Distributed networks What they are, how they work, and how to choose the best for y&r okganizatibn

by STEVE WARNER

W ith much use of the term dis- tributed processing in the computer industry today a

number of questions immediately arise in the minds of many Data Processing Managers:

What is the true definition of distri- buted processing and what options do I have in constructing a net- work? What is the relevance of network architectures? How do I choose whether to distri- bute or centralize? If I decide to distribute what are the major steps to be taken?

In this article I have attempted to answers these questions and provide an initial guide to what is becoming

~1 Figure 1. H~~t~i ‘pee? structure.

Abstract: Distributed data processing {DDP) and network architectures are discussed. The paper points out the advantages and disadmntages of DDP and sets out the major steps that should be taken in ~~~~~ a DDP strategy.

KqrwordF: data processing, computer networks, distributed processing.

Steve Warner is senior consultant, network sys- tems at Honeywell Infomxation Systems Ltd.

one of the most important present-day computing.

aspects of

Definition of distributed pro- cessing and options in network construction

The term distributed processing is fre- quently misused. The correct defini- tion is ‘an integrated set of notation processing facilities implemented in two or more relatively independent computer sites’.

In general terms distributed net- works can be constructed in three forms: horizontal, hierarchical and hybrid. Each has different characteris- tics and the choice between them is simply a case of which one best fits the organization. In each case, application development and operations control can be centralized or distributed according to choice,

Horizontal networks (see Figure 1) comprise a group of computers operat- ing within a ‘peer’ relationship. In other words there is no ‘master’, each has equal rights and runs applications independently. The computers in- volved can be of similar size or can vary considerably - there are no con- straints. The dialogue between com- puters typically includes transfer of data files, transfer of complete jobs for processing (distributed job proces- sing), cooperation of the transaction processing executives on multiple sys- tems in order to satisfy particularly complex user transactions (coopera-

j ‘SatelliteJ’ j

Figure 2. Hierarchical ‘masterfsatellite’ structure.

tive transaction pr~ess~g), and elec- tronic mail.

Hierarchical networks (see Figure 2) comprise a tree structure where a master computer operates in conjunc- tion with a number of satellites. The satellite performs ~st~buted proces- sing for a remote location but still re- lies on the master for some application functions.

Hybrid networks (see Figure 3) are a combination of the previous two types. For example: multiple horizontal com- puters each with a hierarchical net- work of satellites, or the converse: satellites of a hierarchical network in- terconnected into their own horizontal network for certain functions.

While describing types of distri- buted processing it is appropriate to include the distributed database, i.e. the distribution of the data of an or- ganization across a distributed net- work in order to ensure that it is at, or near, the processing location which most needs it. There are three methods of achieving this:

0 replication l extraction l partitioning

Replication involves duplicating part or all of an organizations database at multiple locations, and while it is the easiest approach it has the disadvan- tage of being costly on storage space and presenting difficulties in ensuring data consistency between the various copies.

An alternative to replication is ex- traction, i.e. the extraction of key

28 0011-684X/83/070028-04$03.00 @ 1983 Butterworth & Co (Publishers) Ltd. data processing

Processor \ Processor

,

Processor Processor Processor Processor Processor

Figure 3. Hybrid ‘mixed’ structure.

fields or records from a central data- base and the formation of a subset ‘mini’ database at a distributed site. This saves the space of replication but has the same problem of maintaining data consistency.

cost advantages in using these services in the future. PDNs are of particular relevance to international organiza- tions in that they are interconnected and provide a very effective means of quickly implementing a primary net- work or a backup service for interna- tional distributed processing, without the problems normally associated with international leased lines.

To overcome this, network architectures provide a very structured approach using descrete layers of soft- ware, each carrying out specific distri- buted processing or networking func- tions, and just one set of protocols. These layers are implemented identi- callyon the majority of computer types in each vendor’s range allowing simple yet flexible construction of distributed networks.

Generally of more practical use is partitioning, which places the data associated with each part of the or- ganization on the computer which serves it. Apart from normal backup this is the only copy of the data and the ‘owning’ part of the organization is often made responsibility for its up- keep. Other locations wishing to ac- cess this data do so by means of cross- network terminal access, remote batch access or cooperative transaction pro- cessing. In fact cooperative transaction processing is a very effective way of providing distributed database sup- port under application control.

