects functionMulti-level hierarchyMultiple tiers of managementCentralised authorityTop-down control

Decentralised Organisations

Flatter structure fewer tiers of “middle management”

Less formalised structuresMore local autonomy and accountabilityCustomer/market orientedMore responsive to changing business environment more like bottom-up

Advantages of Distributed Systems

Improved FlexibilityLocal AutonomyIncreased Reliability & AvailabilityImproved PerformanceIsolation of Security Problems

Improved Flexibility

Resources can be located within the organisation where they will be most effective utilised Resources can be relocated as requiredComponents can be added and upgraded independently and incrementallyProvides Scalability the ability of an infrastructure to grow to meet

increasing demand with minimal disruption

Local Autonomy

Allows domains of control to be establishedA domain of control covers purchasing, ownership, IT budgets, operating priorities, IS development, IT management, etcDomains allow decisions about IT resources to be made where they are utilisedAutonomy is recognition of the distributed nature of many organisational activities

Increased Reliability & Availability

Component failure in a centralised system can mean total systems failureIn 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 bottlenecksIncreased transaction processing or volumes of data result in performance degradationDistributed 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 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 secureReduced reliability and availabilityShortage 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 sub-

systems

Elements of a Distributed IS

An Information System (IS) captures, stores, processes and communicates dataInformation Technology (IT) combines computing and communications technology to facilitate ISThree main elements of Distributed IS Distributed IT Infrastructure (Processing) Data Presentation

Distributed IT Infrastructure

The components that make up the “physical” system 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 storageOrganisational data modelDifferent 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 handledUser 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: 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

Distributed Presentation Services

PRESPROC DATA

Client

Network

Server

PRES

Client

PROC DATAPRES

Presentation Protocol (e.g. X-Windows)

Server

Distributed Processing Services

Co-operating processing objects are distributed across both client and server elementsAt 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

PROC DATA

Client

Server

Remote Inter-Process Communication Protocol

Network

Distributed Processing Services

PRES PROC

Client A

PROC DATA

Server

Network

A Distributed Processing System

PRES

Client B

PROC

PRES PROC

Remote Data Access Services

Presentation and processing are client sideData 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 levelMinimises network traffic only processed data is communicated

PRES

Client

PROC

DATA

DBMS

DataBase

Server

Request

Network

Remote Data Access Service

Records

Record Set

Remote File Access Services

Presentation and processing are client sideData objects reside on server(s) server implements some form of file service

Data access is at the file levelWhen a client requests an object: 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

Client

DATA

DBMS

Server

Request

Network

Remote File Access Service

File Block

File Block

FileSystem

PROC DATA

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

DATA

DBMS

DATA

DBMS

Client Logically Centralisedbut

Physically Distributed

Distributed Data Management

Network

Server

DataBase

PROC DATA

Distributed Object Management Services

An object (in this context) is an entity with a clearly defined interface and services invoked by sending messages to itObjects closely couple data and operations Objects act as both clients and serversDISs can use objects as a unit of distributionA Distributed Object Management Service gives the illusion that distributed objects occupy a single “object space”CORBA is an example DOM service

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

Developing Distributed IS