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Channels: quick start

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Containers and channels

Containers are named blocks of data designed for passing information between programs. You can think of them as "named communication areas (COMMAREAs)". Programs can pass any number of containers between each other. Containers are grouped together in sets called channels. A channel is analogous to a parameter list.

To create named containers and assign them to a channel, a program uses EXEC CICS PUT CONTAINER(container-name) CHANNEL(channel-name) commands. It can then pass the channel (and its containers) to a second program using the CHANNEL(channel-name) option of the EXEC CICS LINK, XCTL, START, or RETURN commands.

The second program can read containers passed to it using the EXEC CICS GET CONTAINER(container-name) command. This command reads the named container belonging to the channel that the program was invoked with.

If the second program is invoked by a LINK command, it can also return containers to the calling program. It can do this by creating new containers, or by reusing existing containers.

Channels and containers are visible only to the program that creates them and the programs they are passed to. When these programs terminate, CICS automatically destroys the containers and their storage.

Channel containers are not recoverable. If you need to use recoverable containers, use CICS business transaction services (BTS) containers. The relationship between channel and BTS containers is described in Channels and BTS activities.

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Basic examples

Figure 41 shows a COBOL program, CLIENT1, that:

  1. Uses PUT CONTAINER(container-name) CHANNEL(channel-name) commands to create a channel called inqcustrec and add two containers, custno and branchno, to it; these contain a customer number and a branch number, respectively.
  2. Uses a LINK PROGRAM(program-name) CHANNEL(channel-name) command to link to program SERVER1, passing the inqcustrec channel.
  3. Uses a GET CONTAINER(container-name) CHANNEL(channel-name) command to retrieve the customer record returned by SERVER1. The customer record is in the custrec container of the inqcustrec channel.

Note that the same COBOL copybook, INQINTC, is used by both the client and server programs. Line 3 and lines 5 through 7 of the copybook represent the INQUIRY-CHANNEL and its containers. These lines are not strictly necessary to the working of the programs, because containers and channels are created simply by being named (on, for example, PUT CONTAINER commands); they do not have to be defined. However, the inclusion of these lines in the copybook used by both programs makes for easier maintenance; they record the names of the containers used.

Recommendation

For ease of maintenance of a client/server application that uses a channel, create a copybook that records the names of the containers used and defines the data fields that map to the containers. Include the copybook in both the client and the server program.

Note:
This example shows two COBOL programs. The same techniques can be used in any of the other languages supported by CICS. However, for COBOL programs only, if the server program uses the SET option (instead of INTO) on the EXEC CICS GET CONTAINER command, the structure of the storage pointed to by SET must be defined in the LINKAGE section of the program. This means that you will require two copybooks rather than one. The first, in the WORKING-STORAGE section of the program, names the channel and containers used. The second, in the LINKAGE section, defines the storage structure.

Figure 41. A simple example of a program that creates a channel and passes it to a second program
IDENTIFICATION DIVISION.
PROGRAM-ID. CLIENT1.

WORKING-STORAGE SECTION.

  COPY INQINTC
*                 copybook INQINTC
*  Channel name
*  01 INQUIRY-CHANNEL  PIC X(16) VALUE 'inqcustrec'.
*  Container names
*  01 CUSTOMER-NO        PIC X(16) VALUE 'custno'.
*  01 BRANCH-NO          PIC X(16) VALUE 'branchno'.
*  01 CUSTOMER-RECORD    PIC X(16) VALUE 'custrec'.
*  Define the data fields used by the program
*  01 CUSTNO     PIC X(8).
*  01 BRANCHNO   PIC X(5).
*  01 CREC.
*    02 CUSTNAME   PIC X(80).
*    02 CUSTADDR1  PIC X(80).
*    02 CUSTADDR2  PIC X(80).
*    02 CUSTADDR3  PIC X(80).

PROCEDURE DIVISION.
MAIN-PROCESSING SECTION.
  
*
* INITIALISE CUSTOMER RECORD
*
     ... CREATE CUSTNO and BRANCHNO
*
* GET CUSTOMER RECORD
*
    EXEC CICS PUT CONTAINER(CUSTOMER-NO) CHANNEL(INQUIRY-CHANNEL)
                  FROM(CUSTNO) FLENGTH(LENGTH OF CUSTNO)
                  END-EXEC
    EXEC CICS PUT CONTAINER(BRANCH-NO) CHANNEL(INQUIRY-CHANNEL)
                  FROM(BRANCHNO) FLENGTH(LENGTH OF BRANCHNO)
                  END-EXEC
                  
    EXEC CICS LINK PROGRAM('SERVER1') CHANNEL(INQUIRY-CHANNEL) END-EXEC
    
    EXEC CICS GET CONTAINER(CUSTOMER-RECORD) CHANNEL(INQUIRY-CHANNEL) 
                  INTO(CREC) END-EXEC
                  
*
* PROCESS CUSTOMER RECORD
*
    ... FURTHER PROCESSING USING CUSTNAME and CUSTADDR1 etc...
    
    EXEC CICS RETURN END-EXEC
    
    EXIT.

Figure 42 shows the SERVER1 program linked to by CLIENT1. SERVER1 retrieves the data from the custno and branchno containers it has been passed, and uses it to locate the full customer record in its database. It then creates a new container, custrec, on the same channel, and returns the customer record in it.

Note that the programmer hasn't specified the CHANNEL keyword on the GET and PUT commands in SERVER1: if the channel isn't specified explicitly, the current channel is used--that is, the channel that the program was invoked with.

Figure 42. A simple example of a linked to program that retrieves data from the channel it has been passed. This program is linked-to by program CLIENT1 shown in Figure 41.
IDENTIFICATION DIVISION.
PROGRAM-ID. SERVER1.      

WORKING-STORAGE SECTION.

  COPY INQINTC
*                 copybook INQINTC
*  Channel name
*  01 INQUIRY-CHANNEL  PIC X(16) VALUE 'inqcustrec'.
*  Container names
*  01 CUSTOMER-NO        PIC X(16) VALUE 'custno'.
*  01 BRANCH-NO          PIC X(16) VALUE 'branchno'.
*  01 CUSTOMER-RECORD    PIC X(16) VALUE 'custrec'.
*  Define the data fields used by the program
*  01 CUSTNO     PIC X(8).
*  01 BRANCHNO   PIC X(5).
*  01 CREC.
*    02 CUSTNAME   PIC X(80).
*    02 CUSTADDR1  PIC X(80).
*    02 CUSTADDR2  PIC X(80).
*    02 CUSTADDR3  PIC X(80).

PROCEDURE DIVISION.
MAIN-PROCESSING SECTION.

    EXEC CICS GET CONTAINER(CUSTOMER-NO)
                         INTO(CUSTNO) END-EXEC
    EXEC CICS GET CONTAINER(BRANCH-NO)
                         INTO(BRANCHNO) END-EXEC

       ... USE CUSTNO AND BRANCHNO TO FIND CREC IN A DATABASE

    EXEC CICS PUT CONTAINER(CUSTOMER-RECORD)
                  FROM(CREC)
                  FLENGTH(LENGTH OF CREC) END-EXEC

    EXEC CICS RETURN END-EXEC

    EXIT.                                                        

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