High Availability Concepts: Availability, What is it and how Can it be Measured?

To discuss high availability, it is necessary to first define the concept of availability: Availability is the ability of a system to provide a service during the time for which it has been designed.

In general and its application in industrial systems, we could talk about availability according to the level of service, for example: to ensure the integrity of data in the company ABC in the pharmaceutical sector, the system that records the data is required to be available during the entire production time. If the factory works 16 hours a day, 5 days a week, and only 9 hours of system downtime are allowed (e.g., due to unplanned maintenance of the servers), the level of service required for this system is 16×5, and in percentage, the availability required for this system would be 99.78%. Where does this percentage come from?

Availability measurement:
Normally, when an analysis and negotiation are carried out to determine the availability objective, a measure based on percentages is used:
Availability = ((A-B)/A)x100%)

Where:

• A = Hours of availability committed.
• B = Number of offline hours (corresponds to the “system downtime” hours during the committed availability time).
  In the previous case, the calculations would be as follows: Availability = ((4160-9)/4160)*100% = 99.78%

This availability percentage requires the design of an infrastructure based on high availability principles. In a future post, we will discuss the “nines of availability” and their relationship with real-time industrial systems.