Distributed Systems: The Overall Architecture Chapter 5 Information Systems Management In Practice 5E McNurlin & Sprague

Distributed Systems: The Overall Architecture

Chapter 5

Information Systems Management In Practice 5EMcNurlin & Sprague

Copyright 2002 by Prentice Hall, Inc.

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Attributes of Distributed Systems

Degree to which a system is distributed can be determined by answering four questions:

1. Where is the processing done? 2. How are the processors and other

devices interconnected?3. Where is the information stored?4. What rules or standards are used?


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Attributes of Distributed Systems: Where is the processing done?

Distributed processing is the ability for more than one interconnected processor to be operating at the same time. Goal: move the appropriate processing as close to the user as possible and to use computers that do part of the job the best. Permits interoperability-capability of different computers using different O.S. on different networks to work together on tasks. Two forms of interoperability (capability for different machines to work together on tasks):

communication between systems two-way flow between user applications


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Connectivity among processors means that each processor in a distributed system can send data and messages to any other processor through electronic communication links. Desirable to have at least two independent paths between two nodes to provide automatic alternate routing.

Attributes of Distributed Systems: How are the processors and other

devices interconnected?


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Distributed databases either divide a database and distribute its portions throughout a system without duplicating the data or store the same data at several different locations, with one site containing the master file.

Issue: synchronization of data

Attributes of Distributed Systems: Where is the information stored?


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System wide rules mean that an operating discipline for the distributed system has been developed and is enforced at all times. “Open systems” concept-mix products from vendors using open std. Based on “open-systems” - standardized interfaces that allow products to inter-operate across multi-vendor networks, e.g., UNIX OS. Define the communication between nodes, security, data accessibility, program and file transfers, and common operating procedures. API’s - Application program interfaces: define the way to present data to another system component

Attributes of Distributed Systems: What rules or standards are used?


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When to Distribute Computing Responsibilities

1. Are the operations interdependent? One operation knows what other is doing. If yes,

their planning, development, resources, and operations must be centralized.

2. Are the businesses really homogenous? Have a lot in common; e.g., IT needs for a fast-

food franchise, processing distributed, planning and hardware selection centralized.

3. Does the corporate culture support decentralization? A decentralized business hierarchy might still

centralize finance, HR, and systems planning.


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Two Guiding Frameworks for DS: Organizational

1. Organizational - (see Figure 5-1): Locate processing power and dB at each level in the organization. Top three levels are traditional domain of IS, where computers resided in the past. Bottom three levels are where the bulk of employees are. Intent: give autonomy and decision-making power to better serve customers.


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2. Technical - (see Figures 5-2 & 5-3): Migration of computer power to end users will be driving force for network-based IS. SUMURU - single user, multiple user, remote utility.

A. Processors: SU: single user, stand-alone and connected to LNs;

clients MU: multiple user, serve local groups of users; server.

Also heavy duty computation for SU’s, backups for MU’s, program libraries for SU’s, and dB management.

RU: remote utility, heavy-duty computing, corporate dB management, corporation mainframes and value-added network services

Two Guiding Frameworks for DS: Technical


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B. Networks: LN: local networks, high-speed

information transfer, LAN RN: remote networks, lower transfer

speeds, WAN, MAN, Internet



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C. Services that this network architecture provides: access file transfer e-mail

D. Standards needed in three areas: OS communication protocols: TCP/IP DBMS: SQL



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Types of Distributed System: Host-Based Hierarchy

Six distributed architectures:1. Host-Based Hierarchy (see Figure 5-4): A central, controlling mainframe at the top, PCs at the bottom, minicomputers in between. Master/slave. First data processing distributed system. Host computer central, controlling component; terminals are access systems. Where is the data stored?: could be at any level.


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Types of Distributed System: Decentralized Stand-Alone Systems

2. Decentralized Stand-Alone Systems

(see Figure 5-5): Decentralized but does not form a distributed system, “islands of computing,” little data flow amongst, except upward to corporate systems.


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3. Peer-to-Peer LAN-Based Systems(see Figure 5-6):

No hierarchy, “peer-to-peer” communications, interconnecting LANs rather than hierarchical communications through a central hub. No “superior” computer.

Types of Distributed System: Peer-to-Peer LAN-Based Systems


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4. Hybrid Enterprisewide Systems(see Figure 5-7): Combination hierarchy (mainframe-based, favored for corporate computing) and LAN-based (favored by departments). The structure of choice for many years. Allows company to link “automation islands” and retain IT investments, begin to automate business processes (cooperative processing).

