introduction to cics transaction server3b 102704

© 2004 IBM Corporation

IBM ^

Introduction to CICS Transaction Server

for OS/390 – Unit 3


IBM ^

3A2. Hierarchical databases

Database resource adapter (DRA) is a component of the

CICS/DBCTL interface in the CICS address space. It performs request,

contact and other functions between CICS and DBCTL.

Database-level sharing is an IMS data-sharing type which allows an

application to read data in one region while another application reads or

updates the same data in another region. In this data-sharing type,

CICS can be designated an IMS region.


IBM ^

3B. Functions of CICS attachment facility

CICS TS for OS/390 comes with an

attachment facility that allows you to

operate DB2 with CICS.

Through this facility, CICS

applications can access DB2 data

while operating within the CICS TS

for OS/390 environment. Thus,

the CICS applications can access

both DB2 and CICS data.

CICS coordinates the recovery of

both DB2 and CICS data if

transaction or system failure occurs.


IBM ^


The CICS DB2 attachment facility

provides the following major functions:

An application program interface

that allows you to operate DB2 with

CICS

Attachment commands that display

and control the status of the

attachment facility


IBM ^


Through the DB2 attachment facility API, the connection between CICS and DB2 can be created or terminated at any time. This connection and disconnection can be set up to occur automatically. You can also start and stop CICS and DB2 independently. The DB2 system can be shared by several CICS systems, but an individual CICS system can be connected to only a single DB2 system at a time.


IBM ^

3B. Using the CICS attachment facility

The attachment commands that come with the CICS DB2 attachment

facility are issued using the CICS-supplied DSNC transaction ID.

The attachment commands include:

STRT – Start the connection to DB2

STOP – Stop the connection to DB2

DISP – Display the status of the connection to DB2

MODI – Modify the characteristics of the connection to DB2

DISC – Disconnect threads


IBM ^


CICS application programs make

requests to DB2 in the form of

Structured Query Language (SQL)

statements.

1) When a CICS application issues

an SQL request, the CICS

resource manager processes the

request and passes control to the

CICS attachment facility.


IBM ^


2) A transaction thread is scheduled

by the attachment facility.

3) DB2 checks the authorization for

the request and creates control

blocks for it.

4) When the SQL request has been

completed, DB2 passes data back

to the CICS attachment facility, and

the CICS application program

regains control.


IBM ^


You can use RDO to declare a

particular DB2 database available

to your CICS TS system.

Defining database resources

online means that you do not have

to shut down the CICS-DB2

attachment facility in order to make

changes to CICS-DB2 resources.

This allows continuous availability

for both systems.


IBM ^


Using RDO to define DB2

resources as CICS-managed

resources also has the advantage

of providing a consistent end-user

interface.

This means that users can use

familiar CICS transaction

identifiers to perform tasks such

as installing and manipulating

database entries.


IBM ^

3C. Architecture example overview

In order to get a good understanding of the respective roles played by

CICS and a database system, it helps to look at an example of a

transaction processing setup.

In our example, 1st Bank is a midsize financial institution that has two

clusters of OS/390 systems, each with two MVS system images. Within

the MVS images there are multiple CICS and IMS regions. The OS/390

clusters are located in different cites that are miles apart.


IBM ^



IBM ^


CICS receives transactions and communicates with other regions

through Virtual Telecommunications Access Method (VTAM). VTAM is

an IBM API that is used for communicating with telecommunication

devices.

The systems in our example are connected to remote locations and to

each other through a network. Each system also has VTAM connections

in place for each MVS image accessed from the network via network

control program (NCP).


IBM ^


NCPs are key for routing

communication as they manage

the communication traffic between

terminals and CICS.

NCPs also route information between

the VTAM-serviced OS/390 images

and connected CICS regions using

ISC (intersystem communication).


IBM ^

3C. Transaction initiation

A customer goes to an ATM and

inserts a bankcard. The ATM

requests the PIN number and

other information needed for the

transaction. The customer enters

the information at the PIN pad (1).

