System Design: Non-Functional Requirements

Non-Functional Requirements are those requirements that are not directly related to the product/feature/functionality we want to build. These requirements should be built into the system inherently by the engineering team to make good systems.

Here we discuss the main Non-Functional requirements that need to be considered before designing a system.

Critical non-functional requirements are:

• Availability
• Reliability
• Maintainability
• Scalability
• Fault Tolerance

Availability

The system you design should be readily available to the users which means, whenever a user wants, they should be able to access the system with normal behavior.

What contributes to downtime?

• Software Crashes
• Hardware failures
• Human errors
• Planned Downtime

Measuring Availability

Availability can be visualized as the ratio of the time the system was available to the total time the system was expected to be available.

\[Availability = \frac{Total Time - Service down time}{Total Time} * 100\]

Eg. If a system was running for 24 hours and the system was down for 8 minutes the availability of this system would be: 99.5% availability is also referred to as 2 Nines availability.

\[Availability = \frac{24 * 60 - 8}{24*60} * 100 ~ 99.5%\]

Here is a reference table of service downtime allowed for standard availabilities:

Higher the availability, the higher the nines, the better.

Ideally, our system should aim for high availability, but that comes with a trade-off with consistency in the case of network-partitioned distributed systems (CAP Theorem).

Ensuring Availability

• Making the system fault tolerant
• Removing any single point of failure using Redundancy and Replication

Reliability