Raspberry Pi Electronics Projects for the Evil Genius by Donald Norris

Author:Donald Norris
Language: eng
Format: epub
Publisher: McGraw-Hill Education
Published: 2016-07-22T16:00:00+00:00

Figure 6-1 Project block diagram.

Table 6-1 lists some of the salient features of MQTT that make it so popular as a messaging protocol.

TABLE 6-1 MQTT Features

These features make MQTT very popular for M2M applications, including weather monitoring, stock ticker, smart power grid meters, and even Facebook messaging. It is also a very popular way for cellular services to implement message alerts.

Quality of Service (QoS)

QoS refers to the level assurance that MQTT provides regarding message delivery. There are three QoS levels:

Level 0. This is also known as “fire and forget.” At this level, the publisher sends off messages, and there is no attempt to acknowledge their reception by the broker on behalf of the publisher. It is obviously the quickest message-delivery method, but it is also the least reliable.

Level 1. This is also known as “at least one.” Here messages are sent and resent until the broker receives one acknowledgment from the subscriber. It does provide some assurance that the message did get through to its intended recipient. Level 1 is typically set as the default QoS for a MQTT messaging system.

Level 2. This is also known as “exactly one.” At this level, messages undergo a two-stage process in which there is a definitive acknowledgment between the broker and subscriber ensuring that one and only one message copy was delivered. This QoS level is the slowest among the three levels owing to the additional processing overhead required to establish a high reliability level.

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