Understanding CICS TS 4.1 Very Importnat Doc

© 2004 IBM Corporation

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Introduction to CICS Transaction Server

for OS/390 – Unit 4


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Unit 4: CICS intercommunication

Topics:

� 4A. Intercommunication facilities

� 4B. Interregion and intersystem communication

� 4C. CICS and the Internet


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Unit 4 overview

Many organizations have resources such as terminals, application

programs, and data spread out over one or more systems or CICSPlex

setups.

CICS TS for OS/390 supplies several facilities to allow you to access

CICS resources across a CICSPlex, over an intranet, or over the

Internet. This unit describes these facilities and the advantages that they

offer. It also introduces several products that provide a range of options

in configuring a CICS setup for Web browser access.


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Unit 4 objectives

This is what you should be able to do upon completing this unit:

� Describe the intercommunication facilities that allow the sharing of

CICS resources across regions and systems

� List the advantages of distributing transactions and programs

across regions and systems

� Identify the applications of multiregion operation in CICS TS for

OS/390

� Describe the possible system configurations for setting up

intersystem communication

� List and describe the facilities available in CICS TS for OS/390 for

making CICS available over the Internet or an intranet


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4A. Intercommunication facilities

In previous units, you learned that every CICS setup controls its own

resources, such as files, terminals and local application programs.

However, through CICS intercommunication facilities, a CICS system

can establish links to other systems and gain access to remote

resources. Such intercommunication permits the sharing of resources

and transferring of data across several systems.

CICS employs these methods for communicating between systems:

� Function shipping

� Asynchronous processing

� Transaction routing

� Distributed program link (DPL)

� Distributed transaction programming


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4A. Function shipping

Function shipping allows multiple systems to share resources such as

files, temporary storage or application programs. When function

shipping is used, a program is invoked in a local CICS system and

accesses resources owned by a remote system. The program operates

as though the resources are resident on the local system.

Access gained through function shipping allows both reading from and

writing to the resource, as well as restarting and recovery operations.


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In developing applications,

CICS systems programmers

can specify that a required

resource is not located on the

local or requesting system,

but on a remote or owning

system.


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When a file-control command is

issued for a resource, first the

EXEC interface program

determines where the resource

is located.


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If the resource is on another system, the request is passed to the owningsystem.

The owning system runs the requestin a mirror transaction, a local re-creation of the original request.

Application programs can use EXECCICS commands to performThese function shipping operations:

� Shipping file control requests to another CICS system

� Shipping transient data and temporary storage requests to another CICS system for data transfer

� Initiating transactions in another CICS or IMS system by shipping START commands (called asynchronous processing)


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Once the request is completed,

a reply is sent back to the

originating CICS system.


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Some transactions that involve function shipping:

� EXEC CICS READ FILE (‘RFILE’) – A simple read request is passed

from one CICS system to another and returned.

� EXEC CICS READ UPDATE FILE – A request to read file updates is

passed from one CICS system to another and returned.

� EXEC CICS REWRITE FILE – A request to rewrite a remote file is

passed from one CICS system to another.


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Asynchronous processing is a type of function shipping where a

transaction is initiated in a remote CICS system by shipping an EXEC

CICS START request to a remote system.

The processing is asynchronous because the performance of the

request on the remote system proceeds independently of any

implementation on the initiating system. To return a reply when the

transaction is completed, the remote system can initiate a transaction in

the local system.


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4A. Transaction routing

Another type of intercommunication in CICS is transaction routing,

which allows terminals connected to one CICS system to run

transactions within a remote CICS system. This means that you can

distribute terminals and transactions throughout a network of systems

and still have the ability to run any transaction from any terminal.


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4A. Distributed program link

Distributed program link (DPL)

enables a CICS program to call a

program on a remote system or

CICS region.

The DPL exploits the LINK

command by allowing the linking

and linked-to application to reside

in different systems or regions.

This potentially reduces the

number of data transfers between

systems, thus improving

performance.


