Compression Exit
A Compression Exit is a user-written routine, typically developed in Assembler, COBOL, or PL/I, that is invoked by a z/OS system component, database management system (DBMS), or application to compress or decompress data. Its primary purpose is to allow custom data compression algorithms to be applied, optimizing storage utilization and I/O performance for specific data types or business requirements. A Compression Exit in the z/OS environment is a user-written program, typically in assembler, COBOL, or C, that is invoked by an access method or system component to perform custom data compression or decompression. It allows organizations to implement specific compression algorithms tailored to their data characteristics, often to optimize storage utilization or I/O performance for particular data sets.
Key Characteristics
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- User-Written Code: Developed by the customer to implement a specific compression algorithm tailored to their data characteristics, offering flexibility beyond standard system compression.
- System or Application Invocation: Called by various mainframe components such as DB2, IMS, VSAM, DFSORT, or IBM MQ when data is read from or written to a compressed object.
- Standardized Interface: Must adhere to a predefined interface (e.g., parameter list, register conventions) specified by the invoking system component to ensure proper data exchange and control flow.
- Data Transformation: Performs both compression (encoding data into a smaller form) and decompression (restoring data to its original form) based on the operation requested by the caller.
- Performance Critical: The efficiency of the compression algorithm directly impacts CPU utilization and I/O performance; poorly designed exits can degrade system performance.
- Authorized Library Placement: Often resides in an authorized program library (e.g.,
STEPLIB,JOBLIB,LINKLIST) if it needs to access sensitive system resources or run in a privileged state.
Use Cases
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- DB2 Table Space/Index Compression: Implementing custom compression for DB2 table spaces or indexes where the data characteristics are well-suited for a specific algorithm, potentially achieving better compression ratios than DB2's built-in dictionary compression.
- IMS Database Compression: Applying user-defined compression to IMS segments to reduce storage requirements for large IMS databases, improving I/O efficiency during database access.
- VSAM KSDS Data Compression: Compressing data records within VSAM Key-Sequenced Data Sets (KSDS) to optimize disk space usage and reduce the number of I/O operations needed to retrieve records.
- IBM MQ Message Compression: Compressing message payloads before they are placed on an MQ queue and decompressing them upon retrieval, reducing network bandwidth usage and storage for persistent messages.
- Data Archiving and Backup: Custom compression routines can be used during data archiving or backup processes to minimize the storage footprint of historical or recovery data.
Related Concepts
A Compression Exit is a specialized form of a user exit, providing a hook for custom logic within a system process. It directly relates to data compression strategies, offering a user-controlled alternative or supplement to hardware compression (e.g., zEDC) or