What is High Availability (HA)?
High Availability (HA) is the design principle whereby systems and networks are configured such that business-critical functions remain continuously available, even in the event of failures or component outages.
In industrial automation, HA is essential for systems that must guarantee 24/7 production, process monitoring or safety, such as SCADA, Historian, PLC networks or MES.
🧠 How does High Availability work?
- Redundancy
- Duplicated hardware components such as servers, power supplies, Switches or Firewalls
- Redundant network paths (e.g. RSTP, MRP, PRP)
- Failover mechanisms
- In the event of failure, the system automatically switches to a working component
- Downtime is reduced to seconds or even milliseconds
- Monitoring and detection
- Continuous monitoring of system status
- Automatic recovery or alerting on deviations
HA is not a product but a combination of design choices, configuration and testing.
🏭 Application in industrial networks and automation
- Redundant SCADA servers with synchronised databases
- Failover PLCs or dual-redundant control units
- Redundant communication via Ring Redundancy, DLR, ProfiNET media rings
- HA in data logging via Historian clusters or Time Series Database
- OT Firewalls and Routers in active/passive configuration
Relevant sectors:
- Power plants, water treatment, chemicals, food, pharma, tunnel and rail technology
🔍 Examples of HA architectures
| Architecture | Description |
|---|---|
| Active/Passive | One node active, the other on standby for failover |
| Active/Active | Both nodes active, with load distribution |
| Clustered HA | A group of systems shares workload and state |
| Redundant networks | PRP, MRP, RSTP for continuous data flow |
| Virtualisation with failover | VMs run on different hosts (e.g. via vSphere) |
The right HA solution depends on the criticality of the process and the desired RTO (Recovery Time Objective).
🔐 Security aspects
- Redundancy increases availability, but also requires duplicated Access Control and patch management
- Monitoring and anomaly detection must monitor both nodes
- Backup and Restore procedures must work in mirrored systems
- Audit trails must log both systems
- Failover must not create a security gap (such as automatic open connections)
HA is not a substitute for Security, but should integrate with it.