Cold Start

Enhanced Definition

A cold start refers to the process of initializing a mainframe subsystem (such as CICS, IMS, or DB2) from scratch, without utilizing any previous state information, logs, or checkpoints. It effectively discards any in-flight transactions or recovery data from a prior run, bringing the system up in a completely clean, default state. A Cold Start is a system startup procedure, particularly in environments like CICS or IMS, where the system is initialized without using any previous state, recovery logs, or checkpoint information. It effectively starts the system "from scratch," discarding any in-flight transactions or uncommitted changes from a prior abnormal termination.

Key Characteristics

- No Recovery: A cold start performs no recovery processing; it does not attempt to back out uncommitted transactions or apply committed changes from logs.
- Loss of In-Flight Work: Any work that was active or uncommitted at the time of the previous shutdown or failure is permanently lost.
- Full Initialization: All control blocks, buffers, and system resources are initialized to their default, pristine states.
- Longer Startup Time: Can potentially take longer than a warm or emergency start due to the complete re-initialization of all components.
- Specific Parameters: Typically requires explicit JCL parameters or system commands (e.g., START CICS,TYPE=COLD) to indicate a cold start.
- Data Integrity Assumption: Assumes that external mechanisms (e.g., database recovery, application logic) will handle the integrity of data that might have been affected by the previous system state.

Use Cases

- Initial System Installation: The very first time a CICS region, IMS control region, or DB2 subsystem is brought online after installation.
- Catastrophic Log Corruption: When log files or recovery data become unrecoverable or severely corrupted, preventing a warm or emergency start.
- Development and Testing: To ensure a clean, consistent environment for testing new applications, system configurations, or during specific development cycles where previous state is irrelevant.
- Major System Upgrades: After significant upgrades or changes to the operating system or subsystem itself that might invalidate existing recovery logs.
- Complete Data Refresh: In scenarios where a complete reset of an application's state or a subsystem's operational data is intentionally desired.

Related Concepts

A cold start stands in contrast to a warm start, which uses logs and checkpoints to resume operations from a normal shutdown, and an emergency start (or auto start), which recovers from an abnormal termination by backing out uncommitted work. Unlike these recovery-oriented starts, a cold start completely bypasses log processing and checkpoint restoration, making it a more drastic measure. It impacts how data integrity is maintained, as the application or database system must handle any data consistency issues that arise from the loss of in-flight transactions.

Best Practices:

Avoid in Production: Generally, avoid cold starts in production environments unless absolutely necessary, due to the potential for data loss and increased downtime.
Robust Backup Strategy: Ensure comprehensive backup and recovery procedures are in place for all critical data that could be affected by a cold start.
Thorough Documentation: Document the exact procedures and parameters required for performing a cold start for each subsystem, including any post-startup verification steps.
Coordinate in Data Sharing: In environments like DB2 Data Sharing or IMS Data Sharing, carefully coordinate a cold start of one member with other active members to prevent data inconsistencies.
Test Procedures: Regularly test cold start procedures in non-production environments to ensure they are well-understood and effective.