In context of dependability in software engineering, the ability of the system to deliver services when requested, is called _______ and the ability of the system to deliver services as requested, is called _______ .

Question

Accepted Answer

The correct option is (1): Availability, Reliability

In software engineering, dependability is a critical quality attribute encompassing a system's ability to deliver services that can be justifiably trusted by the user. The statement correctly identifies two core elements of this concept. The ability of the system to deliver services **when requested** refers to the system being operational and ready for use; this is the definition of **Availability**, which is a measure of the system's readiness for correct service, often expressed as a percentage of uptime. Conversely, the ability of the system to deliver services **as requested** focuses on the correctness of the delivered result. This is the definition of **Reliability**, which measures the continuous, error-free operation of the system over a specified period of time. A system must be both available (up) and reliable (correct) to be considered dependable.

The incorrect option is (2): Reliability, Availability

This option incorrectly swaps the definitions of the two core dependability properties. Reliability is concerned with the consistency of correct output over time ("as requested"), not simply the system's operational status at a single point in time ("when requested"). Similarly, Availability addresses whether the system is ready to respond to a request, which means it is operational, not whether the response itself is correct and continuous. Placing Reliability first and Availability second fundamentally misaligns the terms with the industry-standard definitions provided in the question.

The incorrect option is (3): Security, Reliability

Security, while a major component of overall dependability, does not fit the definition given for the first blank. Security is defined as the system's ability to protect itself from accidental or malicious attacks, thus ensuring data confidentiality, integrity, and accessibility. It is a protective measure, not a measure of operational readiness or the "ability to deliver services when requested." Although Reliability is correctly defined for the second blank, the incorrect term for the first slot makes this option wrong.

The incorrect option is (4): Availability, Security

Although Availability is the correct term for the first blank, matching the definition of being ready for service "when requested," Security is an inappropriate term for the second blank. The phrase "ability of the system to deliver services as requested" demands a focus on the service's continuous correctness and adherence to specifications, which is the definition of Reliability. Security is focused on protection and integrity, not the correct functional output of the system as described by the second phrase.