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High availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.[1]

There is now more dependence on these systems as a result of modernization. For example, to carry out their regular daily tasks, hospitals and data centers need their systems to be highly available. Availability refers to the ability of the user to access a service or system, whether to submit new work, update or modify existing work, or retrieve the results of previous work. If a user cannot access the system, it is considered unavailable from the user's perspective.[2] The term downtime is generally used to refer to describe periods when a system is unavailable.

Resilience

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High availability is a property of network resilience, the ability to "provide and maintain an acceptable level of service in the face of faults and challenges to normal operation."[3] Threats and challenges for services can range from simple misconfiguration over large scale natural disasters to targeted attacks.[4] As such, network resilience touches a very wide range of topics. In order to increase the resilience of a given communication network, the probable challenges and risks have to be identified and appropriate resilience metrics have to be defined for the service to be protected.[5]

The importance of network resilience is continuously increasing, as communication networks are becoming a fundamental component in the operation of critical infrastructures.[6] Consequently, recent efforts focus on interpreting and improving network and computing resilience with applications to critical infrastructures.[7] As an example, one can consider as a resilience objective the provisioning of services over the network, instead of the services of the network itself. This may require coordinated response from both the network and from the services running on top of the network.[8]

These services include:

Resilience and survivability are interchangeably used according to the specific context of a given study.[9]

Principles

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Further information: Design for availability

There are three principles of systems design in reliability engineering that can help achieve high availability.

  1. Elimination of single points of failure. This means adding or building redundancy into the system so that failure of a component does not mean failure of the entire system.
  2. Reliable crossover. In redundant systems, the crossover point itself tends to become a single point of failure. Reliable systems must provide for reliable crossover.
  3. Detection of failures as they occur. If the two principles above are observed, then a user may never see a failure – but the maintenance activity must.

Scheduled and unscheduled downtime

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A distinction can be made between scheduled and unscheduled downtime. 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. Examples of unscheduled downtime events include power outages, failed CPU or RAM components (or possibly other failed hardware components), an over-temperature related shutdown, logically or physically severed network connections, security breaches, or various application, middleware, and operating system failures.

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.

Many computing sites exclude scheduled downtime from availability calculations, assuming that it has little or no impact upon the computing user community. By doing this, they can claim to have phenomenally high availability, which might give the illusion of continuous availability. Systems that exhibit truly continuous availability are comparatively rare and higher priced, and most have carefully implemented specialty designs that eliminate any single point of failure and allow online hardware, network, operating system, middleware, and application upgrades, patches, and replacements. For certain systems, scheduled downtime does not matter, for example, system downtime at an office building after everybody has gone home for the night.

Percentage calculation

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Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements often refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable.

Availability % Downtime per year[note 1] Downtime per quarter Downtime per month Downtime per week Downtime per day (24 hours)
90% ("one nine") 36.53 days 9.13 days 73.05 hours 16.80 hours 2.40 hours
95% ("one nine five") 18.26 days 4.56 days 36.53 hours 8.40 hours 1.20 hours
97% ("one nine seven") 10.96 days 2.74 days 21.92 hours 5.04 hours 43.20 minutes
98% ("one nine eight") 7.31 days 43.86 hours 14.61 hours 3.36 hours 28.80 minutes
99% ("two nines") 3.65 days 21.9 hours 7.31 hours 1.68 hours 14.40 minutes
99.5% ("two nines five") 1.83 days 10.98 hours 3.65 hours 50.40 minutes 7.20 minutes
99.8% ("two nines eight") 17.53 hours 4.38 hours 87.66 minutes 20.16 minutes 2.88 minutes
99.9% ("three nines") 8.77 hours 2.19 hours 43.83 minutes 10.08 minutes 1.44 minutes
99.95% ("three nines five") 4.38 hours 65.7 minutes 21.92 minutes 5.04 minutes 43.20 seconds
99.99% ("four nines") 52.60 minutes 13.15 minutes 4.38 minutes 1.01 minutes 8.64 seconds
99.995% ("four nines five") 26.30 minutes 6.57 minutes 2.19 minutes 30.24 seconds 4.32 seconds
99.999% ("five nines") 5.26 minutes 1.31 minutes 26.30 seconds 6.05 seconds 864.00 milliseconds
99.9999% ("six nines") 31.56 seconds 7.89 seconds 2.63 seconds 604.80 milliseconds 86.40 milliseconds
99.99999% ("seven nines") 3.16 seconds 0.79 seconds 262.98 milliseconds 60.48 milliseconds 8.64 milliseconds
99.999999% ("eight nines") 315.58 milliseconds 78.89 milliseconds 26.30 milliseconds 6.05 milliseconds 864.00 microseconds
99.9999999% ("nine nines") 31.56 milliseconds 7.89 milliseconds 2.63 milliseconds 604.80 microseconds 86.40 microseconds
99.99999999% ("ten nines") 3.16 milliseconds 788.40 microseconds 262.80 microseconds 60.48 microseconds 8.64 microseconds
99.999999999% ("eleven nines") 315.58 microseconds 78.84 microseconds 26.28 microseconds 6.05 microseconds 864.00 nanoseconds
99.9999999999% ("twelve nines") 31.56 microseconds 7.88 microseconds 2.63 microseconds 604.81 nanoseconds 86.40 nanoseconds

The terms uptime and availability are often used interchangeably but do not always refer to the same thing. For example, a system can be "up" with its services not "available" in the case of a network outage. Or a system undergoing software maintenance can be "available" to be worked on by a system administrator, but its services do not appear "up" to the end user or customer. The subject of the terms is thus important here: whether the focus of a discussion is the server hardware, server OS, functional service, software service/process, or similar, it is only if there is a single, consistent subject of the discussion that the words uptime and availability can be used synonymously.

Five-by-five mnemonic

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A simple mnemonic rule states that 5 nines allows approximately 5 minutes of downtime per year. Variants can be derived by multiplying or dividing by 10: 4 nines is 50 minutes and 3 nines is 500 minutes. In the opposite direction, 6 nines is 0.5 minutes (30 sec) and 7 nines is 3 seconds.

"Powers of 10" trick

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Another memory trick to calculate the allowed downtime duration for an "-nines" availability percentage is to use the formula seconds per day.

