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Availability in System Design

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In system design, availability refers to the proportion of time that a system or service is operational and accessible for use. It is a critical aspect of designing reliable and resilient systems, especially in the context of online services, websites, cloud-based applications, and other mission-critical systems.


How is availability measured?

Availability is usually measured as a percentage and is often expressed in terms of “uptime” versus “downtime” over a given period. For instance, a system with 99% availability means it is expected to be operational and accessible 99% of the time, while the remaining 1% represents the allowable downtime.

How do we achieve high availability?

High availability is essential for systems where continuous operation is vital, and any disruption could lead to financial losses, reputational damage, or even safety hazards. Commonly, systems with high availability requirements include banking applications, e-commerce platforms, healthcare systems, emergency response services, and cloud infrastructure.

System designers implement various strategies and technologies to achieve high availability, such as:

  • Redundancy: Employ redundant components or servers to ensure that another can take over seamlessly if one fails. This can include redundancy at different levels, such as hardware, networking, and data centers.
  • Load balancing: Distributing incoming requests across multiple servers or resources to prevent overload on any single component and improve overall system performance and fault tolerance.
  • Failover mechanisms: Implementing automated processes to detect failures and switch to redundant systems without manual intervention.
  • Disaster Recovery (DR): Having a comprehensive plan in place to recover the system in case of a catastrophic event that affects the primary infrastructure.
  • Monitoring and Alerting: Implementing robust monitoring systems that can detect issues in real-time and notify administrators to take appropriate action promptly.
  • Performance optimization: Ensuring that the system is designed and tuned to handle the expected load efficiently, reducing the risk of bottlenecks and failures.
  • Scalability: Designing the system to scale easily by adding more resources when needed to accommodate increased demand.

Difference in availability and fault tolerance

AspectAvailabilityFault Tolerance
DefinitionThe proportion of time a system is operational and accessible for use.The ability of a system to continue functioning, albeit with reduced performance, in the presence of faults or failures.
GoalMaximizing the system’s uptime and minimizing downtime.Ensuring the system remains operational despite hardware, software, or network failures
FocusEmphasizes continuous and consistent access to services.Focuses on the system’s ability to handle and recover from failures.
MeasuresTypically expressed as a percentage of uptime over a specific period (e.g., 99.9% uptime per month).It is usually expressed in terms of Mean Time Between Failures (MTBF) and Mean Time to Recover (MTTR).
StrategiesRedundancy, load balancing, failover mechanisms, disaster recovery planning, etc.Use of redundant components, data replication, failover mechanisms, and graceful degradation of performance in case of faults.
Goal AchievementHigh availability is achieved by minimizing the impact of potential failures.Fault tolerance is achieved by detecting and recovering from failures in a way that doesn’t lead to system-wide outages.
User ExperienceFocuses on providing a consistent and reliable user experience with minimal disruption.Focuses on maintaining the overall system functionality and preventing complete system failures.
Use CasesCritical for systems that need to be accessible and operational at almost all times (e.g., e-commerce, banking).Important in safety-critical systems, aerospace, healthcare, and other scenarios where system failure can lead to severe consequences.
Redundancy LevelHigh availability may involve some redundancy, but it may not eliminate all single points of failure.Fault tolerance often requires a higher degree of redundancy to provide backup mechanisms for various components.

Last Updated : 11 Sep, 2023
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