ILLUSTRATIVE DISTRIBUTED SYSTEMS The features of data-processingj communications

Teleprocessing-The Modern Marriage of Computers and Communications

ILLUSTRATIVE DISTRIBUTED SYSTEMS The features of data-processingj communications

systems thus span a wide spectrum that almost defies illustration. Nevertheless, we'll try to illustrate the fact that systems of today are distributed in two ways:

first in a hierarchical fashion and then in a peer fashion. The degree of centralization and distribution will first be illustrated by examining typical systems in three industries:

1. Airline reservation systems using a centralized data base

2. A banking system, where the data base is centralized but some of the message processing is distributed

3. A retail system, where more of the processing is distributed

Then we will consider how, in addition, each of these systems might involve the use of peer processors and peer data bases.

Centralized Reservations.

One of the pioneering developments in on-line, interactive, data-base-oriented teleprocessing systems has been for airline systems. Out of the development of PARS (the Programmed Airline Reservation System) came a generalized Airlines Control Program (ACP) that was optimized for short standard messages. fixed formatted file records. and hi2'h trans-action~ rates. Today, a typical ACP syste~ might consist of 2000 to 5000 terminals. Some ACP systems also exist with a few hundred terminals, and 10,000-terminal systems have been envisaged for the near future.

Often, these networks span a large area, typically nationwide, connecting agents in the major cities of a

R.J. Cypser, Communications Architecture for Distributed Systems,

Reprinted with permission.

country to a centralized database. Thus, any agent can sell, change, or cancel a reservation for any flight segment in the system and know that all information is accurate and current to that instant.

In airline reservation systems, long-distance communi-cation lines are shared among many agents through the use of concentrators at key locations. Traffic to and from a number of agents is multiplexed by the concentrator onto a single long-distance line (see Figure 3). A number of these concentrators may all share a single 2400-bitjsec (bps) communication line to the central site; polling manages this sharing by allowing each concentrator, in turn, to use that line.

The agent work stations may be locally attached to the concentrator or remotely attached via communication lines operating at 2400, 1200, or 148.8 bps. (A still higher level of sharing may be done by the telephone company, in which many such individual lines share a broadband transmission facility for the intercity and long-distance traffic. This sharing, however, is completely transparent to the subscriber).

[ Agent Work Stations]

Figure 3. Centralized airline reservation system [From "A Case Study: Airlines Reservation Systmes" by J.R. Knight, Proc.

IEEE 60 (November 1972), pp. 1423-1431. Reprinted by permission. ]

ot:OO(,U"\IIf"TII"\II.I DDI"\UIDITCn

CS10-150-106 Basic Concepts

Teleprocessing-The Modern Marriage of Computers and Communications As an example, a system using an IBM S/360 Model

195 was designed to process 180 typical reservation messages each second, with the central processing unit operating at 85 percent utilization. The average response time was designed to be within two seconds, the response time at the 90th percentile to be within four seconds, and the average processing time per message to be less than 4.7 milliseconds.

Centralized Data and Distributed Processing.

M any financial institutions are using distributed pro-grammable units to handle transactions locally. We will describe a hypothetical but representative system in which large numbers of work stations, spread over large areas, operate on-line in this type of distributed-function network.

Each work station typically is composed of the following terminal facilities:

1. A programmable keyboard

2. A reader of prerecorded information in magnetic stripes

3. An alphanumeric character display (for example, a 240-character gas display panel)

4. A receipt and journal printer (for example, a 30-character! second, 80-column printer)

Alternatively, a work station might be a higher-speed administrative line printer. A group of such work stations is managed by a programmable cluster controller, as shown in Figure 4. The work stations are connected to the programmable controller via private, on-premises loops at 1200, 2400, or 4800 bps, or via common carrier at-say, 1200 bps.

I

Keyboard

8 II

Printerl Work Station

Figure 4. Multiple work stations attached to loops from a programmable cluster controller

R.J. Cypser, Communications Architecture for Distributed Systems,

Reprinted \vith permission.