For example, 90% ("one nine") yields the exponent , and therefore the allowed downtime is seconds per day.

Also, 99.999% ("five nines") gives the exponent , and therefore the allowed downtime is seconds per day.

"Nines"

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Main article: Nine (purity)

Percentages of a particular order of magnitude are sometimes referred to by the number of nines or "class of nines" in the digits. For example, electricity that is delivered without interruptions (blackouts, brownouts or surges) 99.999% of the time would have 5 nines reliability, or class five.[10] In particular, the term is used in connection with mainframes[11][12] or enterprise computing, often as part of a service-level agreement.

Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then "five", so 99.95% is "three nines five", abbreviated 3N5.[13][14] This is casually referred to as "three and a half nines",[15] but this is incorrect: a 5 is only a factor of 2, while a 9 is a factor of 10, so a 5 is 0.3 nines (per below formula: ):[note 2] 99.95% availability is 3.3 nines, not 3.5 nines.[16] More simply, going from 99.9% availability to 99.95% availability is a factor of 2 (0.1% to 0.05% unavailability), but going from 99.95% to 99.99% availability is a factor of 5 (0.05% to 0.01% unavailability), over twice as much.[note 3]

A formulation of the class of 9s based on a system's unavailability would be

(cf. Floor and ceiling functions).

A similar measurement is sometimes used to describe the purity of substances.

In general, the number of nines is not often used by a network engineer when modeling and measuring availability because it is hard to apply in formula. More often, the unavailability expressed as a probability (like 0.00001), or a downtime per year is quoted. Availability specified as a number of nines is often seen in marketing documents.[citation needed] The use of the "nines" has been called into question, since it does not appropriately reflect that the impact of unavailability varies with its time of occurrence.[17] For large amounts of 9s, the "unavailability" index (measure of downtime rather than uptime) is easier to handle. For example, this is why an "unavailability" rather than availability metric is used in hard disk or data link bit error rates.

Sometimes the humorous term "nine fives" (55.5555555%) is used to contrast with "five nines" (99.999%),[18][19][20] though this is not an actual goal, but rather a sarcastic reference to something totally failing to meet any reasonable target.

Measurement and interpretation

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Availability measurement is subject to some degree of interpretation. A system that has been up for 365 days in a non-leap year might have been eclipsed by a network failure that lasted for 9 hours during a peak usage period; the user community will see the system as unavailable, whereas the system administrator will claim 100% uptime. 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.

Availability must be measured to be determined, ideally with comprehensive monitoring tools ("instrumentation") that are themselves highly available. If there is a lack of instrumentation, systems supporting high volume transaction processing throughout the day and night, such as credit card processing systems or telephone switches, are often inherently better monitored, at least by the users themselves, than systems which experience periodic lulls in demand.

An alternative metric is mean time between failures (MTBF).

[edit]

Recovery time (or estimated time of repair (ETR), also known as recovery time objective (RTO) is closely related to availability, that is the total time required for a planned outage or the time required to fully recover from an unplanned outage. Another metric is mean time to recovery (MTTR). Recovery time could be infinite with certain system designs and failures, i.e. full recovery is impossible. One such example is a fire or flood that destroys a data center and its systems when there is no secondary disaster recovery data center.

Another related concept is data availability, that is the degree to which databases and other information storage systems faithfully record and report system transactions. Information management often focuses separately on data availability, or Recovery Point Objective, in order to determine acceptable (or actual) data loss with various failure events. Some users can tolerate application service interruptions but cannot tolerate data loss.

A service level agreement ("SLA") formalizes an organization's availability objectives and requirements.

Military control systems

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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.

System design

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On one hand, adding more components to an overall system design can undermine efforts to achieve high availability because complex systems inherently have more potential failure points and are more difficult to implement correctly. While some analysts would put forth the theory that the most highly available systems adhere to a simple architecture (a single, high-quality, multi-purpose physical system with comprehensive internal hardware redundancy), this architecture suffers from the requirement that the entire system must be brought down for patching and operating system upgrades. More advanced system designs allow for systems to be patched and upgraded without compromising service availability (see load balancing and failover). High availability requires less human intervention to restore operation in complex systems; the reason for this being that the most common cause for outages is human error.[21]

High availability through redundancy

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On the other hand, redundancy is used to create systems with high levels of availability (e.g. popular ecommerce websites). In this case it is required to have high levels of failure detectability and avoidance of common cause failures.

If redundant parts are used in parallel and have independent failure (e.g. by not being within the same data center), they can exponentially increase the availability and make the overall system highly available. If you have N parallel components each having X availability, then you can use following formula:[22][23]

Availability of parallel components = 1 - (1 - X)^ N

10 hosts, each having 50% availability. But if they are used in parallel and fail independently, they can provide high availability.
10 hosts, each having 50% availability. But if they are used in parallel and fail independently, they can provide high availability.

So for example if each of your components has only 50% availability, by using 10 of components in parallel, you can achieve 99.9023% availability.

Two kinds of redundancy are passive redundancy and active redundancy.

Passive redundancy is used to achieve high availability by including enough excess capacity in the design to accommodate a performance decline. The simplest example is a boat with two separate engines driving two separate propellers. The boat continues toward its destination despite failure of a single engine or propeller. A more complex example is multiple redundant power generation facilities within a large system involving electric power transmission. Malfunction of single components is not considered to be a failure unless the resulting performance decline exceeds the specification limits for the entire system.

Active redundancy is used in complex systems to achieve high availability with no performance decline. Multiple items of the same kind are incorporated into a design that includes a method to detect failure and automatically reconfigure the system to bypass failed items using a voting scheme. This is used with complex computing systems that are linked. Internet routing is derived from early work by Birman and Joseph in this area.[24][non-primary source needed] Active redundancy may introduce more complex failure modes into a system, such as continuous system reconfiguration due to faulty voting logic.

Zero downtime system design means that modeling and simulation indicates mean time between failures significantly exceeds the period of time between planned maintenance, upgrade events, or system lifetime. Zero downtime involves massive redundancy, which is needed for some types of aircraft and for most kinds of communications satellites. Global Positioning System is an example of a zero downtime system.