Finally, when considering the con- struction of distributed networks it is important to appreciate that while logically the horizontal, hierarchical, hybrid structures exist, in reality the actual communications links do not have to conform to these structures but can be meshed using either dedicated switching nodes or the distributed processors themselves as switches. Alternatively, dependent on com- munications load and cost justifica- tion, public data networks (PDNs) such as British Telecom PSS can be used for either primary links or as a low-cost backup service.

What is the relevance of network architectures? Network architectures - SNA, IPA, DSA, etc. - are the response by each computer vendor to the need for a single structured approach to distri- buted processing, and are a major step forward for the potential distributed processing user. Prior to network architectures the networking and, albeit limited, distributed processing software of each vendor had evolved like ‘Topsy’ over many years. It was confusing, inflexible and expensive to implement and maintain. For exam- ple, IBM had 30 alternative com- munications access methods available

Unfortunately, the majority of ven- dors have implemented their own different versions of a network architecture which, while solving problems in their own product ranges, does not allow users the freedom of choice to select equipment from multi- ple vendors. Nor does it facilitate direct communication with external organizations such as banks, sup- pliers, Customs and Excise etc., a clear requirement of future integrated com- puting.

To tackle this problem the fnterna- tional Standards Organization (ISO) are in the process of defining a stan- dard architecture for ‘open systems in- terconnection’. The IS0 work has been able to benefit from the vices and virtues of earlier architectures. It has now reached an advanced stage and has significant support throughout the computer industry although it will

I I \ Application

Presentation

Session

Applications

Message management

Transport

Support of PDNs and the planned integrated services digital network (ISDN) should be looked for in any present-day distributed processing product as there are going to be many

Network

Data link

, Communications management

Physical link t

Figure 4. Proposed IS0 architecture for open systems interconnection.

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to users prior to announcement of SNA. Early software was also heavily inclined towards centralized comput- ing.

Network

Vendor B

Figure 5. IS0 open systems interconnection in practice.

obviously take time for those vendors with early architectures to come into line.

Within the IS0 architecture the functions of each computer (from user applications through to communica- tions hardware) are divided into seven distinct layers organized into three groups (see Figure 4). The top group defines the logical ‘rules’ to which ap- plications conform when communicat- ing. The bottom, communications management, group controls the physical exchanges across the com- munications network. The middle, message management, group forms an all-important buffer between applica- tions and communications guaran- teeing future flexibility by eliminating the need for applications to ‘know’ the physical location (or characteristics) of applications or terminals.

Communications Management also incorporates within its lower layers the recommended standards of the CCITT for communications services, such as the X.21 and X.25 recom-

30

mendations used for public data net- works.

Figure 5 shows how multiple sys- tems from the same or different ven- dors would interconnect using the proposed standards.

The proposed IS0 standard architecture has many potential be- nefits for the distributed processing user: Applications are separated from com- munications protecting major applica- tions from the need for modification caused by changes in the network - the most volatile areas of any computer service. Unnecessary modifications to applications mean expense, risk of business disruption and delay in other areas while the modifications are being made. This separation also frees ap- plications staff from the need to con- sider or understand the characteristics of the network, enabling them to con- centrate entirely on providing quickly, and at minimum cost, the computer applications required by their orga- nizations.

The architecture creates no dependencies on a specifi site and allows entirely open- ended networks. This gives resilience in failure situations, ensuring that the fail- ure of any one computer cannot impact others. If offers unlimited growth potential, and provides the ability quickly and easily to respond to changes required in the network struc- ture, for example, by major expan- sion, a merger or a change in markets.

The modular, layered structure of the architecture allows new technologies to be easily absorbed with minimum impact.

Finally, the inclusion of CCITT recom- mendations ensures that the full range of public carrier services is available, giving the freedom to choose the most cost-effective service for each part of the network.

How do I choose whether to distribute or centralize?

There is no simple answer. Despite the present day emphasis on distributed processing it is not the answer to every organization’s problems. A decision has to be made according to each orga- nization’s priorities and requirements. The following comments can, howev- er, be used as a guide:

The advantages of distributed pro- cessing centre around its suitability for ‘distributed’ organizations, flexibility of growth, increased business surviva- bility, and potential cost savings.

Distributed processing can be poli- tically more suitable to business orga- nizations which adopt a practice of loc- al management autonomy, local profit and loss responsibility and high local spending authority. Each part of the organization can be provided with a system entirely responsive to their needs. Local management can set their own priorities and no compromises with other parts of the organisation need be suffered.

