If every programmer of cloud-hosted apps possessed exceptional technical capability and endless patience, the DevOps environment (also known as Platform-as-a-Service, or PaaS) would perhaps become irrelevant. However, the reality is almost always the opposite case. Hence, IT engineers dream of a reliable and usable DevOps environment that can substantially facilitate their developments and simplify their operations. Current DevOps environments include Google App Engine, Docker, Kubernetes, Mesos, and so forth. In other words, PaaS bridges the gap between vivid IT engineers and stiff cloud systems. In this paper, we comprehensively examine state-of-the-art PaaS solutions across various tiers of the cloud-computing DevOps stack. On this basis, we identify areas of consensus and diversity in their philosophies and methodologies. In addition, we explore cutting-edge solutions towards realizing a more fine-grained, full-stack DevOps environment. From this paper, readers are expected to quickly grasp the essence, current status, and future prospects of PaaS.