Fault instrumentation can be used in systems with limited redundancy to achieve high availability. Maintenance actions occur during brief periods of downtime only after a fault indicator activates. Failure is only significant if this occurs during a mission critical period.

Modeling and simulation is used to evaluate the theoretical reliability for large systems. The outcome of this kind of model is used to evaluate different design options. A model of the entire system is created, and the model is stressed by removing components. Redundancy simulation involves the N-x criteria. N represents the total number of components in the system. x is the number of components used to stress the system. N-1 means the model is stressed by evaluating performance with all possible combinations where one component is faulted. N-2 means the model is stressed by evaluating performance with all possible combinations where two component are faulted simultaneously.

Reasons for unavailability

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A survey among academic availability experts in 2010 ranked reasons for unavailability of enterprise IT systems. All reasons refer to not following best practice in each of the following areas (in order of importance):[25]

  1. Monitoring of the relevant components
  2. Requirements and procurement
  3. Operations
  4. Avoidance of network failures
  5. Avoidance of internal application failures
  6. Avoidance of external services that fail
  7. Physical environment
  8. Network redundancy
  9. Technical solution of backup
  10. Process solution of backup
  11. Physical location
  12. Infrastructure redundancy
  13. Storage architecture redundancy

A book on the factors themselves was published in 2003.[26]

Costs of unavailability

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In a 1998 report from IBM Global Services, unavailable systems were estimated to have cost American businesses $4.54 billion in 1996, due to lost productivity and revenues.[27]

See also

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Notes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

High availability

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High availability (HA) is a critical characteristic of computer systems, networks, and applications designed to ensure continuous operation and accessibility with minimal downtime, often targeting uptime levels of 99.9% or higher through mechanisms such as redundancy and failover to mitigate failures in hardware, software, or infrastructure. This approach eliminates single points of failure and enables seamless recovery from interruptions, maintaining service reliability in demanding environments like data centers and cloud platforms. The importance of high availability stems from its role in supporting business continuity and user expectations in mission-critical sectors, where even brief outages can result in significant financial losses or safety risks, as seen in , healthcare, and applications. Availability is typically measured in "nines," representing the percentage of uptime over a year—for instance, three nines (99.9%) allows about 8.76 hours of annual downtime, while five nines (99.999%) limits it to roughly 5.26 minutes. In , HA is essential for sustaining customer trust and preventing revenue impacts from service disruptions. Key techniques for achieving high availability include hardware and software , such as deploying primary and standby resources across fault domains or availability zones to enable automatic failover during component failures. Clustering and load balancing distribute workloads to prevent overloads, while geographic —pairing systems at separate locations—protects against site-wide issues like power outages or . These methods draw from fault-tolerant design principles developed since the late , emphasizing empirical and repair strategies to enhance overall system reliability. In modern contexts, high availability has evolved with cloud-native architectures and middleware solutions that automate recovery and scaling, ensuring resilient performance for distributed applications. For example, in , controller clustering provides HA by synchronizing states across nodes to maintain network service continuity. Overall, HA remains a foundational for IT infrastructures aiming to deliver uninterrupted services.

Fundamentals

Definition and Importance

High availability (HA) refers to the design and implementation of computer systems, networks, and applications that ensure continuous operation and minimal , even in the presence of hardware , software errors, or other disruptions. It focuses on maintaining an agreed level of operational performance, typically targeting uptime of 99.9% or higher, to support seamless service delivery over extended periods. This approach integrates , mechanisms, and monitoring to prevent single points of from halting services. The scope of HA extends across hardware components like servers and storage, software architectures such as distributed applications, network infrastructures for connectivity, and operational processes for maintenance and recovery. Unlike basic reliability, which measures a system's probability of performing its functions correctly without failure over time, HA proactively minimizes interruptions through built-in resilience, emphasizing rapid detection and recovery to sustain user access. HA is critically important in sectors reliant on uninterrupted operations, including , healthcare, , and , where downtime can incur massive financial losses, regulatory penalties, and risks. In , for example, a 2012 software at Knight Capital resulted in $440 million in losses within 45 minutes due to unintended stock trades. Healthcare systems face similar threats; the 2024 cyberattack on led to over $2.45 billion in costs for and widespread disruptions in claims processing and patient care. In , brief outages at platforms like Amazon can cost around $220,000 per minute in foregone sales. These examples underscore how HA safeguards revenue, compliance, and trust in mission-critical environments.

Historical Context

The origins of high availability (HA) in computing trace back to the mid-20th century, driven by the need for reliable systems in military and critical applications. In the 1950s and 1960s, the (SAGE) air defense system, developed by and for the U.S. , represented an early milestone in design. SAGE employed dual AN/FSQ-7 processors per site, with one on hot standby to ensure continuous operation despite the unreliability of vacuum tubes, achieving approximately 99% uptime through redundancy and marginal checking to detect failing components before total breakdown. This emphasis on influenced subsequent mainframe developments, such as IBM's System/360 in the 1960s, where modular designs and error-correcting memory began addressing (MTBF) that were often limited to hours in early systems. By the 1970s, commercial HA systems emerged, exemplified by ' NonStop architecture, introduced in 1976. The Tandem/16, deployed initially for banking applications like Citibank's , featured paired processors with lockstep execution and automatic , enabling continuous operation without data loss in fault-tolerant environments. The 1980s and 1990s saw significant advancements in distributed and storage technologies. Unix-based clustering gained traction, with systems like DEC's VMS Cluster (evolving from the 1970s) and ' early work in the 1980s enabling shared resources across nodes for improved resilience. Concurrently, the introduction of Redundant Arrays of Inexpensive Disks (RAID) in 1987 by researchers at UC Berkeley provided a framework for data redundancy, with the 1988 paper outlining levels like RAID-1 () and RAID-5 (parity) to enhance storage availability against disk failures. Hot-swappable hardware also proliferated in this era, particularly in mid-1990s rackmount servers from vendors like and HP, allowing component replacement without system downtime to support enterprise HA. The 2000s marked a pivotal shift influenced by the boom and , where downtime directly impacted revenue, prompting the widespread adoption of service level agreements (SLAs) with explicit uptime guarantees, often targeting 99.9% or higher availability. A key catalyst was the 1988 , which infected thousands of Unix systems, causing 5-10% of the early to go offline and underscoring the vulnerabilities in networked environments, thereby accelerating investments in resilient architectures and the formation of the for incident response. Post-2000, technologies transformed HA practices; 's Workstation, released in 1999, enabled x86-based virtual machines, paving the way for clustered virtualization features introduced in Virtual Infrastructure 3 (2006), which automated VM migration and to minimize outages and evolved into vSphere (introduced 2009). The 2010s ushered in the cloud era, with (AWS), launching EC2 in 2006, and , debuting in 2010, popularizing elastic HA through auto-scaling groups, multi-region replication, and managed services that abstracted complexity for global-scale availability. These platforms shifted HA from hardware-centric to software-defined models, enabling dynamic resource provisioning to meet SLA commitments in distributed environments.