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4A. Distributed program link


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4A. Distributed transaction processing

Distribution transaction

processing (DTP) is the

distribution of transaction

functions over several programs

within a network of systems.

Application programs involving

DTP are complex to develop

because they are able to

initiate and control their own

intercommunication between

transactions.


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DTP differs from the other

intercommunication facilities

discussed thus far in that it

employs synchronous processing.

The communication is dynamic,

and not predefined by transaction

definitions. However, transactions

in DTP are designed to

communicate with other

transactions as efficiently as

possible.


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In large or high-traffic Parallel Sysplex setups, complex CICS

innovations such as DTP serve to reduce the potential problems

associated with simpler, more transparent and slower transaction

facilities.

DTP serves to streamline and synchronize transactions, and allows

complex programming conversations between CICS systems. This

helps to prevent heavy system use from resulting in sluggish

performance or system errors.


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DTP is not suited to all systems.

For smaller and less heavily used

systems, DTP might not be

necessary. The complexity of DTP

might demand too much technical

expertise to set up and maintain it

in some user environments.


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4B. Multiregion operation

CICS multiregion operation (MRO) enables CICS systems that are running in the same OS/390 system or within a Parallel Sysplex setup to communicate with each other.

MRO does not enable communication between a CICS setup and a non-CICS system, such as an IMS or DB2 system.

MRO provides all five of the intercommunication facilities listed at the right.


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These are some of the uses of

multiregion operation (MRO):

� Program development

� Providing reliable database access

� Setting up department-specific

regions

� Multiprocessing


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MRO allows for easy and risk-free

testing of newly written applications

by making it possible to set up a

separate CICS system that is

designated specifically for testing.


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MRO allows you to set up your system toreduce the possibility that unreliableapplications will disable the system orother applications. You can also use it to increase the reliability of databaseaccess.

You could divide your system into multiple regions, dedicating one region to less reliable applications, and thus separating them from more reliable applications. Theapplications in one region can still accessresources in other regions through interregion communications (IRC).

To increase the reliability of database access, you can designate a database-owning region that contains no user applications, and thus might not be subject to the effects of an unstable application.


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MRO allows you to set up a

CICSPlex where each department

of an organization has its own

CICS system. IRC allows one

department to access another

department’s resources and data.

With transaction routing, any

terminal can run a transaction on

any system. Thus, terminals do

not need to be designated as

belonging to a specific department.


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4B. Intersystem communication

Intersystem communication (ISC)

is the communication between a

CICS system and another system

or set of systems that are

networked together.

Communication between systems

in ISC uses the telecommunication

protocols of Systems Network

Architecture (SNA).


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4B. Intersystem communication

There are three basic ways in which subsystems can be connected for intersystem communication:

� ISC within a single operating system� ISC between physically adjacent operating systems� ISC between physically remote operating systems

Communication can take place between CICS and non-CICS systems. For example, intersystem communication can take place between a CICS Transaction Server for OS/390 system and an IMS system.

Intersystem communication can make use of the same facilities asMRO, including function shipping, transaction routing, DLP and DTP.


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4C. CICS from a browser

CICS TS for OS/390 provides you

with the facilities for making

transaction-processing services

available over the Internet or an

intranet.

There are several different means

by which a client application

program or an Internet browser

can communicate with CICS and

request CICS services.


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Web browsers can communicate

directly with CICS application

programs without an intermediate

gateway or Web server through

CICS Web support (CWS).


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Instead of accessing CICS directly

through CWS, you can also

access CICS services through a

server on the OS/390 platform.

For example, you can obtain a

secure connection with CICS

through an IBM WebSphere

Application Server for OS/390.


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You can also access CICS TS for

OS/390 via a non-OS/390 Web

server that is connected via a

network.

Communicating with CICS in this

way uses the External CICS

Interface (EXCI). EXCI is a form of

function shipping, where the

incoming request is processed by

a mirror transaction in the linked-

to system.


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Another way to access CICS from

a Web browser is through the CICS

Transaction Gateway.

