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Reliability vs. Availability: What’s The Difference?

Reliability and availability have different meanings when it comes to software. What are the differences and what is the importance of each?

What is the Difference between Reliability vs. Availability?

Availability refers to the percentage of time a system is available to users. Reliability refers to the likelihood that the system will meet a certain level of performance based on user needs within a certain time frame.

While reliability sounds very similar to availability, there are certain differences when you look closely. For example, if two services have the following availability percentages, which one of them will be considered more reliable?

A graph showing the availbility percentages of two services
A graph showing the availbility percentages of two services

At a quick glance, it looks like service A is more reliable, but a closer look shows otherwise. Users don’t access every page on the site equally. Every user must visit the log in page, about 90% of them visit the catalog, and the Settings page is accessed by only 30% of the users. Considering that, service B will be perceived as more reliable, as reliability is defined based on user experience.

What is Availability?

Availability, also known as uptime, describes the percentage of the time that a service is functioning. It’s the simplest building block of reliability and is often confused with being equivalent to reliability. Being available is a broad term and different organizations may define it differently. For example, one organization may consider an outage when it affects a certain percentage of the users while another may consider it when certain instances are unavailable regardless of the number of affected users.

Additionally, you shouldn’t merely aim for being “available”. The service should be able to perform its intended operations even under varying conditions. In distributed systems, you can utilize chaos engineering to experiment with the resiliency of your service. 

How to Measure Availability?

Here’s how you can calculate the availability percentage of your service:

  1. Determine the total length of time to be assessed
  2. Subtract the total amount of time that the service was unavailable
  3. Divide the remaining time by the total time

Percentage of Availability = (Total Elapsed Time - Sum of Downtime)/Total Elapsed Time

The Nines of Availability

A graph showing the allowed downtime for different 9s of availability
A graph showing the allowed downtime for different 9s of availability

How to Improve Availability of a Service?

  • Deploy the application across various geographical locations, worldwide, reducing single points of failure.
  • Utilize chaos engineering practices to experiment and find system vulnerabilities.
  • Use load balancers efficiently to reroute requests.
  • Improve your incident management process to reduce downtime caused by incidents

What is Reliability?

A system’s reliability is the probability that it will meet certain performance standards and produce the correct output at a specific time. It can be used to understand how well the service will operate under various real-world circumstances.

How to Measure Reliability?

Since reliability is the duration a system operates without failure, we can measure it using the Mean Time Between Failure (MTBF) metric. 

  1. Determine number of failures
  2. Find the total length of time assessed
  3. Divide total time by number of failures

Mean Time Before Failures (MTBF) = Total Operation Time (hours)/Number of Failures

Reliability can also be measured using the failure rate of a service:

  1. Determine number of failures
  2. Find the total length of time assessed
  3. Divide total failures by the total time in service

Failure Rate = Number of failures/Total Time in Service

Here is a table summarizing the distinctions between availability, maintainability, and reliability:

Keep in mind that, although these formulas look simple, properly defining what failure is for your system is the difficult part. Since reliability is based on user experience, you need to use processes such as SLIs to understand what an acceptable service level is.

How to Improve the Reliability of a Service?

  • Test the system and deploy runbook automation.
  • Test new features thoroughly before deployment. 
  • Have a proper incident management process in place. 

Relationship between Reliability and Availability

The reliability of a system depends on whether it will deliver the right output when required. It’s not the same as availability, which is being available at all times. However, availability and reliability are interconnected. You can say that availability is the basic building block of reliability. Reliability and availability go hand in hand as one is not possible without the other. Only if something is available can we determine if it is reliable. So, high levels of reliability lead to high levels of availability. 

What is Maintainability and How Does it Relate to Availability and Reliability?

Maintainability (sometimes referred to as serviceability) is the measure of a service’s ability to be retained or restored to its previous condition after maintenance. It factors into availability by defining how effectively downtime is resolved. In case of an incident, maintainable services can be easily restored or retained quickly. Maintainability can be either proactive or reactive. 

  • Proactive maintainability involves building a service with an easy-to-understand and easy-to-change codebase. Proactive maintenance also involves practices like testing and quality assurance (QA). 
  • Reactive maintainability refers to a system’s ability to restore after an incident. Since incidents are inevitable, it’s best to have a robust incident response process in place.

Reliability Vs. Innovation

Whether you’re offering a product or a service, reliability is very important, but so is innovation. You can’t stay competitive without improvement and innovation means change, hence some instability. However, it’s vital to balance reliability with innovation. On the other hand, offering the best features in the market can go in vain if your consumer cannot access them. Maintaining a good balance of reliability and development velocity is hard, but ultimately having good reliability will increase velocity, through practices like error budgeting.

There will always be change, instability, and uncertainty, and that is why it’s pointless to pursue perfection. For this reason, you should always aim for a practical uptime (which is never 100%). Past a certain point, users won’t be able to notice increases in reliability or availability. The effort you spend improving reliability past that point would be better spent elsewhere.  The best-in-class enterprise organizations often offer 99.999% availability, also known as the five nines availability with a yearly downtime of only 5.256 minutes. 

The availability percentage of a system guaranteed to users is usually mandated in the service level agreement (SLA). The best way to deal with downtime is to be ready and prepared for any incident. Having a solid incident management process and appropriate error budget (acceptable unreliability) is vital for every organization.

How Can Blameless Help?

Considering how important availability and reliability are for your service, you can’t rely on half measures. Blameless can help take your service reliability to the next level with various tools and resources. It can help you understand your availability metrics and improve incident response and retrospectives. We offer services like Comprehensive Reliability Insights tool, Automated Incident Response, Incident Retrospectives, and SLO Manager that help you understand and improve your service. To learn more about Blameless, request a demo or sign up for our newsletter below.

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