  Recently, fog computing has been introduced as a modern distributed paradigm
and complement to cloud computing to provide services. Fog system extends
storing and computing to the edge of the network, which can solve the problem
about service computing of the delay-sensitive applications remarkably besides
enabling the location awareness and mobility support. Load balancing is an
important aspect of fog networks that avoids a situation with some under-loaded
or overloaded fog nodes. Quality of Service (QoS) parameters such as resource
utilization, throughput, cost, response time, performance, and energy
consumption can be improved with load balancing. In recent years, some
researches in load balancing techniques in fog networks have been carried out,
but there is no systematic review to consolidate these studies. This article
reviews the load-balancing mechanisms systematically in fog computing in four
classifications, including approximate, exact, fundamental, and hybrid methods
(published between 2013 and August 2020). Also, this article investigates load
balancing metrics with all advantages and disadvantages related to chosen load
balancing mechanisms in fog networks. The evaluation techniques and tools
applied for each reviewed study are explored as well. Additionally, the
essential open challenges and future trends of these mechanisms are discussed.
