In the modern world of microservices and cloud-native architectures, the quality of Application Programming Interfaces (APIs) is essential to maintain the system reliability and performance that leads to the success of the entire business. A quality of software application is mainly based on Non-Functional Requirements (NFRs), named as quality attributes. Conversely, the quality standard like ISO/IEC 25010:2023 specifies nine NFRs along with their sub-quality factors influencing the quality of a software product or in our case APIs. However, they lack the comprehensive guidance to handle the quality of APIs, though they outline a wide range of NFRs but extensively compatible with the quality of traditional software. Thus, the aim of our research is to introduce two novel NFRs, Observability and Resiliency, which significantly meet the quality requirements of an API. As a part of methodology, we employed a Systematic Literature Review (SLR) by following the strict guidelines to select and assess 927 articles, which were collected from various reputable journals. A comprehensive review of 86 articles further reduced to 15 most relevant studies reveals the significance of quality factors of emerging NFRs, such as fault tolerance, failure recovery, logging, and monitoring are predominantly influencing API quality. Additionally, the Observability and Resiliency provide a novel addition to the existing standards, explicitly aligning them with API quality management. This study supports both researchers as well software industry to build a solid understanding towards API quality and meet the complex challenges during API design.
