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The ultimate guide to highly available systems

|Availability

The ultimate guide to highly available systems

By Rachel Scott and Jarrod Chesher

Availability is arguably the most important metric in asset heavy industries. When your systems are unavailable, you can't deliver services, and unexpected unavailability often comes at a significant opportunity cost. That's why it's so important to build highly available systems. 

To learn more about measuring availability effectively, explore our guide: The ultimate guide to asset management KPIs.

What is availability?

Availability is a measure of how often a system is operational and able to be used. It's usually expressed as a percentage, and can be thought of as "uptime." For example, if a system has an availability of 99%, that means it's operational 99% of the time.

Types of availability

Inherent availability

Inherent Availability only considers downtime that is associated with corrective maintenance. This type of availability is easy to calculate, but it doesn't take into account other factors that can impact availability, such as degradation, spikes in demand or logistics delays.

Achieved availability

Achieved Availability considers both downtime due to corrective maintenance and planned outages. This type of availability is more accurate than Inherent Availability, but it can be difficult to calculate because you need to track both types of downtime. This can be particularly difficult before you have developed a planned maintenance program for your asset.

Operational availability

Operational Availability takes into account all types of downtime, including unplanned outages, planned outages, and admin and logistics delays. This is the most accurate measure of availability, but it can be difficult to calculate and track. 

For insights into optimising maintenance tasks, read our article: How to package maintenance tasks for maximum efficiency

Instantaneous availability

Instantaneous Availability only considers whether the system is operational at a specific moment in time. Instantaneous Availability is of particular use when using modelling and simulation (e.g. Monte Carlo) to understand the expected Availability throughout specific missions and operations. Knowing when and how often Availability is going to drop below required levels will allow planning of sparing and other support arrangements.

Steady state availability

Steady State Availability only considers downtime that occurs after the system has reached its required performance level. This can be useful for long-term planning, as it allows you to understand how often your system will need to be replaced or upgraded. However, it doesn't take into account downtime that occurs during the start-up or commissioning phase of your asset.

How much availability is enough?

The answer to this question depends on your specific industry and business requirements. In general, the higher the stakes, the higher the availability needs to be. For example, in the utility industry, a power plant might need to have an availability of 99.999%, because even a brief loss of power can have major consequences.

It's important to note that availability is often a trade-off with cost. The higher the availability, the more expensive it will be to achieve. To determine the level of availability required, it is important to understand the cost of achieving that availability, as well as the opportunity cost of both expected and unexpected downtime.

Types of high availability

There are two main types of high availability:

Active-active systems

These are systems in which all components are operational at all times. This provides the highest level of availability, but also the highest cost. Active-active systems are typically used in mission critical applications, such as power plants or air traffic control.

Active-passive systems

These are systems in which only one component is active at any given time, with the other components standing by in case of failure. This provides a lower level of availability than active-active systems, but is also less expensive.

Building a highly available system

There are several key steps to build a highly available system:

  1. Redundancy: Having multiple components that can perform the same function
  2. Fault tolerance: Designing your system to continue operating even if components fail
  3. Load balancing: Distributing workload across multiple servers to prevent single points of failure
  4. Backup and recovery: Having robust data recovery plans in place
  5. Monitoring and management: Implementing proactive system health monitoring

Conclusion

Building a highly available system is crucial for ensuring operational continuity. By understanding the different types of availability, implementing appropriate redundancy measures, and maintaining effective monitoring systems, organisations can develop robust systems that meet their availability requirements while balancing costs and risks. Contact us to see how we can help your business and its systems. 

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