In this episode of MQTT Essentials series, we will talk about MQTT Retained Messages.

👉 What is a Retained Message in MQTT?
A retained message in MQTT is a topic's last known good value persisted by the broker. The broker stores the last retained message and the corresponding QoS for that topic. Each client that subscribes to a topic pattern that matches the topic of the retained message receives the retained message immediately after they subscribe. The broker stores only one retained message per topic.

Retained messages
- are useful when a start state is required
- ensure that clients get the last stored value immediately after subscription

Sending retained messages
From the perspective of a developer, sending a retained message is quite simple and straight-forward. You just set the retained flag of an MQTT publish message to true. Typically, your client library provides an easy way to set this flag.

- subscribers receive the message immediately (even if they were offline when the message was published)
- this solves the empty start value problem
- only one retained message per topic is stored by the broker

Queued vs. retained messages
Retained messages: work on a topic level, newly-connected subscribers to a topic receive messages immediately
Queued messages: work in a client context, the broker queues undelivered messages for a specific client

Deleting retained messages:
- not necessary in most cases
- a new retained message overwrites the old one
- to delete: send a retained message with an empty payload

*** Chapters / Table of Contents ***
00:00 Introduction to MQTT Retained Messages
00:22 What is a Retained Message in MQTT?
01:07 Sending retained messages in MQTT
02:08 Queued vs. retained messages in MQTT
03:41 How to delete a retained message in MQTT
04:31 Announcing part 10 - Last Will & Testament in MQTT

👉 If you missed Part 1 - What is MQTT, watch it now: [ Ссылка ]

👉 We also explain the MQTT Retained Messages in detail in the HiveMQ blog here: [ Ссылка ]

👉 We also explain the core of MQTT concepts, its features, and other essential information on the HiveMQ blog: [ Ссылка ]

👉 Get your own MQTT Essentials in a comprehensive, free E-Book: [ Ссылка ]

👉 To know about HiveMQ: [ Ссылка ]

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👉 We're looking forward to seeing you next week for MQTT Essentials part 10 - Last Will & Testament. [ Ссылка ]

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