ETR - External Time Reference

Enhanced Definition

The External Time Reference (ETR) is a hardware component used in IBM mainframe environments to synchronize the Time-of-Day (TOD) clocks of multiple z/OS systems. It ensures that all connected processors operate with a consistent and unified time, which is crucial for data integrity and consistency across a multi-system complex.

Key Characteristics

- Hardware Component: The ETR is a physical hardware device, typically a standalone unit, connected to multiple mainframe processors via dedicated cables.
- Time Synchronization: Its primary function is to provide a common, synchronized time source to all attached z/OS systems, preventing time drifts between individual CPCs (Central Processor Complexes).
- Time-of-Day (TOD) Clock: It directly influences and synchronizes the internal TOD clocks of the connected mainframe processors, ensuring they all tick in unison.
- Predecessor to Sysplex Timer: The ETR was an earlier technology for time synchronization, largely superseded by the IBM Sysplex Timer (9037-002) and later by Server Time Protocol (STP) in modern Parallel Sysplex environments.
- Critical for Data Consistency: Without synchronized time, events recorded on different systems could appear out of order, leading to data integrity issues, especially in shared data environments.

Use Cases

- Multi-System Data Sharing: Essential for environments where multiple z/OS systems share common data resources, such as DB2 Data Sharing groups or IMS Data Sharing, to ensure transaction ordering and data consistency.
- Sysplex Environments (Legacy): Provided the fundamental time synchronization required for early Sysplex configurations, enabling coordinated operations and resource management across multiple LPARs or CPCs.
- Disaster Recovery and High Availability: Facilitates consistent logging and recovery operations across geographically dispersed or redundant systems by ensuring a unified timeline for events.
- Auditing and Compliance: Ensures that timestamps on audit logs and system records are consistent across all participating systems, which is vital for regulatory compliance and forensic analysis.

Related Concepts

The ETR is foundational to the concept of a Sysplex, particularly in its early implementations, by providing the necessary time synchronization for coordinated operations. It directly synchronizes the Time-of-Day (TOD) clocks of individual processors, which are critical for event sequencing. The ETR was later succeeded by the IBM Sysplex Timer, which offered enhanced capabilities and reliability, and ultimately by Server Time Protocol (STP), a software-based solution that eliminated the need for dedicated external hardware for time synchronization in modern Parallel Sysplex environments. Its role is paramount for maintaining data integrity in shared data environments like DB2 Data Sharing.

Best Practices:

Redundancy: Implement at least two ETRs (or Sysplex Timers/STP configurations) for redundancy to ensure continuous time synchronization even if one unit fails.
Regular Monitoring: Continuously monitor the status and health of the ETR and its connections to all connected CPCs to detect and address any synchronization issues promptly.
Physical Security: Secure the physical ETR hardware in a controlled environment to prevent unauthorized access or tampering, which could compromise system time integrity.
Migration to Modern Solutions: For new or upgraded environments, prioritize migrating from ETRs to Sysplex Timers or, preferably, to Server Time Protocol (STP) for improved functionality, manageability, and reduced hardware footprint.
Configuration Verification: Regularly verify the ETR configuration within the HMC (Hardware Management Console) and z/OS SYSPLEX parameters to ensure all systems are correctly referencing the synchronized time source.