The CICS Transaction Gateway

uses the CICS External Call

Interface (EXCI) to pass requests

to CICS.


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4C. CICS Transaction Gateway

IBM’s CICS Transaction Gateway for OS/390 provides a set of Java-based Web server facilities allowing access to CICS applications from a Web browser.

These facilities include:

� A JavaGateway application that interfaces with CICS applications running on CICS servers. This communication takes place through the CICS External Call Interface (EXCI).

� A CICS Java class library that provides an application programming interface (API), and facilitates communication between the gateway application and a Java application or applet.


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The Java classes and JavaBeans

supplied by CICS Transaction

Gateway allow you to write Java

Web server code to handle

browser requests for CICS

services.

You can also use these facilities

to code applets.


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One of the Java classes included

in the CICS Java class library is

the JavaGateway class, which is

used to establish communication

with the gateway process.

Another class, the ECIRequest

class, is used to specify external

call interface (ECI) calls.


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The steps involved in accessing

CICS using the CICS Transaction

Gateway from a Web browser:

1. Browser calls a Web server

using HTTP to get HTML pages.

2. Browser finds an applet tag for

the applet that will communicate

with CICS, and calls Web server to

get the applet and classes that it

requires.

3. After the browser receives an

applet, it runs the applet.


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4.The implemented applet creates a

JavaGateway object, which calls

the CICS Transaction Gateway for

OS/390.

The applet also creates an

ECIRequest object to represent its

request for a CICS program.


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5.The ECIRequest object is passed

to the gateway, which then calls

the CICS program.

6.When the program is finished, the

results are returned to the

browser by way of the gateway.


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4C. CICS Web Interface

The CICS Web support (CWS)

is a set of CICS services that

support direct access to CICS

from a Web browser over a

TCP/IP connection.

CWS allows customers to use

the transaction processing

facilities of CICS for

transactions conducted over

the Internet without an

intermediate gateway to

separate Web server.


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4C. CICS Web Interface

With CICS you can build CICS

HTML application utilities and

provide Internet or intranet access

to CICS without a Web server.

However, the facilities provided by

CWS do not replace those

performed by a Web server. You

should use a separate server if

you need facilities that provide

GIFs, applets, or other items that

might be referenced from the

CICS pages.


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4C. CICS Web interface

CWS accepts two types of requests:

� Requests from Web browsers connected via a TCP/IP port that is designated for CWS

� Requests from Web browsers connected to IBM WebSphere Application Server for OS/390.

With CICS you can build application utilities and provide Internet or intranetaccess to CICS without a Web server.

However, the facilities provided by CWSdo not replace those performed by a Web server. You should use a separateserver if you need facilities that provide GIFs, applets, or other items that may bereferenced from the CICS pages.


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4C. CICS Web interface

For requests from Web browsers

connected via a TCP/IP port that is

designated fro CWS, a user-replaceable

application translates the HTTP request

into a request for CICS transaction

processing services.

For requests from Web browsers

connected to IBM WebSphere Application

Server for OS/390, A CICS-supplied

WebServer plug-in operating within the

server relates the HTTP request to the

transaction processing services.


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Unit 4 summary

This is what you learned in this unit:

� MRO allows communication between different CICS regions within a

single system or CICSPlex.

� ISC connects separate systems, including non-CICS systems, for

intercommunication.

� CICS TS for OS/390 provides five facilities for intercommunication:

function shipping, asynchronous processing, transaction routing, DPL

and DTP.

(continued)


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Unit 4 summary

� Intercommunication permits the sharing of resources across systems

and regions.

� All five types of intercommunication are available in both MRO and

ISC setups.

� There are several possibilities for making CICS transaction

processing services available over the Internet or an intranet: CICS

Web Support provides access to CICS with or without an intermediate

Web server or gateway; CICS Transaction Gateway serves as an

intermediary between CICS and a Web browser.

Documents

Understanding CICS TS 4.1 Very Importnat Doc