Types of Distributed System: Hybrid Enterprisewide Systems


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Case Example: Northwest Airlines

Purpose: to audit all the redeemed tickets to calculate revenue accurately.

The system uses a cooperative processing architecture and integrates expert systems, image processing, relational databases, high resolution UNIX workstations, servers, and LANs.


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Case Example: Northwest Airlines (cont.)

Lessons learned: Benchmark and prototype new

technologies to verify vendors’ claims. An open architecture works on mission-

critical applications. Large distributed system projects need a

vendor coordinator. Use of CASE was mandatory.


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5. Client-Server Systems: The 90s version of distributed systems, splits the computing workload between the client, which is a computer used by the user and can sit on the desktop or be carried around, and the server, which answers the request. Trend: three-tierarchitecture (see Figure 5-9).

Types of Distributed Systems: Client-Server Systems


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Tier 3: the superserver/mainframe. Allows to include legacy applications, short-lived and fast-changing data, and integrity rules.

Tier 2: specialized servers, dedicated to housing databases or middleware-software to ease connection between client and server. Also, department-specific data that does not change often.

Tier 1: clients, connected through network.



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Figure 5-8 shows the possibilities for splitting work between clients and servers. Three components being split are: Presentation software: what user sees

(P) Application software (A) Data (D)



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Spectrum: Distributed: D, A, P at remote server. Leave

mainframe legacy in place w/ graphical output Remote presentation: P on client, D & A on

server. Also preserve legacy systems Distributed: P client, D server, A split C/S.

Complex, coordination to split application. Appropriate for A as packaged software (s/s,WP) client as corporate application. Also mobile computing.



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Spectrum: (cont.) Remote data mgt.: P & A on client, D on

server. Popular, all A in one place - fat client and takes advantage of large processing capacity on desktop.

Distributed database: P & A & some D on client, remaining D on server. complex, important for mobile computing, local and less dynamic data, fat client.



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Benefits of Client-Server Computing: Better access to information: improved

customer service, ability to communicate customer needs, and anticipate customer needs. Reduce cycle times and allow companies to compete better.

Empowered employees: blend autonomy of PCs with system wide rules and connectivity of traditional IS. Shifts focus of computing to users.



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Benefits of Client-Server Computing (cont.): Increases organizational flexibility: allows new

technology to be added more easily without affecting rest of system; streamlines work flow between functional areas; encourages people to work together via networks.

Drawbacks: not lower in cost than mainframes; entail

much coordination



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6. Internet-Centric Systems: “thin” client machines, run like telephones, no hard-disk but browser, memory, kb, modem. For consumer applications and hand-held devices.

E.g., for extranet applications, such as global broker-dealer

Types of Distributed Systems: Internet-Centric Systems


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Describing the Overall Architecture

To describe the IS architecture, look at the roles people and components play: Rows: Views must be taken into account

when building complex products: planner (scope statement), owner (model of the enterprise), designer (model of the information system), builder (technology model), subcontractor (description of the components), and user (functioning system).


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Describing the Overall Architecture

Columns: IS components - data models (what it is made of), functional models (how it works), and network models (where the components are located); represent physical manifestations of the system. Also people (who), time (when), and motivation (why).

Use of the framework: When IS users bring in a package that follows a data model inconsistent with the rules of the company, a lot will be spent fixing the package.


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The IT Infrastructure

Four types of investments (see Fig. 5-14):1. Strategic investments: aim to change the

way a firm competes, intends to increase revenues, longer term so income stream is difficult to estimate

2. Informational investments: provide a firm with information employees need to manage and control the organization, e.g., accounting, EIS, medium term horizon, depend on lower two levels


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3. Transactional investments: support operational management; intended to cut operating costs by substituting capital for labor, handle larger volume

4. Infrastructure investments: provide foundation of IT capability in a firm; supports applications

IT infrastructure includes networks, DW, computing facilities, EDI, applications sit on top


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IT investments can provide: Economies of scale (utility):

infrastructure cost as an administrative expense, minimize expense

Support for business programs (dependent): infrastructure treated as business expense, measured by short-term business benefits, infrastructure planning in current business plan



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Flexibility to meet changes in the marketplace (enabling): primary benefit long-term flexibility, intended to provide the foundation for changing direction in the future, IT cost seen as business investment



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Conclusion

Distributed systems dominate the computing environment

They create an enterprise architecture to cope with complexity of change.

IT infrastructures should provide the platform for allowing interconnection and organizational flexibility.

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Distributed Systems: The Overall Architecture Chapter 5 Information Systems Management In Practice 5E McNurlin & Sprague