The information is routed to the

remote NCP (2), which in turn

takes it through the network to the

mainframe switch and then to the

system NCPs (3), which are tied

to miniprocessors.


IBM ^


The system NCP interfaces with VTAM, which is tied to all applications

on the mainframe. VTAM recognizes the transaction header and routes

it through the security translator application. Every remote terminal,

such as an ATM, is defined as a resource in a CICS TOR. VTAM

establishes a session with the TOR where the initiating remote terminal

is defined. CICS then takes over the transaction routing.

Throughout processing, CICS will invoke VTAM again if any cross-

region communication is needed.

Each CICS system has a TOR with a CSD file where all resources are

defined, including remote terminals such as ATMs.


IBM ^


After VTAM establishes a session

with the CICS TOR, CICS gives the

transaction a name and sends it

back through VTAM to the

appropriate AOR, which might or

might not be in the same CICS

system.

After CICS determines the source of

the transaction and names it, the

application required for processing

the transaction is located by the

AOR.


IBM ^


The AOR also initiates updates by

scheduling a program

specification block (PSB).

The PSB is sent by CICS to IMS,

where it is accepted by DBCTL.

The DBCTL controls the different

functions involved in the database

update.


IBM ^


Like CICS, IMS/DBCTL can be

split into regions, which improves

speed and efficiency in updating

and accessing data.

A multiregion setup consists of

these four regions:

Control region, which performs

the DBCTL functions contained

in the other regions.


IBM ^


DL/I region, which actually carries out the update. It contains the DL/I

code, control blocks and buffers for IMS databases, and performs the

accessing and updating of data through DL/I calls.

DBRC region, which is where logging and recovery information is kept

in case there are problems with the update.

IRLM region, which functions as a gatekeeper during the actual

database update executed by DBCTL, preventing any accessing of

the database segments being used by the update.


IBM ^

3C. Transaction completion

Once DBCTL receives the transaction request, it executes the

transaction, retrieves the database segment information and sends the

information back to CICS.

If there are any problems at the database level, they will be logged by

the DBCTL region containing DBRC, which can then be accessed by

CICS in problem determination.

The IRLM prevents any corruption by other transactions requesting the

same database segment as the update. The DL/I region runs and

receives back the completed program to pass on to the DBCTL control

region.


IBM ^


Now the transaction path reverses itself.

The completed update is sent back to the CICS AOR, where it is

acknowledged and renamed.

The transaction is then sent on to the original receiving AOR, retracing

its path.

The receiving AOR records the transaction name, logs its completion

level and sends the information on to CICS TOR.


IBM ^


A VTAM session is once again initiated, this time from the CICS side.

VTAM takes over the transaction routing, through the security

translation application and out to the system NCP and miniprocessors.

The transaction is then sent out over the network to the remote NCP

and from there to the ATM, where the customer is waiting.

The ATM then issues the requested service, such as a cash withdrawal

or bank balance, to the waiting customer.


IBM ^

3C. Transaction summary

The role of CICS in this complex transaction can be summarized as follows: CICS resource definition allows

VTAM to establish a successful session with CICS TOR.

The interregion interface between CICS TOR and AORs allows CICS to recognize, name and pass the transaction to the appropriate region where IMS programs are located.


IBM ^

3C. Transaction summary

The CICS/IMS interface, involving CICS resource definition, resident

utilities and commands, allows efficient handling of the transaction

both going and coming from IMS/DBCTL.

The CICS information logged during the inward-bound path of the

transaction facilitates the outward handling to VTAM and from there to

the originating remote terminal (ATM).


IBM ^

Unit 3 summary

This is what you learned in this unit: CICS supports both relational and hierarchical database structures. Relational databases, such as DB2, structure data in terms of tables

containing predefined data categories.

Hierarchical databases, such as IMS, structure data in a hierarchy where data lower in the hierarchy is dependent upon data higher in the hierarchy.

CICS receives transactions and communicates with other regions through

VTAM.

IMS/DBCTL, an important interface between CICS and IMS databases, is often structured into several different regions, including a control region, a DL/I region, a DBRC region and an IRLM region.

introduction to cics transaction server3b 102704

Documents