Core Principles

Reliability and Resilience

Reliability in high availability systems refers to the probability that a system or component will perform its required functions without failure under specified conditions for a designated period of time. This concept is foundational to ensuring consistent operation, drawing from established principles that emphasize the prevention of faults through robust design and material selection. Core metrics for assessing reliability include Mean Time Between Failures (MTBF), which quantifies the average operational time between consecutive failures in repairable systems, and (MTTR), which measures the average duration required to restore functionality after a failure. Higher MTBF values indicate greater system dependability, while minimizing MTTR supports faster recovery, both critical for maintaining service continuity in demanding environments like data centers or . Resilience, in contrast, encompasses a system's capacity to anticipate, withstand, and recover from adverse events such as hardware malfunctions, software bugs, or cyberattacks, while adapting to evolving threats without complete loss of functionality. This involves principles like graceful degradation, where the system reduces non-essential operations to preserve core services during overload or partial failure, ensuring partial operability rather than total shutdown. Complementing this are self-healing mechanisms, which enable automated detection, diagnosis, and remediation of issues, such as restarting faulty components or rerouting traffic, thereby minimizing human intervention and in dynamic IT ecosystems. These elements allow resilient systems to maintain essential capabilities even under stress, as outlined in cybersecurity frameworks. The interplay between reliability and resilience lies in their complementary roles: reliability proactively minimizes the occurrence of failures through inherent strengths, while resilience reactively limits the consequences when failures inevitably arise, creating a layered defense for high . For instance, in , bridge s incorporate reliable structural materials to prevent collapse (high MTBF) alongside resilient features like flexible joints and redundant supports that absorb shocks from earthquakes, allowing the structure to deform without and recover post-event. Adapted to IT, this means building systems with reliable hardware (e.g., fault-tolerant processors) that, when combined with resilient software protocols (e.g., automatic ), ensure minimal disruption—preventing minor glitches from escalating into outages. Such integration not only enhances overall system robustness but also serves as a prerequisite for accurate availability measurement by clearly delineating "available" as a state of functional performance despite perturbations.

Redundancy Fundamentals

Redundancy is a foundational strategy in high availability (HA) systems, involving the duplication of critical components, processes, or to prevent any (SPOF) from disrupting overall system operation. By incorporating backup elements that can seamlessly take over during failures, ensures that services remain accessible and functional, minimizing and supporting continuous business operations. This approach is essential for eliminating SPOFs, where a single component failure could otherwise cascade into widespread unavailability. Common types of redundancy configurations include active-active, active-passive, and setups. In an active-active configuration, multiple systems operate simultaneously, sharing the workload and providing mutual support without idle resources. An active-passive setup, by contrast, maintains one primary active system handling all operations while a secondary passive system remains on standby, activating only upon failure detection to assume responsibilities. The model provisions one extra unit beyond the minimum required (N) to handle normal loads, allowing the system to tolerate the loss of any single component while preserving capacity. The primary benefits of redundancy lie in its ability to eradicate SPOFs and enhance reliability through mechanisms. For instance, hardware redundancy examples include supplies in servers, which ensure uninterrupted power delivery if one supply fails, and redundant network interface cards to maintain connectivity despite link failures. In software contexts, mirrored databases replicate across multiple nodes, enabling immediate access to backups if the primary instance encounters issues, thus preventing or service interruption. These implementations directly support resilience by establishing alternative paths for operation, allowing systems to recover swiftly from faults without user impact. Despite its advantages, redundancy introduces notable challenges, particularly in terms of increased system complexity and operational costs. Duplicating components requires additional resources for , , and monitoring, elevating overall expenses while complicating and . Synchronization across redundant elements poses further difficulties, such as maintaining consistency in replicated systems, where asynchronous updates can lead to temporary discrepancies or conflicts during . These issues demand careful design to balance gains against the added overhead.

Measurement and Metrics

Uptime Calculation

Uptime in high availability systems is quantified using the basic formula for availability: Availability = (Total Time - Downtime) / Total Time, typically expressed as a percentage. This metric represents the proportion of time a system is operational over a defined period, such as a month or year. To convert availability percentages to allowable downtime, the equation Downtime (hours per year) = 8760 × (1 - Availability) is commonly applied, assuming a non-leap year with 365 days × 24 hours. For leap years, the total time adjusts to 8784 hours, slightly reducing allowable downtime for the same percentage (e.g., 99.9% availability permits approximately 8.76 hours in a non-leap year but 8.78 hours in a leap year). The "nines" system provides a shorthand for expressing high availability levels, where each additional "nine" after the decimal point indicates greater reliability. For instance, three nines (99.9%) allows about 8.76 hours of downtime per year, while five nines (99.999%) permits roughly 5.26 minutes annually. This system emphasizes the exponential decrease in tolerable outages as nines increase. A common mnemonic for five nines is the "five-by-five" approximation, recalling that 99.999% equates to approximately 5 minutes of downtime per year. Additionally, the "powers of 10" approach aids quick estimation: each additional nine divides the allowable downtime by 10, as unavailability scales from 0.1 (one nine) to 0.00001 (five nines) of total time. The following table details allowable annual downtime for availability levels from one to seven nines, based on 8760 hours in a non-leap year:
Nines (%) (days) (hours) (minutes) (seconds)
19036.5---
299-87.6--
399.9-8.76--
499.99--52.56-
599.999--5.256-
699.9999---31.536
799.99999---3.1536
Service level agreements (SLAs) frequently incorporate these calculations to define contractual uptime guarantees. For example, Amazon Web Services (AWS) commits to 99.99% monthly uptime for Amazon EC2 instances in each region, translating to no more than about 4.32 minutes of downtime per month.

