Mainframes

Basic Concepts of CICS(Customer Information and Control System)

In CICS environment, the tasks can be executed by using the transaction id.

Transaction:

Transaction is a logical unit of work that terminal user can invoke(సహాయం కోరు).
It is unique identifier which is used to execute or start particular task.
CICS is called to be transaction driven, because every transaction or program can be invoked with the help of a transaction-id.
Transaction-id is identified by a four OR 4 character id, for e.g. TXN1 and it is initiated, usually by typing the transaction id
in the top left left-hand corner of a blank screen.
A transaction may take several tasks to complete it. A Simple update transaction may take several tasks for validations.
A Transaction is to update a record in a file but terminal user may enter incorrect data and then program validates sends error
message to the screen.
Each time terminal users corrects the data, a new task is generated. So this update transaction may complete in several tasks.

TASK:

➤ A Task is a unit of work.

➤ Task is an instance(ఉదాహరణకు) of the execution of a particular transaction. It is a logical unit of Work.

➤ In Pseudo conversation task starts with procedure division and ends with RETURN TRANSID or RETURN commands.

➤ A Transaction from a terminal completes with multiple tasks.

➤ The same transaction can be executed from 100 terminals, in this case 100 tasks are created at the same time in the main memory.

➤ Each terminal user may execute that transaction in several tasks.

Multi Tasking:

➤ Mutli tasking means that the Operating System allows execution of more than one task concurrently regardless of whether the task

uses the same program or different programs. CICS provides a multi tasking environment where more than one CICS task or

transaction can run at the same time.

➤ When CICS receives a request to run a program, CICS start running that program instance for that user.

We will call this task for that particular user.

➤ Now let us see what is multi tasking in CICS, When CICS receives a request to run the same program or different program.

CICS starts running instance of requested program for that user. Now, CICS receives another request to run the same program or

another program. Again, CICS starts running instance of requested program for that user.

➤ In this manner, CICS can able to handle thousands of requests, can able to run multiple tasks simultaneoulsy.

➤ This Capability is called multi tasking

Multi Threading:

➤ Multi Threading is sub-concept of Multi-Tasking.

➤ It is the system environment where the tasks are sharing the same program under the multi tasking environment.

➤ Under Multi threading environment, a program is shared by several tasks concurrently.

➤ For each task,the program must work as if it were executing instructions exclusivley for each task.

➤ Therefore it requires special considerations such as reentrancy.

What is Single Threading

➤ When Transaction Server(TS) receives a request to run a program, TS starts running that program instance for that user.

During this time, TS receives another request from different user to run the same program.

In single threading model TS has to wait till first instance of program to be completed and then start executing the same program

for second user.

Multi Threading is a technique that allows a single copy of a program to be processed by several transactions concurrently.

For Example:

One transaction may begin to execute an application program. While this happens, another transaction may then execute the same copy

of the application program.

Reentrancy

➤ A Re-entrant program is one which doesn't modify itself so that it can re-enter to itself and continue processing after an interuption

by the operating system and Batch Programs are non re-entrant.

Quasi-Reentrancy

➤ Program should be quasi-re-entrant, in order to make multi threading possible for that program.

➤ i.e;Program should not be in modified state, when CICS start using the copy of the program.

➤ For example: When CICS receive 1st request to process a program, CICS starts executing that program,

at the same time another request 2nd request by CICS which has more priority, then CICS halt processing of 1st request

& preserve its state (all changed variable data, or changed application logic for 1st request to a separate storage area and

initialize the program (to get fresh copy of that program),

Now, start executing the 2nd request. Once second request is over.

➤ It initializes the program again and restores 1st request's state (the data stored in separate storage area) to this program

and start executing the program,where had stopped.

➤ In this manner,CICS can able to execute same program for many user requests. The feature of storing the program data in a separate storage area,

when there is an interruption from CICS is called Quasi-Reentrancy capability.

➤ Program which supports this feature is called Quasi-Reentrant program.

What I need to do as a programmer to make my program Quasi-Reentrant?

It is simple, use RENT option while comppiling program.

➤ In simple manner, we can say Reentrancy in the following manner

➤ In non-CICS environment, i.e,; OS executes the program & OS interrupt the programs for multi threading purposes.

In these cases same Quasi-Reentrancy is called as Reentrany.

➤ If you talk Quasi-Reentrancy concept on OS envionment directly. It is called Reentrancy.

Conversation:

➤ Program can communicate with the user by pair of SEND and RECEIVE commands.

But in between the SEND and RECEIVE, that is during the human response time all the resources are held by the program.

Once the user entered the information in the screen, then program proceeds further. This is called Conversation.

Pseudo Conversation:

➤ Program can communicate with the user by pair of SEND and RECEIVE commands.

But in between the SEND and RECEIVE, that is during the human response time,it releases all the resources are held by the program and

those resources are used for another transaction purpose.This is called Pseudo Conversation.

➤ In Conversational transaction, user sends the request, operating system executes the program for that request and sends back reply.
.
Wait for user response, once user responded, again process the user request and sends back reply and wait for user response.

In this scenario, total time for waiting for user to respond is more than executing the requested program.

In Conversational transaction, system resources spend lot of time in waiting for user response.

➤ In non-Conversational transaction, system receives request from user, process it and send back reply to user,

and terminates. System doesn't required to wait for users response in this mode.

➤ Pseudo Conversatioal transaction contains a series of non-Conversational transactions that look to the user like a Single Conversational transaction.

But how a user get a feel that it is a single conversational transaction.

He get a feel that system is waiting for his/her to respond.But system is not waiting for his response.

When user sends request, system excutes the program and sends back reply and terminates it.
This is non-converation transaction.

