Checkpoint

Enhanced Definition

In the context of IBM mainframe systems, a **checkpoint** is a mechanism used to capture and save the current state of a running program, transaction, or system component at a specific point in time. Its primary purpose is to enable efficient restart and recovery after an interruption, such as an `ABEND` (abnormal end), system failure, or planned shutdown, without having to reprocess the entire workload from the beginning.

Key Characteristics

- State Capture: A checkpoint typically records the program's execution point, the contents of working storage, register values, file positions, and other critical control information necessary to resume processing.
- Recovery Point: It establishes a known, consistent point from which processing can be restarted, significantly reducing the amount of work lost and the time required for recovery.
- Programmatic or System-Initiated: Checkpoints can be explicitly coded within application programs (e.g., using CHKP calls in COBOL/PLI for IMS or CICS) or implicitly managed by system components like IMS, DB2, or CICS for transaction and database recovery.
- Storage Location: Checkpoint information is typically written to a dedicated checkpoint dataset, a system log, or a database recovery log, ensuring its persistence even if the main application fails.
- Overhead: While crucial for recovery, frequent checkpoints can introduce overhead due to I/O operations and state saving. Their placement must be strategically balanced against potential recovery time.
- Scope: Checkpoints can range in scope from a single application program's progress to the state of an entire subsystem or database.

Use Cases

- Long-Running Batch Jobs: In JCL batch applications that process large volumes of data, checkpoints are inserted periodically to allow the job to be restarted from the last successful checkpoint rather than from the beginning if it abends.
- IMS Transaction Recovery: IMS (Information Management System) uses checkpoints extensively to ensure the atomicity and durability of transactions. If an IMS region fails, transactions can be recovered to their last committed state using checkpoint and log records.
- CICS Syncpoints: While CICS uses the term syncpoint (synchronization point), it serves a similar purpose to a checkpoint, marking a point at which all changes made by a transaction become permanent and recoverable.
- Database Recovery (DB2/IMS DB): Database management systems like DB2 and IMS DB use checkpoints to establish consistent points in their logs, facilitating faster forward recovery or backout operations after a system crash.
- Restarting After System Outages: In the event of a z/OS system crash or LPAR shutdown, subsystems like IMS or DB2 use their last system checkpoint to determine the state of active transactions and databases for subsequent restart and recovery.

Related Concepts

Checkpoints are fundamental to the broader concepts of recovery and restart on z/OS. They work in conjunction with logging mechanisms (e.g., IMS logs, DB2 logs, CICS journals) which record changes made between checkpoints, allowing for precise forward recovery or backout. The RESTART parameter in JCL (RESTART=stepname or `RE