CICS Channel Management

Channels are the modern way to pass data between CICS programs and transactions. A channel is a named collection of containers that provides a flexible and efficient mechanism for inter-program communication.

What is a CICS Channel?

A channel is a logical grouping of containers that can be passed between programs using LINK, XCTL, or START commands. Unlike the traditional COMMAREA, channels can carry multiple named data items (containers) of varying sizes and types.

Channel Characteristics

Named Identification

Each channel has a name that can be up to 16 characters long. Channel names help identify the purpose and content of the data being passed.

Container Collection

A channel can contain multiple containers, each with its own name and data. This allows you to pass complex data structures with multiple related pieces of information.

Task Scoped

Channels are associated with a specific task and exist for the duration of that task. They are automatically deleted when the task terminates.

Creating and Using Channels

Implicit Channel Creation

Channels are typically created implicitly when you create the first container on a new channel:

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EXEC CICS PUT CONTAINER('CUSTOMER-ID')
    FROM(WS-CUSTOMER-ID)
    FLENGTH(LENGTH OF WS-CUSTOMER-ID)
    CHANNEL('ORDER-CHANNEL')
END-EXEC.

Checking for Channel Existence

You can check if a channel exists and retrieve its name:

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EXEC CICS GET CHANNEL(WS-CHANNEL-NAME)
    RESP(WS-RESPONSE)
END-EXEC.
 
IF WS-RESPONSE = DFHRESP(NORMAL)
    DISPLAY 'Current channel: ' WS-CHANNEL-NAME
ELSE
    DISPLAY 'No current channel'
END-IF.

Passing Channels Between Programs

Pass a channel to another program using the CHANNEL option on LINK or XCTL:

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* Create containers on channel
EXEC CICS PUT CONTAINER('INPUT-DATA')
    FROM(WS-INPUT-DATA)
    FLENGTH(LENGTH OF WS-INPUT-DATA)
    CHANNEL('WORK-CHANNEL')
END-EXEC.
 
* Link to program passing the channel
EXEC CICS LINK PROGRAM('SUBPROG')
    CHANNEL('WORK-CHANNEL')
END-EXEC.
 
* Retrieve response from container
EXEC CICS GET CONTAINER('OUTPUT-DATA')
    INTO(WS-OUTPUT-DATA)
    CHANNEL('WORK-CHANNEL')
END-EXEC.

Current Channel

When a program is invoked with a channel, that channel becomes the "current channel" for that program. Container commands can omit the CHANNEL parameter when working with the current channel:

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* Working with current channel
EXEC CICS GET CONTAINER('INPUT-DATA')
    INTO(WS-INPUT-DATA)
    * No CHANNEL parameter needed
END-EXEC.
 
EXEC CICS PUT CONTAINER('OUTPUT-DATA')
    FROM(WS-OUTPUT-DATA)
    * Still working with current channel
END-EXEC.

Channel Operations

Creating New Channels

Create a new channel explicitly if needed (though usually done implicitly through containers):

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EXEC CICS GET CHANNEL('NEW-CHANNEL')
END-EXEC.

Deleting Channels

Delete an entire channel and all its containers:

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EXEC CICS DELETE CHANNEL('TEMP-CHANNEL')
END-EXEC.

Starting Transactions with Channels

Start a new transaction and pass a channel to it:

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EXEC CICS PUT CONTAINER('REQUEST-DATA')
    FROM(WS-REQUEST-DATA)
    CHANNEL('REQUEST-CHANNEL')
END-EXEC.
 
EXEC CICS START TRANSID('PROC')
    CHANNEL('REQUEST-CHANNEL')
END-EXEC.

Best Practices

1. Use Descriptive Channel Names

Choose channel names that clearly indicate their purpose and content. Examples: 'ORDER-PROCESSING', 'CUSTOMER-INQUIRY', 'BATCH-CONTROL'.

2. Design Channel Structure

Plan what containers will be on each channel and document the structure. This helps maintain consistency across programs and makes debugging easier.

3. Check Channel Availability

Always check if a channel was passed before attempting to access it:

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EXEC CICS GET CHANNEL(WS-CHANNEL-NAME)
    RESP(WS-RESPONSE)
END-EXEC.
 
EVALUATE WS-RESPONSE
    WHEN DFHRESP(NORMAL)
        PERFORM PROCESS-WITH-CHANNEL
    WHEN DFHRESP(CHANNELERR)
        PERFORM PROCESS-WITHOUT-CHANNEL
END-EVALUATE.

4. Clean Up Temporary Channels

Delete channels that are no longer needed, especially in long-running or frequently-executed programs.

5. Handle Missing Containers Gracefully

Design programs to handle optional containers that may or may not be present on a channel.

Channel vs. COMMAREA

Advanced Channel Features

Cross-Region Channels

Channels can be passed across CICS regions using distributed program link (DPL). The system automatically handles the transfer of all containers on the channel.

BTS and Channels

Business Transaction Services (BTS) uses channels extensively for passing data between activities. BTS containers can persist beyond task termination.

Web Services and Channels

CICS web services can map between XML/JSON and channel/container structures, making it easy to build modern API interfaces.

Error Handling

Common channel-related response codes:

CHANNELERR

No current channel exists or the specified channel wasn't found. Check if your program was invoked with a channel.

INVREQ

Invalid request, such as trying to pass both a COMMAREA and a CHANNEL on the same LINK command.

LENGERR

Channel name too long (maximum 16 characters) or other length-related issues.

Migration from COMMAREA to Channels

When migrating existing programs from COMMAREA to channels:

• Create a container to hold what was in the COMMAREA
• Add new containers for additional data that couldn't fit in COMMAREA
• Update LINK/XCTL commands to use CHANNEL instead of COMMAREA
• Test thoroughly, especially program invocations and data passing
• Consider maintaining both interfaces during transition period