Failover

Enhanced Definition

Failover is a critical process in mainframe environments that involves automatically or manually switching to a redundant or standby system, component, or data center when the primary one becomes unavailable. Its primary purpose is to ensure continuous availability, business continuity, and disaster recovery for mission-critical applications and data on z/OS.

Key Characteristics

- Redundancy Requirement: Failover inherently relies on the existence of redundant hardware, software, or data paths, such as multiple LPARs, sysplex members, or replicated storage.
- Automatic vs. Manual: It can be triggered automatically by monitoring systems detecting a failure, or manually initiated by operators during planned outages or disaster recovery scenarios.
- Impact on RTO/RPO: The effectiveness of a failover strategy directly determines the Recovery Time Objective (RTO) and Recovery Point Objective (RPO) for critical business services.
- Data Consistency: Maintaining data integrity and consistency across primary and standby systems is paramount, often achieved through synchronous or asynchronous data replication technologies.
- Application Transparency: Ideally, applications should be designed to be largely unaware of a failover, automatically reconnecting to new resources or having their workload redirected.
- Pre-configured Procedures: Requires extensive planning, configuration, and testing of recovery procedures to ensure a smooth and predictable transition.

Use Cases

- Data Center Disaster Recovery: Shifting an entire production workload from a primary data center to a geographically remote backup data center following a catastrophic event.
- Sysplex Member Failure: When an LPAR within an IBM Parallel Sysplex fails, Workload Manager (WLM) can automatically redirect work and resources to other healthy LPARs in the sysplex.
- Database High Availability: Switching from a primary DB2 or IMS database instance to a standby or replicated instance to maintain data access and transaction processing.
- CICS Region Failure: In a CICSplex, if a CICS region fails, CICS transaction routing can redirect incoming transactions to other available regions.
- Hardware Component Failure: Utilizing redundant components like power supplies, network adapters, or storage controllers to seamlessly switch operations without interrupting the system.

Related Concepts

Failover is a cornerstone of High Availability (HA) and Disaster Recovery (DR) strategies on the mainframe. It is deeply integrated with the IBM Parallel Sysplex architecture, which provides the underlying shared data and resource management capabilities necessary for many z/OS failover scenarios. Technologies like GDPS (Geographically Dispersed Parallel Sysplex), PPRC (Peer-to-Peer Remote Copy), and XRC (Extended Remote Copy) are fundamental for data replication and orchestration of failover across distances.

Best Practices:

Regular Testing: Conduct periodic, realistic failover drills (e.g., GDPS drills) to validate recovery procedures, RTO/RPO targets, and staff readiness.
Automate Where Possible: Automate as many steps of the failover process as feasible to reduce human error, improve recovery speed, and ensure consistency.
Robust Monitoring: Implement comprehensive monitoring across all critical components to detect failures quickly and accurately, enabling timely failover initiation.
Ensure Data Integrity: Utilize appropriate data replication technologies (PPRC, XRC, DB2 Data Sharing, IMS Shared Queues) and recovery mechanisms to guarantee data consistency and prevent loss or corruption during a failover.
Document Procedures Thoroughly: Maintain clear, concise, and up-to-date documentation for all failover procedures, including manual steps, contact lists, and verification checks.