To calculate the availability of multiple system components, you need to understand the availability of each individual component and how they interact within the system. Here’s a step-by-step explanation:
1. Define Availability
Availability is a measure of the proportion of time a system is operational and accessible when required for use. It is typically expressed as a percentage and can be calculated using the formula:
Each component in the system will have its own availability percentage. For example, if you have a load balancer with 99.99% availability and a web server with 99.95% availability, these are the individual availabilities you will use in your calculations.
3. System Configuration: Series vs. Parallel
The overall system availability depends on how the components are configured:
Series Configuration: All components must be operational for the system to be available. The total availability is the product of the individual availabilities.
For example, if you have two components in series:
$$
\text{Total Availability} = A_1 \times A_2
$$
Where $$ A_1 $$ and $$ A_2 $$ are the availabilities of the two components.
Parallel Configuration: The system is available if at least one component is operational. This configuration increases overall availability.
4. Calculating Availability for Series Configuration
For a series configuration, multiply the availability of each component. For instance, if you have a load balancer (99.99%) and a web server (99.95%) in series:
$$
\text{Total Availability} = 0.9999 \times 0.9995 = 0.9994 \text{ or } 99.94%
$$
5. Calculating Availability for Parallel Configuration
For a parallel configuration, the formula is:
$$
\text{Total Availability} = 1 - \prod_{i=1}^{n} (1 - A_i)
$$
Where $$ A_i $$ is the availability of each component.
For example, if you have two VMs each with 99.9% availability in parallel:
$$
\text{Total Availability} = 1 - (1 - 0.999) \times (1 - 0.999) = 1 - 0.0001 \times 0.0001 = 1 - 0.00000001 = 0.99999999 \text{ or } 99.999999%
$$
6. Hybrid Systems
Many systems are neither purely series nor purely parallel but a combination of both. For these systems, you need to apply the series and parallel formulas to each subsystem and then combine the results.