The programmed cluster controllers execute ap-plication-oriented programs and store data pertinent to local operations. They can be programmed to act as an "electronic journal," maintaining local totals, logging transaction performed on attached terminals, and providing a detailed audit trail. They can also be programmed to capture transactions during off-line operation for later transmission to a central computer site. In one type of controller, a removable random-access diskette can store up to 560K bytes of data.

In addition, certain members of that controller family have nonremovable disk storage of up to 9.3 million bytes. M any transactions, however, may also draw data in real time from the central database to which each programmable controller is connected. The programmable controllers are, in effect, local coordinators and preliminary processors for the operations at the multiple work stations.

Each of the programmable cluster controllers in a typical installation will be connected to a central host site via lines of 1200-4800 bps. In some applications, the central site contains the central data base that is updated in real time by certain transactions entered at each work station. Every transaction across the entire network thus can draw on information that is accurate up to that instant, regardless of the number and! or location of the transactions. An illustrative duplexed configura-tion for a central site is given in Figure 5, showing dual processors, shared disk storage, shared tape files, and shared communications controllers.

Although an I 10 device may be shared, only one processor, with the required amount of equipment dedicated to it, would be on-line at any given time.

Another part of the financial network may involve high-speed data collection during a brief period each day. Data is collected from the batch-process-ing centers at the dispersed locations to the above-mentioned central site. The batch-processing sites could be connected, via high-speed lines, to the central site. High-speed tapes, operating in the range of 470K-J 250K bytes per second, would receive the batch input from these high-speed lines.

The batch input from the dispersed locations provides the daily confirmation of the central data base, which then is incremented in real time during the day, as described previously.

Semiautonomous Distributed Processing.

Examples of distributed processing, where still greater autonomy is exercised at each processor, are found in the retail industry. Here programmable controllers operate autonomously, for the common types of transaction, in each store. With over a hundred thousand bytes of high-speed storage in the cluster controller, multiple applications can be run at

JUNE 1979

Teleprocessing-The Modern Marriage of Computers and Communications

Work Stations

Remote

Branch -1 Branch x Branch - 2

Branch y Branch - 3

Work Stations

~

Communications ^DUPleXed ^~ '----r---~ Controllers

Duplexed Hosts

Shared 110

Random Access Data Base

Figure 5. Illustrative duplexed central processing site and work stations on remote cluster controllers

the store level. For other types, an interaction with a central site is used. Let us examine one of these "in-store" systems.

Sales personnel use a "point-of-sale" terminal for sales transactions, credit authorization, and some inquiry functions. Data entry may be through a magnetic or optical wand, whose passage over a label reads the identity of the item, or through a numeric and function-key keyboard. Instructions to the operator and data being entered are displayed; data provided in response to an inquiry may be printed.

Cash transactions are handled solely by the interactions of the terminal and a programmed cluster controller located in each store. In this role, the programmed controllers operate autonomously.

Credit and check-cashing authorization, on the other hand, involve a check against a master file at a central computer location, Also; once a day, another central computer application draws data from all of its connected controllers so as to establish register balances and conduct an overall sales audit.

R.J. Cypser, Communications Architecture for Distributed Systems,

Reprinted with permission.

Another set of applications concerns the flow of inventory, and relies on a few separate display terminals per store. Order entry is the creation of purchase orders and the input to the purchase-order data base. The receiving application controls the movement of merchandise received and checked.

Accounts payable includes the entry of invoice data into the data base, the calculation of cost and retail sales dollars, and information verification. These types of application are executed partly in the controller and partly in the central processor. The interaction is from the display terminal via the same controller that handles the sales transactions to the central computer.

Let us take as an example a chain of stores located throughout several states. In this installation, a group of 20 department stores is being brought on-line, with one programmed controller in each store and a central computer to coordinate them all. In at least one case, several stores can share a single pro-grammed controller.

In our example, terminals are connected to the programmed store controller via a 2400- or 9600-bps transmission loop. The controllers, in turn, are each connected to the central computer by a separate 4800-bps telephone line. Each programmed controller manages from 60 to 120 point-of-sale terminals plus a display terminal and a printer. These terminals may handle from 20 to 30 transactions per hour, and the programmed controller in a store may handle 2000 to 3000 transactions per hour during a peak sales period. Response times at a point-of-sale terminal probably average less than a second, and less than ten percent of the responses should take more than, say, 1.5 seconds.

