With the explosive growth of mobile devices and the advent of many new applications, mobile network traffic volume has been growing exponentially. The traditional centralized network architecture cannot accommodate such user demands due to heavy burden on the backhaul links and long latency. Therefore, new architecture, which brings network capability to the edge of network, is proposed, i.e., mobile edge computing (MEC). MEC provides lightweight cloud computing and caching capabilities at the edge of cellular networks. In this survey, an exhaustive review on the state-of-the-art research efforts on mobile edge computing is carried out. First, an overview of mobile edge computing, including development process, research hotpots, and key enablers, is given. Then, a comprehensive survey of issues on mobile edge computing architecture, computation offloading, edge caching and service orchestration at the mobile edge computing is presented. The applications and use cases of mobile edge computing, such as augmented reality, virtual reality, dynamic content delivery, Internet of vehicles, and Internet of things, are discussed. Finally, from the prspectives of function enhancement, quality of service assurance, security, and availability, the open research challenges and future direction of mobile edge computing are presented as well.