Commarea - Communication Area
A `Commarea` (Communication Area) is a contiguous block of memory used in CICS (Customer Information Control System) applications to pass data between programs within a transaction or between transactions in a pseudo-conversational flow. It serves as a temporary, shared data buffer for application-specific information. A CICS Communication Area (`COMMAREA`) is a block of contiguous storage used to pass data between CICS application programs, typically across different transactions in a pseudo-conversational flow, or between linked programs within the same transaction. It serves as a temporary data buffer for maintaining application state or sharing information.
Key Characteristics
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- Fixed-size: The size of the
Commareais defined by the sending program and remains constant for the duration of its use, up to a maximum of 32,767 bytes (32KB). - Program-to-program data transfer: It is primarily used to pass small amounts of data between CICS application programs, often within the same logical transaction using
EXEC CICS LINKorEXEC CICS XCTLcommands. - Pseudo-conversational state management: Critical for maintaining application state across multiple screen interactions in a pseudo-conversational transaction, where the transaction ends and restarts.
- Defined by
DFHCOMMAREA: In COBOL programs, theCommareais typically defined in theLINKAGE SECTIONasDFHCOMMAREAand mapped to a01level data structure. - Transient and volatile: Data in the
Commareais transient; it exists only for the duration of the program or transaction link and is not persistent storage.
- Fixed-size: The size of the
Use Cases
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- Passing user input: A common use is to pass data entered by a user on a screen from a map-processing program to a business logic program within the same transaction.
- Maintaining state in pseudo-conversational transactions: Storing essential transaction identifiers, flags, or partial results between screen displays to allow the application to resume processing correctly after a user interaction.
- Sharing common parameters: Passing configuration settings, error codes, or small lookup values between different modules of a CICS application.
- Inter-transaction communication: Using
EXEC CICS RETURN TRANSIDto pass aCommareato a subsequent transaction, enabling a multi-step process across different CICS transactions.
Related Concepts
The Commarea is fundamental to CICS application design, working closely with CICS programs and transactions. It is often used in conjunction with CICS maps (BMS) to process user input and display output. While Commarea is suitable for smaller data transfers, CICS Channels and Containers provide a more flexible and scalable mechanism for passing larger or variable-sized data between programs, especially in modern CICS applications.
- Keep it small: Design
Commareasto be as small as possible (e.g., under 4KB) to minimize overhead and improve performance, as largerCommareasconsume more system resources. - Consistent definition: Ensure the
Commareadata structure is defined identically in all sending and receiving programs to prevent data corruption or misinterpretation. - Initialize data: Always initialize
Commareafields before use, especially when receiving aCommareafrom another program or transaction, as its contents are not guaranteed to be clean. - Error handling for length: Implement logic to check the actual length of the received
Commarea(usingEIBCALENin the EIB) against the expected length to handle potentialLENGERRconditions gracefully. - Consider
Channels and Containersfor large data: For transferring data exceeding a few kilobytes, or for more complex data structures, leverage CICSChannels and Containersinstead ofCommareato avoid performance bottlenecks and improve flexibility.