Interpreting Availability Levels

The Uptime Institute's Tier Classification System categorizes data center infrastructure into four levels, each defining escalating standards for reliability and redundancy that translate to specific availability percentages. Tier 1 represents basic infrastructure with no redundancy, delivering approximately 99.671% availability and permitting up to 28.8 hours of annual downtime. Tier 4, by contrast, incorporates fault-tolerant components with comprehensive dual-path redundancy, achieving 99.995% availability and restricting downtime to roughly 26 minutes per year. These tiers guide organizations in aligning infrastructure investments with targeted availability goals, emphasizing that higher tiers exponentially increase complexity and cost to minimize unplanned outages. In practice, interpreting availability levels involves assessing feasibility and inherent trade-offs, particularly as targets approach five nines (99.999%), which equates to no more than 5.26 minutes of annually. Attaining this requires global-scale , such as multi-region data replication and automated across geographically dispersed sites, to withstand disasters or network partitions. Yet, , which contributes to 66% to 80% of all incidents according to recent industry analyses, poses a persistent challenge, often undermining even robust designs through misconfigurations or procedural lapses, making six or more nines increasingly impractical without extensive and rigorous training. Contextual factors heavily influence the interpretation of these levels, as the tolerance for downtime varies by use case. For consumer-facing web applications, 99.9% availability—allowing about 8.76 hours of yearly downtime—is typically adequate, balancing user expectations with manageable costs in dynamic cloud environments. In contrast, safety-critical applications like air traffic control systems mandate six nines (99.9999%), permitting only 31.5 seconds of annual downtime, where even brief interruptions could endanger lives and require redundant, real-time synchronized architectures. Monitoring tools play a crucial role in validating and interpreting in real time, enabling proactive detection of deviations. Nagios offers comprehensive host and service monitoring with threshold-based alerting to track uptime across infrastructure components. , designed for cloud-native ecosystems, collects time-series metrics for distributed services, facilitating queries and dashboards that reveal availability patterns beyond simple binary states. Traditional availability metrics, often derived from uptime calculations for monolithic systems, reveal significant gaps when applied to modern distributed architectures, where partial failures or user-specific degradations defy single-point assessments. In microservices-based environments, end-to-end availability may appear high overall but mask localized issues, such as latency spikes affecting subsets of traffic, necessitating advanced practices like distributed tracing to capture holistic system health.

Design and Implementation

Architectural Strategies

High availability (HA) architectures emphasize designs that minimize single points of and ensure continuous operation through structured approaches to organization. Traditionally, monolithic architectures integrated all components into a single deployable unit, which, while simpler for small-scale applications, posed risks to HA due to their tight and limited fault isolation; a in one module could cascade across the entire . In contrast, distributed architectures, particularly , decompose applications into independent, loosely coupled services that can be scaled, updated, and recovered individually, thereby improving resilience and enabling higher availability levels by containing faults to specific services. This shift from monolithic to microservices-based designs has become a for achieving HA in modern s, as it facilitates better and rapid recovery without affecting the whole application. A layered approach to HA integrates redundancy and fault tolerance across distinct system strata, ensuring comprehensive coverage from foundational infrastructure to user-facing components. At the network layer, protocols like (BGP) provide routing redundancy by maintaining multiple paths and enabling automatic during link or router failures, which is essential for sustaining connectivity in large-scale networks. In the application layer, adopting stateless designs—where applications do not retain session data between requests—allows for seamless load balancing and horizontal scaling across servers, reducing from instance failures as any server can handle any request without state synchronization overhead. For the storage layer, replicated databases employ techniques such as chain replication, where data is synchronously mirrored across a chain of nodes to guarantee high throughput and availability even if individual nodes fail, maintaining data consistency and accessibility. This stratified implementation ensures that HA is not siloed but holistically addresses potential disruptions at each level. Key best practices in HA architectures promote flexibility and automation to sustain operational continuity. between components minimizes interdependencies, allowing isolated updates and failures without propagating issues, as demonstrated in service-oriented designs that enhance overall system resilience. Automation through Infrastructure as Code (IaC) treats infrastructure configurations as version-controlled software, enabling reproducible deployments and rapid recovery from misconfigurations or outages via automated provisioning tools. Zero-downtime deployments, such as strategies, maintain two identical production environments—one active (blue) and one staging (green)—switching traffic instantaneously upon validation to eliminate interruptions during updates. Redundancy fundamentals underpin these practices by providing the necessary duplication of resources to support . Standards like integrate HA into broader business continuity management systems (BCMS) by requiring organizations to identify critical IT dependencies, implement resilient architectures, and conduct regular testing to ensure operational continuity amid disruptions. This standard emphasizes a systematic approach to aligning HA designs with organizational risk profiles, fostering proactive measures that extend beyond technical layers to encompass policy and recovery planning.

Key Techniques for HA

Failover and failback are essential mechanisms in high availability systems, enabling automatic switching from a primary component to a redundant backup upon failure detection, followed by restoration to the original setup once resolved. This process minimizes downtime, with failover typically completing in seconds through predefined scripts or automated tools that redirect traffic or workloads. Heartbeat monitoring underpins failure detection by exchanging periodic signals between nodes; if signals cease within a timeout period, the system initiates failover to prevent service interruption. High-availability clustering groups multiple nodes to provide and shared resources, ensuring continuous operation if one node fails. In environments, tools like the High Availability Add-On with Pacemaker and Corosync form clusters that manage resource fencing and to avoid split-brain scenarios. Corosync serves as the underlying messaging layer, facilitating reliable communication for cluster state synchronization. Load balancing within clusters distributes incoming requests across nodes to optimize performance and availability; DNS round-robin achieves this by cycling IP addresses in responses to evenly spread traffic, while hardware solutions like F5 BIG-IP use advanced algorithms for topology-aware distribution and . Emerging techniques leverage for , using to forecast potential failures before they impact availability; in 2025, over 70% of operators trust AI for sensor data analysis and maintenance prediction, reducing unplanned outages in . Container orchestration platforms like enhance HA through auto-scaling features, such as the Horizontal Pod Autoscaler, which dynamically adjusts pod replicas based on CPU or custom metrics to maintain performance under varying loads. (HCI) simplifies redundancy by integrating compute, storage, and networking into software-defined clusters, enabling seamless scaling and built-in without dedicated hardware silos. To validate HA implementations, introduces controlled failures in production environments, testing system resilience against real-world disruptions. Netflix's Chaos Monkey exemplifies this by randomly terminating instances, compelling services to recover automatically and ensuring at scale.