With distributed processing, grea- ter flexibility of growth is also achiev- able in that distributed units of hard- ware are of a lower order of cost, and can be more directly matched to the

data processing

communications

financial justification of a new applica- tion than is the case with the units of growth of a large centralized compu- ter. Many DP stagers of centralized facilities are familiar with the situation where one additional small application causes a major upgrade or even a swap- out, at considerable cost and disrup- tion.

Increased business su~ivab~ity is also a merit of distributed processing. Failures are implicitly localized and there is no reliance on a potentially vulnerable, single, central computer complex. If a high level of failsafe is required in a distributed processing environment, it is practical to selec- tively duplicate just the computers in key areas rather than duplicate an en- tire central computer complex.

Finally, distributed processing has the potential of reducing communica- tions costs by moving processing clos- er to the end-user. In some cases a link can be reduced to an off-peak dialup or occasional PDN access, From a finan- cial planning point of view the more that communications costs can be minimized the better, as they are one of the few really uncontrollable and unpredictable elements in present day computing.

Having described the advantages of distributed processing, a centralized approach still remains the right choice for some organizations, particularly those with a strong policy of central management, or where sign~cant re- sources are needed for particular com- puting tasks. If an organization has a policy of strong centralized manage- ment and administration, centralized computing is often politically and operation~ly the best approach. In addition, economy of scale is an impor- tant issue which can still favour cen- tralized computing.

In the 1970s Herb Grosch, the noted industry personality, was responsible for ‘Grosch’s Law’ which stated that ‘for twice as much money you could buy a processor with four times as much computing power as a smaller machine’. Despite improvements in

technology and price/performance ratios, Grosch’s Law still applies to- day, and if your business requires sig- nificant processing resources and data- base storage for single computing tasks, then the argument is strongly in favour of a centralized approach.

Finally, economy of scale in staf- fing, and the avoidance of the need for expertise at remote locations, is an argument for centralized computing. This argument is, however, now re- ducing as cross-compilers, remote de- velopment tools, remote operator and ~ten~ce facilities, make it possi- ble to retain centralized development and operations staff in a distributed environment.

If1 decide to distribute what are the major steps to be taken? These fall into two categories: the in- ternal decisions to be made and the major things to look for in an equip- ment vendor. Internally within an organization there are four major steps:

First, decide where the equipment choice, spending authority, applica- tion development and operational con- trol are to be placed. Remember that total local autonomy is impractical. Standards must be defined to ensure adequate compatibility between distri- buted sites and consistency of the organization’s data.

Second, decide which distributed processing structure you are going to use (horizontal, hierarchical, etc.> and how you will split and locate data. Re- gardless of the degree of distribution the presence of a database administra- tor to ensure data consistency over the entire org~ization is advisable.

Third, if a high degree of distribu- tion and autonomy is intended, ensure that adequate training is carried out, and that sufficient computer aware- ness exists at remote locations. Invest- ment in the beginning can make the difference between a successful and an unsuccessful distributed processing environment.

Finally, carefully consider the wide

range of communication services and equipment available today. In the fu- ture, costs and service levels within an organization will be influenced con- siderably by careful communications planning.

From the point of view of vendor selection there are a number of strategic criteria which should be care- fully evaluated:

First, is the vendor’s equipment range specifically designed for distri- buted processing rather than having accidentally evolved a limited capabil- ity?

Second, are the applications associ- ated with distributed processing, i.e. local applications with database, file transfer, remote batch, cooperative transaction processing, distributed database support, electronic mail, etc., available and compatible on all elements of the vendor’s range, and is there sufficient growth potential in each element to allow response to change without end user disruption?

Finally, is a network architecture available, one which protects invest- ment in applications, is open-ended, and has no dependencies on the availa- bility of any one computer in the net- work? If your investment is to be truly protected in the long term, the architecture must have adequate sup- port of current and future communica- tions services and should be in line with the proposed IS0 standards.

In selection of a vendor it is essential to retain freedom of choice to imple- ment the solution which is best for your organization. Artificial con- straints imposed by the limited capabi- lities of the vendor must, and can, be avoided.

In summary, the approaches and the decision criteria surrounding distri- buted processing are now well tried and proven. If it is right for your orga- nization you can, today, implement an effective distributed processing net- work. cl

Honeywell Information Systems Ltd, Boundary House, Cricket Field Road, Uxbridge, Mid- dlesex UB8 IQK, UK. Tel: (0895) 71221.

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