What is High Availability? How HA Works & How To Achieve It

The second primary classification for availability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. Mi [1998] gives some comparison results of availability considering inherent availability. An important consideration in evaluating SLAs is to understand how well it aligns with business goals. The resulting strategy is often a tradeoff between cost and service levels in context of the business value, impact, and requirements for maintaining a reliable and available service. This means that in most verticals, especially software-driven services, a high availability architecture makes a lot of sense.
Lie, Hwang, and Tillman [1977] developed a complete survey along with a systematic classification of availability. Proper planning and cloud visualization can help you address faults quickly so that they don’t become huge problems that keep people from accessing your cloud offerings. The cloud makes it easy to build fault-tolerance into your infrastructure.

  • One way to measure this performance is to evaluate the reliability of the service that is available to consume.
  • For example, OSes such as Microsoft Windows 365 include built-in features that automatically detect and fix computer issues, and antivirus software and spyware autoprotect features include detection and removal programs.
  • A model of the entire system is created, and the model is stressed by removing components.
  • In this case it is required to have high levels of failure detectability and avoidance of common cause failures.
  • Achieving anything higher than 99% availability in-house requires expensive backups and a dedicated maintenance team.
  • Similarly, it is important to mention the difference between high availability and disaster recovery here.

No matter what size and type of business you run, any amount of downtime can be costly. Each hour of service unavailability costs revenue, turns away customers, and risks business data. From that standpoint, the cost of downtime dramatically surpasses the costs of a well-designed IT system, making investments in high availability a no-brainer decision if you’ve got the right use case. Like all other components in a HA infrastructure, the load balancer also requires redundancy to stop it from becoming a single point of failure. A high availability system must have sound data protection and disaster recovery plans. Data backup strategy is an absolute must, and a company must have the ability to recover from storage failures like data loss or corruption quickly.
Availability must be measured to be determined, ideally with comprehensive monitoring tools («instrumentation») that are themselves highly available. If users can be warned away from scheduled downtimes, then the distinction is useful. But if the requirement is for true high availability, then downtime is downtime whether or not it is scheduled. There are three principles of systems design in reliability engineering which can help achieve high availability. For critical infrastructure, such as hospital emergency rooms or power supply to nuclear power cooling plants, even the six-nines could potentially risk human lives.
No system is entirely failsafe—even a five-nines setup requires a few seconds to a minute to perform failover and switch to a backup component. Some systems are self-monitoring and use diagnostics to automatically identify and correct software and hardware faults before more serious trouble occurs. For example, OSes such as Microsoft Windows 365 include built-in features that automatically detect and fix computer issues, and antivirus software and spyware autoprotect features include detection and removal programs. Ideally, maintenance and repair operations cause as little downtime or disruption as possible.

High availability

In contrast, a high availability solution takes a software- rather than a hardware-based, approach to reducing server downtime. Instead of using physical hardware to achieve total redundancy, a high availability cluster locates a set of servers together. Multiple systems operate in tandem to achieve fault tolerance, identically mirroring applications and executing instructions together. When the main system fails, another system should take over with no loss in uptime. To achieve high availability, first identify and eliminate single points of failure in the operating system’s infrastructure. Any point that would trigger a mission critical service interruption if it was unavailable qualifies here.

It is highly cost-effective compared to a fault tolerant solution, which cannot handle software issues in the same way. High availability and fault tolerance both refer to techniques for delivering high levels of uptime. However, fault tolerant vs high availability strategies achieve that goal differently. These include recovery time, and both scheduled and unscheduled maintenance periods. Use PhoenixNAP’s backup and restore solutions to create cloud-based backups of valuable data and ensure resistance against cyberattacks, natural disasters, and employee error.

What Are High Availability Clusters?

