Dependency

Enhanced Definition

In the mainframe context, a dependency refers to a required relationship where one component, program, job, or resource relies on another to function correctly, complete its execution, or be available. This reliance dictates the order of operations, resource allocation, or data flow within the z/OS environment. In mainframe computing, a dependency refers to a required relationship where one component, process, or resource relies on another to function correctly or complete its execution. This reliance means that the dependent item cannot proceed or succeed until its prerequisite is met, available, or completed.

Key Characteristics

- Precedence: Often defines the order in which tasks must be executed; for example, Job B cannot start until Job A completes successfully.
- Resource-based: A program or transaction might depend on the availability of a specific dataset, DB2 table, IMS database, or CICS region.
- Data-driven: A reporting job might depend on the successful update of a master file by a previous batch job, ensuring data integrity.
- Software/Module-based: A COBOL program might depend on a specific copybook, load module, or dynamically linked library being present in the STEPLIB, JOBLIB, or LINKLIST.
- System-level: A CICS region might depend on DB2 being active and available, or an IMS control region might depend on specific datasets being mounted.
- Explicit vs. Implicit: Dependencies can be explicitly defined (e.g., in a JCL scheduler) or implicitly understood through application logic or operational procedures.

Use Cases

- Batch Job Scheduling: Defining the execution order for a series of JCL jobs, where subsequent jobs depend on the successful completion and output of preceding jobs.
- Application Deployment: Ensuring that a new version of a COBOL program is deployed only after its associated copybooks, DB2 DBRMs, or CICS resource definitions have been updated.
- Database Operations: An online transaction processing (OLTP) application depends on the availability and integrity of its underlying DB2 tables or IMS databases.
- System Startup/Shutdown: The controlled sequence in which z/OS subsystems (like CICS, DB2, IMS) are started or stopped, often involving complex dependencies to maintain data consistency and service availability.
- Data Replication/ETL: A data extraction, transformation, and loading (ETL) process might depend on source system logs being available and target systems being ready to receive updates.

Related Concepts

Dependencies are fundamental to workload management, job scheduling, and system automation in z/OS. They are often managed by Workload Schedulers (like IBM Z Workload Scheduler, formerly OPC/TWS) which use dependency definitions to build execution plans and automate job submission. Dependencies are closely related to resource management, as a dependency often implies a need for a specific resource (e.g., a dataset, a DB2 subsystem). Understanding dependencies is crucial for change management and problem determination, as a failure in one component can cascade due to unmet dependencies.

Best Practices:

Document Dependencies: Explicitly document all known dependencies, especially for critical batch streams and application components, to aid in troubleshooting, change management, and disaster recovery planning.
Automate with Schedulers: Utilize z/OS workload schedulers to define, manage, and enforce job and task dependencies, ensuring correct execution order, automated recovery, and efficient resource utilization.
Minimize Unnecessary Dependencies: Design applications and batch processes to be as modular and independent as possible to reduce complexity, improve flexibility, and minimize potential points of failure.
Monitor Dependency Status: Implement robust monitoring tools to track the status of critical dependencies, providing proactive alerts when a required component is unavailable or fails, enabling swift intervention.
Plan for Failure: Design recovery procedures that explicitly account for dependency failures, ensuring that dependent processes can be restarted, rerouted, or handled gracefully to maintain business continuity.