Causes of Unavailability

Types of Downtime

Scheduled downtime refers to intentional interruptions in system availability that are planned in advance to perform essential , upgrades, or optimizations. These periods allow organizations to apply operating system patches, conduct hardware swaps, or deploy software updates without compromising overall operations. Typically announced through notifications to users and stakeholders, scheduled downtime is timed for low-traffic hours, such as nights or weekends, to limit business impact. Unscheduled downtime, on the other hand, involves unexpected and unplanned system outages resulting from sudden failures. Common categories include power outages that disrupt data centers, hardware malfunctions like disk failures, or software bugs that cause application crashes. These events occur without prior warning, often requiring immediate intervention to restore service and can cascade into broader disruptions if not addressed swiftly. The distinction between these types profoundly influences recovery durations, particularly through their effect on (MTTR), which measures the average time needed to restore functionality after an incident. Unscheduled downtime generally prolongs MTTR due to the additional steps involved in diagnosing root causes and implementing fixes under pressure, whereas scheduled downtime benefits from predefined procedures and pre-staged resources, enabling faster resolutions—often measured in minutes rather than hours. For context, measurement focuses on total unavailability periods, as explored in the Uptime Calculation section. Statistics underscore the dominance of unscheduled downtime in high availability challenges, with cyber threats accounting for a growing share. A 2025 Splunk survey revealed that 76% of business leaders in and 75% in attributed unplanned outages to cybersecurity incidents, highlighting the escalating role of such threats in causing disruptions. Mitigation planning for scheduled centers on structured frameworks to curb potential escalations into unscheduled events. These practices include risk assessments, testing in staging environments, and establishing mechanisms before . By adhering to such protocols, organizations can minimize impacts; industry analyses indicate that approximately 80% of unplanned outages stem from poorly managed changes, emphasizing the value of rigorous processes.

Common Failure Reasons

High availability systems, designed to minimize , nonetheless encounter unavailability due to a range of predictable and unpredictable sources spanning hardware, software, human factors, and external events. These often cascade, amplifying their impact on service delivery, and underscore the need for proactive identification of causes. While and fault-tolerant designs mitigate risks, understanding prevalent triggers remains essential for maintaining system resilience. Hardware failures, though less dominant in modern systems compared to other causes, continue to contribute to outages through component degradation or environmental stressors. Disk crashes represent a primary hardware issue, accounting for approximately 80.9% of server hardware malfunctions due to mechanical wear, read/write errors, or power fluctuations that corrupt . Overheating exacerbates these problems, as excessive thermal loads from dense server configurations or inadequate cooling can induce processor throttling, errors, or complete node shutdowns, leading to unplanned disruptions in centers. Network-related hardware faults, such as cable cuts from accidents or damage, sever connectivity and isolate segments of the , often resulting in widespread and service inaccessibility. Software failures frequently arise from inherent defects or deployment issues, forming a significant portion of outages in large-scale services. Bugs in application code, including data races or memory leaks, caused 15% of analyzed outages between 2009 and 2015, as these errors manifest under load or during recovery processes, halting operations across distributed nodes. Configuration errors compound this risk, responsible for 10% of such incidents through misaligned settings in load balancers, , or tools that propagate inconsistencies and trigger cascading s. In high availability environments, these software faults often evade initial testing, surfacing during peak usage and underscoring the dominance of software over hardware as a source, with ratios as high as 10:1 in contemporary systems. Human and external factors introduce variability that challenges even robust designs, often amplifying other failure modes. Operator errors, such as procedural lapses during or upgrades, account for 33% to 45% of user-visible failures in large services, as manual interventions inadvertently disrupt mechanisms or introduce inconsistencies. , including floods, earthquakes, and storms, initiate complex outages by damaging power supplies or physical infrastructure, with severe weather events contributing to over $383 billion in cumulative U.S. damages for severe storms since 1980 and increasing outage durations by an average of 6.35%. vulnerabilities exemplify external risks, as seen in the 2021 incident, where attackers injected malicious code into software updates distributed to over 18,000 organizations, compromising tools and enabling persistent access that evaded detection for months. Cyber threats have escalated as deliberate causes of unavailability, particularly with evolving tactics in 2025. Distributed denial-of-service (DDoS) attacks dominate incident reports, comprising 76.7% of recorded cases by overwhelming resources and rendering services inaccessible, with global peak traffic exceeding 800 Tbps in the first half of the year. incidents surged in frequency and sophistication, locking critical systems and demanding payment for restoration, while AI-enhanced attacks—such as deepfakes in or automated scanning—facilitated 16% of breaches, often targeting by encrypting or flooding endpoints. Recent trends highlight the prominence of certain failures in environments, where misconfigurations drive a substantial share of disruptions. According to , 99% of security failures through 2025 stem from customer errors, predominantly misconfigurations that expose resources or weaken access controls. These account for 23% of security incidents overall. In data centers, power issues remain the leading outage cause, but IT-related problems—including software and configuration faults—have risen, with errors contributing to 58% of procedural lapses in 2025 reports. These patterns align with broader analyses showing operator actions and software faults as top contributors, far outpacing hardware at 1-6% across service types. Preventing recurrence of these failures relies heavily on continuous monitoring and root cause analysis (RCA). Monitoring tools detect anomalies in real-time, such as rising temperatures or unusual traffic patterns, enabling preemptive interventions before outages escalate. RCA complements this by systematically dissecting incidents to identify underlying triggers—whether a buggy script or procedural gap—using techniques like to implement targeted fixes and reduce future risks by up to 70% in recurrent scenarios. Together, these practices transform reactive recovery into proactive resilience, addressing the multifaceted nature of unavailability without relying solely on architectural redundancies.

