Yes. MQTT is just a protocol and supports three types of Quality of Service (QoS): - QoS 0: Fire and forget - QoS 1: At least once - QoS 2: At most once

You can choose to send messages via any one of these QoS which suits your use-case. Apart from this, standard MQTT implementations also involve a "retain" flag for a topic and a message which means that the brokers will maintain the last sent message on the given topic. When new clients subscribe on this topic with "retain" flag set as true, they will instantly receive the last retained message.

Apart from these, there are more extensions that you can build to add persistence to your system. This depends on the specific MQTT implementations which you might have to find out.

Thanks a lot. Do you know of any real world mqtt deployments that deploy QoS 1 or 2 with success on non trivial scale? Can you recommend a broker impl. that is well tested in those scenarios?

There are some big players that use MQTT although i am not sure about the QoS levels. I know WhatsApp used MQTT, although i am not sure if they still do it not or what QoS they used. My bet would be they used QoS 1.

There are multiple brokers for MQTT: Mosquitto, HiveMQ (Java), VerneMQ (Erlang), EMQ: the one we used (Erlang) etc. You can find out more brokers that are available as well.

Sure. We have already launched the Unicast functionality in Beta and are monitoring it closely. We will be sharing the Unicast post as well soon.

Thanks! The blog outlines our journey for the last 8 months. We ended up sending more than 250Billion+ messages over a Live Cricketing tournament in India. (VIVO IPL 2019)