Dependability Properties of Systems: Principles & Purpose

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

In this lesson, we will define dependability (trustworthiness) of an information system. You will be able to explain the five facets of dependability and the importance of this measure.

Nobody Wants a Lemon

Have you ever owned a lemon? Be it a car, truck, bicycle, dishwasher or computer, it just doesn't ever work quite right. The engine light is always on, the system is always crashing, or it makes horrible grinding noises after so-called repairs. When it comes to computer systems, the last thing we want is a lemon. Instead, we want a dependable and trustworthy system. So what does that mean in computer terms?

From users to programmers, dependability is the most important facet of a computer system. It is the trustworthiness of the system. Not only does dependability refer to confidence in the system, but also outlines reliability, security, and availability.

What's the Harm?

If you have a lemon for a car, you know the risks you face: A breakdown on a busy freeway or in the middle of nowhere. It is not safe, and recovering from a breakdown can be expensive. Thus it is with computer systems. Unreliable systems have the following consequences:

  • User rejection: "I will never use that payroll system again, and instead do it all on paper!"
  • Data loss or physical damage: Corrupt data, melted motherboards, etc.
  • Economic costs/losses: A company may have to spend millions to recover from a failure.

What Causes Failure?

There are a few points in a computer system that can fail:

  • Hardware: Bad design, manufacturing flaws and components eventually wear out and fail.
  • Software: Buggy code, poor design misuse or faulty implementation.
  • User: Computers only do what humans tell them. A user could enter bad data or act maliciously.

We now have an understanding of what dependability is and those things that jeopardize dependability. The focus of this lesson however, is on the good. What are those elements that define dependability?

Dimensions of Dependability

There are five key pillars of dependability in a system as shown in Figure 1. Security, Availability, Reliability, Safety and Resilience.


Figure 1: System Dependability Key Pillars
System dependability


Security

Given prominent security breaches in the news, we've placed security at the top of the list. Security is a property that denotes the ability for a system to shield itself from attack. Today's systems are connected to the Internet, which means the threat from external attack is far greater. The highest risk to a given system, however, is its users. Through social engineering, weak passwords, and inadvertent or deliberate sharing of system information, employees can be the breech in the dam.

When it comes to the security of a system, think of the acronym CIA:

  • C - Confidentiality: Access is only granted to authorized users.
  • I - Integrity: You can't alter data if you are not the owner.
  • A - Availability: The data in the system is consistently accessible.

Security is inexorably linked to the next three facets of dependability. If a system is not secure, then availability, reliability and safety measures go right out the window.

Availability

Availability is a probability measure. It outlines the likelihood that the system will be operational at any given point in time in order to fulfill requests. You can't trust a system that is frequently down, or the data you seek is consistently unavailable.

When measuring availability, you must take into account the time it takes to repair failures, and the average time between repairs. The resultant equation is uptime / (uptime + downtime). Availability does not account for preventative maintenance, however, because this is built into the life cycle of the system.

Reliability

You may think that availability and reliability are the same, but they differ. Reliability is a quantitative measurement (as is availability), but it outlines the probability that the system will run without failure over a given time. This does not mean that the right data is served, only the probability that the system will continue to run.

Technically, you could have a lemon of a system that is still available. You may need to add a quick fix or patch to get it up and running, but it still contains the data.

Safety

Today, almost everything has some sort of computer or software component. Examples include smart phones, thermostats, fridges and medical devices. Safety is a property that indicates a system's ability to run without posing harm to humans or to the system itself.

As opposed to stated requirements (e.g. what a system WILL do), safety specifications state what the system will NOT do. For example, there will not be a single point of failure, the system will not administer medication if any input is corrupt, etc.

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