Economic Impacts

Costs of Downtime

Downtime in high availability systems incurs substantial direct financial losses, primarily through lost revenue during periods of unavailability. Studies, including reports from the Ponemon Institute and , estimate the average cost of IT downtime for organizations at around $5,600 to $9,000 per minute, driven by interrupted transactions and operational halts. For larger enterprises, these figures escalate, with the 2024 ITIC report estimating costs exceeding $14,000 per minute due to the scale of affected revenue streams. A 2024 study further corroborates this, placing the global average at $9,000 per minute across industries. Indirect costs amplify these impacts, including damage to brand and increased customer churn as users shift to competitors during outages. A 2024 Oxford Economics study estimates total costs for Global 2000 enterprises at $400 billion annually, averaging $200 million per company, including impacts from reputational harm, customer churn, and other factors. Legal and regulatory penalties add another layer, particularly under frameworks like the GDPR, where system unavailability compromising data access can result in fines up to 4% of an organization's global annual turnover or €20 million, whichever is greater. In 2024, GDPR enforcement saw total fines exceeding €1.2 billion, with several cases tied to service disruptions affecting data protection obligations. Sector-specific variations underscore the disproportionate burden in revenue-sensitive industries. In , outages during peak periods can cost platforms $500,000 to $1 million or more per hour in foregone sales, as seen in historical incidents like Amazon's one-hour disruption totaling $34 million in losses. faces even steeper penalties from production halts, with a 2024 Splunk report estimating average annual costs at $255 million per organization due to idle machinery and interruptions. Cyber-related outages, often involving or breaches, exacerbate these figures; the 2024 Cost of a Data Breach Report notes that such incidents average $4.88 million globally—about 10% higher than the prior year—owing to extended and recovery complexities. The 2025 report updates this to a global average of $4.45 million, a 9% decrease from 2024, attributed to faster breach detection and AI-assisted responses.

Value of HA Investments

The return on investment (ROI) for high availability (HA) systems is typically calculated using the ROI = (Value of Avoided - HA Costs) / HA Costs, where the value of avoided represents the financial losses prevented by maintaining higher uptime levels. This approach quantifies the economic justification for HA investments by comparing the tangible benefits of reduced outages against the expenses incurred. For high-stakes operations, such as processing, analysis often shows that achieving 99.99% (four nines) yields positive ROI when annual costs exceed $1 million, as the incremental uptime prevents revenue losses that outweigh deployment expenses. In mission-critical environments like issuer processing, this level of has demonstrated ROI through minimized disruptions, with systems recovering in under 52 minutes annually while supporting high transaction volumes. HA investments encompass distinct cost components, including initial outlays for hardware redundancy, such as duplicated servers and failover mechanisms, and ongoing expenses for monitoring tools, software licenses, and personnel training. Total cost of ownership (TCO) models integrate these elements over the system's lifecycle, factoring in indirect costs like security compliance and scalability upgrades to provide a holistic view of long-term financial impact. Higher initial investments in robust HA architectures can lower TCO by reducing maintenance needs and downtime-related productivity losses. Key benefits of HA investments include enhanced service level agreements (SLAs) that guarantee uptime targets, such as 99.999% (five nines), fostering customer trust and enabling contractual penalties for breaches. This reliability provides a competitive edge by differentiating organizations in sectors like , where consistent access drives user retention and . In 2025, hybrid cloud setups have illustrated these advantages, with private cloud integrations reducing HA costs by 30-60% compared to public cloud alternatives through fixed hardware pricing and efficient for redundant workloads. However, HA investments exhibit trade-offs, with diminishing returns beyond five nines (99.999% availability) for non-critical systems, as the engineering effort and complexity required to limit to under 5.26 minutes annually often exceed proportional . In such cases, the escalating costs of advanced and testing yield marginal uptime gains that do not justify the expense for lower-priority applications.

Modern Applications

Cloud and Distributed Systems

In cloud computing environments, high availability (HA) is achieved through architectural designs that distribute workloads across multiple Availability Zones (AZs), such as those provided by Amazon Web Services (AWS). Multi-AZ deployments ensure that applications and data remain accessible even if one AZ experiences an outage, as each AZ operates independently with isolated power, networking, and cooling infrastructure. For instance, Amazon RDS Multi-AZ configurations automatically fail over to a standby replica in another AZ during primary instance failures, providing enhanced durability and 99.95% availability for production workloads. Complementing multi-AZ strategies, auto-scaling groups in AWS dynamically adjust the number of compute instances across AZs to maintain and under varying loads or failures. These groups distribute instances evenly to avoid single points of failure, automatically launching replacements if an instance becomes unhealthy, thereby supporting fault-tolerant architectures without manual intervention. In distributed systems, challenges like data consistency arise when prioritizing over strict , as seen in databases such as . Cassandra employs , where replicas converge on the same data value over time through mechanisms like hinted handoffs and read repairs, allowing high availability in large-scale clusters even if some nodes are temporarily unavailable. This tunable model balances the theorem's trade-offs, enabling writes and reads to succeed with configurable levels for replication factor of three or more. Service meshes like Istio address similar issues in by providing , automatic , and , ensuring resilient communication across distributed components without altering application code. As of 2025, trends emphasize built-in HA, with platforms like inherently deploying functions across multiple AZs for automatic and , eliminating the need for manual provisioning while achieving high availability through managed . Multi-cloud strategies further enhance resilience by distributing workloads across providers like AWS, Azure, and Google Cloud, mitigating risks and improving overall system uptime via standardized abstractions and hybrid integrations. For example, hybrid cloud setups combine on-premises resources with public clouds to enable seamless data replication and workload migration, bolstering resilience against regional outages. Orchestration tools like play a central role in managing HA for containerized distributed systems, supporting multi-master etcd clusters and pod replication across nodes to prevent single points of failure. The 2024 CrowdStrike incident, where a faulty software update caused widespread outages affecting millions of systems, underscored the importance of rigorous testing, phased rollouts, and diversified update mechanisms in cloud environments to maintain HA. Lessons from this event highlight the need for isolated deployment pipelines and multi-cloud redundancies to limit cascading failures in interconnected ecosystems.

