IoT applications run on IoT devices and can be created to be specific to almost every industry and vertical, from small devices to large ones, including healthcare, industrial automation, smart homes and buildings, automotive, and wearable technology. The possibilities are limitless. Increasingly, IoT applications are using AI and machine learning to add intelligence to devices. Among all of the variables in the IoT ecosystem, one common theme is the need to be able to handle the constrained operating environment, such as unreliable network connectivity, limited bandwidth, low battery power, and so on. We will take a look into the MQTT protocol, how it has evolved from its early days which was intended for the connection of oil pipelines via satellite, to now the ever-increasing demand in IoT and M2M applications, to how this protocol will evolve to meet the modern needs especially in the current cloud computing era. We will study a few outstanding MQTT libraries that are available in the market, such as the Java-based HiveMQ, and open source libraries such as Eclipse Mosquitto and Apache licensed VerneMQ.