In this Pseudo Conversatioal transaction, program not only send reply, but also sends the information to the user terminal,
directing terminal control to call the system, when user completes his data entry and type any function keys or ENTER.
Then, system terminates the program and start working on other task. It won't wait for user.

User presented with the reply sent by the system, now, user feels that system is waiting for his response in background and
enter the data and press any FUNCTION KEY or ENTER.Now terminal control call the system with required information.
Now system again starts a non-conversational transaction and excutes corresponding program and return back with reply along with "additional information"
required to call back the system, when user press some key.

In CICS, we have EXEC CICS RETURN command to send back control terminal.
If you want terminal control to call any particular program (transaction), when user pressed any Functional Keys or ENTER.
Then you need to use TRANSID option in EXEC CICS RETURN command.
Using this transid, we tell terminal control, which transaction id to be invoked, when user want to submit the form(by pressing PF, PA keys or ENTER key).

In above discussion, we talked about transaction ID, This is a 4 byte word attached to a program.
When user submitted his map or form termianl control sends back this transactionID, CICS identifies corresponding program and excute that program.

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Explanation of Identification Division

A Division is a block of code, which contains one or more sections or set of sections or set of sentences or block of statements.

It begins with division name and ends at the beginning of the next division or at the end of the program.

Divisions are known as system defined words.

All Divisions should be coded in AREA A.

Identification Division provides the program name and other items used to uniquely identify the program.
This division is required in every COBOL program.

In other words, the identification is used to identify the programmer who has write program and it may contain optional
entries like Author,Installation, Date-written, Date-compiled and security depending on the requirement.

PROGRAM-ID is a keyword and it must be specified immediately after the identification division and it is mandatory
in COBOL PROGRAM.

Program name can be up to 8 Characters MAXIMUM.

All the entries in Identification Division should be coded in AREA A.

The Complete syntax of the Identification Division is as follows.

Syntax:

[Mandatory] Identification Division.

[Mandatory] Program-id. Name of the Program.

[Optional] Installation. Development-Center.

[Optional] Date-Written. mm/dd/yy.

[Optional] Date-Compiled. mm/dd/yy. HH:MM:SS.

[Optional] Security. Program-type.


Program- id:

The name of the program will be updated with program-id.

The Length of the program name is up to 8 characters maximum.

By using the program name provided with program-id, the load module will be created after the compilation.

To Execute the program, the name provided with program-id needs to be provided in JCL or in PPT enteries.

The actual source name and name provided with program-id may not be same in all cases.

This is a Mandatory entry(Program-id).


Author:

It describes the name of the programmer who written the program.

This is comment entry which is set of character from the computer character set.

The Comment entry can be written any number of lines before starting another statement in AREA A.

This is optional in Identification Division.


Installation:

It describes the name of the location or company where they installed.

This is also a comment entry.

This is optional in Identification Division.


Date-Written:

It describes the actual date of the program written.

This is also a comment entry.

This is optional in Identification Division.


Date-Compiled:

It describes the actual date of the program compiled.

Date-Compiled details are automatically written by the compiler during the compilation time.

This is also a comment entry.

This is optional in Identification Division.


Security:

The Security describes the confidentiality level of the program.

This is also a comment entry.

This is optional in Identification Division.


Practical Example:

A Simple Program with all entries of identification division was coded like below.


Program Code:

Identification Division.

Program-id. FIRST.

Author Ajay.

Installation SEPT 2015.

Date-Written 06/06/2016.

Date-Compiled. 06/06/2016.

Security Private.

Environment Division.

Data Division.

Procedure Division.

Display 'Identification Division with all entries'.
stop run.

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Structure of COBOL Program

The Structure of the Cobol applies for COBOL Source Program which has set of syntactically correct set of statements.

In other words, COBOL Source Program contains set of syntactically correct set of statements.

Cobol Programs contains hierarchical in structure which was shown in the above diagram.

The COBOL Program hierarchy consists of Divisions, Sections, Paragraphs, Sentences and Statements.

In other words, Each division may contain one or more sections, Each section may contain one or more paragraphs,
Each paragraph contains one or more sentences and each section contains one or more statements.

Each element of the hierarchy consists of one or more subordinate elements.

Divisions➤Sections➤Paragraphs➤Sentences➤Statements.

Discuss them in reverse order for better understanding

Statements:

It consists of a COBOL verb and an operand or operands describes about COBOL verb process on the operands either
it may be a conditional or unconditional.

A statement should have COBOL verb with an operand and may or may not marked with period at the end.

All statements should be code in the AREA B and should not in AREA A.

There are four categories of statements in COBOL.

1. Imperative

2. Conditional

3. Delimited Scope

4. Compiler directing

Example: ADD A TO B GIVING C.

Sentences:

As stated above, a sentence contains one or more statements and those are marked with a period at the last line.

If those are marked with a period in middle, then it will be treated as statement and those are not considered
as sentence.

All sentences also should code in AREA B not in AREA A.

Example:

Move P to Temp
Move Q to P
Move temp to Q.



Paragraphs:

It is a block of code which contains one or more sentences or a set of statements.

A Paragraph begins with the paragraph name and ends at the next paragraph starting.

A Paragraph can be ended in another way at another section starts or at the end of the program.

All paragraphs should code in the AREA A.

Syntax: Paragraph-name.


Sections:

It is a block of code which contains one or more paragraphs or a set of sentences or a block of statements.

Section begins with the section name and ends at another section starting or at the end of the program.

All Sections should code in the AREA A.

Syntax: (section-name) SECTION.


Divisions:

It is a block of code which contains one or more sections or a set of sections or set of sentences or a block of statements.

Division begins with the division name and ends at another division starting or at the end of the program.

All divisions should code in the AREA A.

There are four divisions in COBOL

1. Identification Division.

2. Environment Division.

3. Data Division.

4. Procedure Division.

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