Edge Computing and Critical Infrastructure

High availability in emphasizes low-latency redundancy to support IoT deployments, where mechanisms like enable rapid switching between network paths to minimize disruptions in real-time applications. (MEC) integrates processing closer to data sources, reducing end-to-end latency to under 10 milliseconds for mission-critical IoT tasks such as industrial automation. Hyper-converged infrastructure (HCI) further bolsters this by consolidating compute, storage, and networking across distributed edge nodes, allowing automated and resource orchestration to sustain availability above 99.99% in decentralized setups. In , high availability safeguards systems like power grids and autonomous vehicles against outages through robust and cybersecurity measures aligned with NIST standards. For power grids, NIST's Cybersecurity Guidelines recommend redundant control systems and intrusion detection to maintain operational continuity during cyber threats, ensuring in the face of distributed denial-of-service attacks. Autonomous vehicles rely on NIST-developed performance metrics and frameworks, incorporating protocols for sensor data and communication links to prevent single-point failures in safety-critical operations. Military applications of high availability have evolved from 1960s-era control systems, which used basic redundant analog circuits for command reliability, to 2025 drone swarms employing AI-driven resilience for coordinated operations. The F-35 Lightning II jet exemplifies this progression with its integrated and redundant architectures, featuring automated fault detection and self-healing networks that support drone control in contested environments. Modern drone swarms leverage AI algorithms for predictive rerouting and collective redundancy, allowing groups of up to 100 unmanned aircraft to maintain operational integrity despite individual losses. Emerging 2025 trends in edge high availability include AI for that forecast failures in IoT nodes, enabling proactive adjustments to achieve near-zero in low-latency scenarios. Quantum-resistant cryptographic designs are also advancing secure communications in edge-critical systems, incorporating post-quantum algorithms to protect against future threats while preserving data in distributed networks. However, challenges persist in harsh environments, such as extreme temperatures and vibrations in industrial or remote deployments, necessitating ruggedized with reinforced hardware enclosures and fault-tolerant designs to ensure edge nodes operate reliably without centralized intervention.

Fault Tolerance and Disaster Recovery

Fault tolerance refers to the ability of a system to continue performing its intended function correctly in the presence of faults, such as hardware failures or software errors, without interrupting service. This is achieved through mechanisms like at the component level, ensuring seamless operation even when individual parts fail. For example, Error-Correcting Code (ECC) memory detects and corrects single-bit errors in data storage, preventing corruption in critical applications like and servers. In contrast to high availability (HA), which emphasizes overall system uptime through and , fault tolerance focuses on internal resilience, allowing the system to mask faults proactively without external intervention. Disaster recovery (DR), on the other hand, involves strategies to restore system functionality after a major disruptive event, such as natural disasters, cyberattacks, or widespread outages, where alone may not suffice. Key metrics in DR planning include the Recovery Time Objective (RTO), which defines the maximum acceptable before recovery, and the Recovery Point Objective (RPO), which specifies the maximum tolerable measured in time (e.g., the age of the last ). Common DR techniques encompass regular , offsite replication, and to secondary sites. For instance, geo-redundancy replicates across geographically distant locations to enable quick restoration if the primary site is compromised, minimizing both RTO and RPO. While HA and fault tolerance address minor, localized issues to prevent downtime, DR targets catastrophic failures requiring full system reconstitution, often integrating with HA for layered protection. Hybrid approaches, such as Disaster Recovery as a Service (DRaaS), leverage cloud providers to automate replication and recovery, offering scalable options that align with HA goals by reducing manual intervention. Fault tolerance is inherently proactive and internal, exemplified by RAID configurations (e.g., RAID 1 mirroring for disk fault tolerance), whereas DR is reactive and external, focusing on post-event recovery like restoring from geo-redundant backups. This distinction ensures comprehensive resilience, with redundancy mechanisms overlapping to support both.

Scalability and Performance

High availability (HA) focuses on ensuring system uptime and minimizing disruptions from failures, whereas scalability addresses the capacity to handle increasing workloads without degradation, and performance emphasizes metrics like latency and throughput. While HA prioritizes redundancy and fault resilience to maintain 99.99% or higher availability, scalability enables growth by adding resources dynamically, often complementing HA by preventing overload-induced downtime. Performance, in turn, measures efficiency in processing requests, where HA mechanisms can introduce overhead if not optimized. These concepts intersect in modern systems, where scalable designs enhance HA by distributing loads, but trade-offs exist in balancing cost and speed. Scalability in HA contexts typically involves horizontal scaling, which adds more nodes or instances to distribute , improving as in one node do not affect others, unlike vertical scaling that upgrades a single node's resources but risks single points of and eventual limits. Horizontal scaling is preferred for HA because it supports redundancy across multiple availability zones, enabling seamless , while vertical scaling suits simpler, low-variability workloads but requires for upgrades. Elastic scaling, a form of horizontal approach, automatically adjusts instance counts based on demand metrics like CPU utilization, ensuring HA by maintaining capacity during traffic spikes without manual intervention. HA designs, such as load balancers, distribute traffic to optimize performance by reducing latency—the time for request completion—and maximizing throughput—the number of requests handled per unit time—while preserving through health checks and . For instance, application load balancers can decrease response times in distributed setups by evenly spreading loads, though improper configuration may add minimal latency from decisions. These mechanisms ensure that HA does not compromise speed, as balanced distribution prevents bottlenecks that could lead to cascading failures. Synergies between HA and are evident in auto-scaling groups, which ensure during peak loads by provisioning additional resources proactively, thus avoiding overload-related outages, while scaling down during lulls to control costs. However, over-provisioning in these setups can lead to higher expenses, as resources remain idle, creating a where aggressive scaling maintains HA but increases operational costs by 20-30% in some environments. Balancing this involves predictive algorithms to minimize excess capacity without risking under-provisioning. In 2025, trends in AI-optimized scaling for edge-cloud hybrids leverage and neural networks to forecast demand and automate , reducing latency by up to 28% in AI inference services while enhancing HA through decentralized decisions. These approaches integrate edge devices for low-latency processing with cloud scalability, achieving 35% better load balancing efficiency in hybrid setups.

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