Each credit authorization requires only one or possibly two messages to the central computer.

However, transactions of the inventory-flow applications may involve four or five messages to the central computer per transaction. The central computer, then, must be capable of handling in the order of eight to ten messages per second during peak sales periods, even though ail cash transactions are handled locally, using the in-store programmed cluster controller.

When the day's transactions are batched from all the store controllers to the central computer, the trans-mission must take place in a short time, say, 0.5-1.5 hours. The records for tens of thousands of transactions must be transmitted in this mode, and the central computer must be capable of handling an equivalent of 10 to 20 messages per second during this time.

QI=PQ()nllrTI()N PQ()I-IIRIT~n

l..;:) , v- , ::>v- ,va Basic Concepts

Teleprocessing-The Modern Marriage of Computers and Communications Multiple Peers

The preceding exampies illustrate a hierarchical distribution of functions among three levels: the intelligent terminals, the programmed cluster controllers, and the central processing unit. Both data bases and processing capabilities can be so dis-tributed.

Given this hierarchy of distribution, one can, in addition, have multiple servers that operate as peers.

To illustrate, any of the central processing units in the above examples might be replaced by multiple CPUs and multiple databases. These might be at different locations. Different types of operation with such peers can be identified as follows.

I. Transaction routing to peer data bases. In some cases, the database is partitioned by geography or function, and separate databases are managed by different processors. These are peers of one another that can be coupled together. An illustrative configuration for systems ~ith peer coupli!1g is shown in Figure 6. It may be desIrable that termmals at any location be able to access any database and that the terminal user be unaware of the database partition-mg.

In such systems, if a request arrives at any CPU it should be rerouted automatically to the site where the appropriate database is located. With transac-tion routing, the routing to the correct database is based on a transaction code in the user's request.

Similarly, a request from the database to any terminal can be routed to that terminal, via an intermediate host if necessary, using the terminal

Data Base - 1 Dat. Base - 2 Data Base - 3

Distributed Data Base

Central Processing Units

TP Network

Figure 6. Configuration with peer-coupled distributed databases

R.J. Cypser, Communications Architecture for Distributed Systems,

Reprinted with permission.

name in the request. In this example, the routing is achieved in an application-like program by examin-ing the contents of the request that is provided by the user of the network.

2. Job routing to peer processing units. This is another form of transaction routing in which the work scope is a job (that is, an application program).

As before, special fields within the user's request that accompanies the job can be used to achieve the routings. These fields can be interpreted by a so-called Job Entry Subsystem (JES), which functions as a pseUdo-application program. JES performs the routing and coordinates the scheduling of jobs at multiple CPUs.

In one implementation, for example, the submitter of a job may specify the host upon which a job is to be executed and also the destination of the output resulting from job execution. A job may be entered into the network from any job entry station that is local to one of the hosts or from a remote terminal. A job may also be entered into the network via any of the internal job queues within any of the hosts. Jobs may be transmitted directly from an originating host to an execution host, without incurring store-and-forward overhead at intermediary hosts. When the job has been received at the execution host, it is queued to await execution. During executi~n ?f the job, output data sets are queued for transmISSIOn to the destination specified by the submitter of the job.

3. Transaction-routing network service. In the two cases cited above, the routing of the transaction (or job) is performed by a subsystem that operates as an application program external to the network. An alternative is to build the system so that the routing function is a part of the network services, even though examination of the content of the request is involved.

4. Connection to alternate peers. Quite a different approach is to build into the network architecture an ability to achieve logical connections to any program that may be located in any CPU without examination of the content of each of the user's requests. The connection (or session) usually pertains to the exchange of a series of bidirectional messages, which may proceed for a short or an extended length of time. Such a connection involves separate set-up messages to establish an initial connection. At that point, the user of the network specifies the name of the desired destination, for example, to which program subsequent messages will be sent. The sub-sequent dialogue employs addressing facilities that use headers supplied by transmission services of the network (rather than fields within the user's request).

More than one of these four types of operation may coexist in the same system. 0

JUNE 1979

The Special Requirements of Data

Im Dokument COMMUNICATIONS SOLUTIONS (Seite 42-46)