Typically, scheduled downtime is a result of maintenance that is disruptive to system operation and usually cannot be avoided with a currently installed system design. Scheduled downtime events might include patches to system software that require a reboot or system configuration changes that only take effect upon a reboot. In general, scheduled downtime is usually the result of some logical, management-initiated event. Unscheduled downtime events typically arise from some physical event, such as a hardware or software failure or environmental anomaly.
What does availability mean in technology
The RAS concept is particularly important when designing a data center. High availability is one of the primary requirements of the control systems in unmanned vehicles and autonomous maritime vessels. If the controlling system becomes unavailable, the Ground Combat Vehicle (GCV) or ASW Continuous Trail Unmanned Vessel (ACTUV) would be lost. Muhammad Raza is a Stockholm-based technology consultant working with leading startups and Fortune 500 firms on thought leadership branding projects across DevOps, Cloud, Security and IoT. Reliability refers to the probability that the system will meet certain performance standards in yielding correct output for a desired time duration.

However, given the true definition of availability, the system will be approximately 99.9% available, or three nines (8751 hours of available time out of 8760 hours per non-leap year). Also, systems experiencing performance problems are often deemed partially or entirely unavailable by users, even when the systems are continuing to function. Similarly, unavailability of select application functions might go unnoticed by administrators yet be devastating to users – a true availability measure is holistic. Many computing sites exclude scheduled downtime from availability calculations, assuming that it has little or no impact upon the computing user community.
What does availability mean in technology
For such specific use cases, several redundant layers of IT system and utility power infrastructure are deployed to reach High Availability figures close to 100%, such as nine-nines or perhaps, even better. Availability percentage is calculated over a significant duration where typically at least one downtime incident has occurred. This can be a few hours, days, or even months, especially since IT incidents can occur for a variety of distinct causes. It then gives the duration of downtime that can be expected with a particular percentage of Availability.

Failure is only significant if this occurs during a mission critical period. Online shopping stores are expected to sell products regardless of time zone, business hours, and holidays—the last is even the source of the largest revenue streams globally. Social media outlets keep users engaged because their friends and connections are online and available for communication any time of the day. Each layer of a highly available system will have different needs in terms of software and configuration. However, at the application level, load balancers represent an essential piece of software for creating any high availability setup.
What does availability mean in technology
A service level agreement («SLA») formalizes an organization’s availability objectives and requirements. The idea is to make your products, services, and tools available to your customers and employees at any time from anywhere using any device with an internet connection. Cloud computing scalability refers to how well your system can react and adapt to changing demands. As your company grows, you want to be able to seamlessly add resources without losing quality of service or interruptions. As demand on your resources decreases, you want to be able to quickly and efficiently downscale your system so you don’t continue to pay for resources you don’t need. Availability is the assurance that an enterprise’s IT infrastructure has suitable recoverability and protection from system failures, natural disasters or malicious attacks.

definition of availability


High availability minimizes or (ideally) eliminates service downtime regardless of what incident the company runs into (a power outage, hardware failure, unresponsive apps, lost connection with the cloud provider, etc.). The term reliability refers to the ability of computer hardware and software to consistently perform according to certain specifications. https://www.globalcloudteam.com/ More specifically, it measures the likelihood that a specific system or application will meet its expected performance levels within a given time period. In computing, the term availability is used to describe the period of time when a service is available, as well as the time required by a system to respond to a request made by a user.
Moving up in the system stack, it is important to implement a reliable redundant solution for your application entry point, normally the load balancer. To remove this single point of failure, as mentioned before, we need to implement a cluster of load balancers behind a Reserved IP. Corosync and Pacemaker are popular choices for creating such a setup, on both Ubuntu and CentOS servers. For the load balancer case, however, there’s an additional complication, due to the way nameservers work. Recovering from a load balancer failure typically means a failover to a redundant load balancer, which implies that a DNS change must be made in order to point a domain name to the redundant load balancer’s IP address. A change like this can take a considerable amount of time to be propagated on the Internet, which would cause a serious downtime to this system.

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