Retained Messages

The "retained" flag in MQTT is used when publishing messages to indicate whether the message should be retained by the broker and made available to new subscribers. The decision to use the retained flag depends on the specific requirements and behavior you want in your MQTT application. Here are some scenarios where using the retained flag when publishing messages can be useful:


1. **Initial State or Configuration:** If you want new subscribers to receive the most recent or initial state of a particular topic, you can use the retained flag. When a message with the retained flag is published on a topic, the broker stores that message and delivers it to any new subscriber that subsequently subscribes to that topic. This is particularly useful when you want new devices or applications joining the system to quickly receive relevant information upon connection.


2. **Status or Last Will Messages:** Retained messages can be used to convey important status or "last will" information. For example, a device can publish a retained message with its status (e.g., online, offline, error) so that any subscribers joining later can immediately see the current state of the device. Similarly, the "last will" message can be published with the retained flag, which will be automatically delivered by the broker to subscribers when a client unexpectedly disconnects.


3. **Configurable Topics:** If you have topics that represent configurable settings or parameters, you can use retained messages to store and communicate the current value of those settings. Subscribers can retrieve the retained message upon subscription to know the current state of the topic.


4. **Presence Information:** Retained messages can be used to indicate the presence or availability of a device or entity. For example, a device can publish a retained message on a specific topic when it becomes available, and subscribers can receive this message to determine the presence of the device.

Comments

Post a Comment

Popular posts from this blog

Message Limitation

The maximum length of a publish message on an MQTT topic depends on the MQTT broker implementation and any specific limitations imposed by the broker or the MQTT protocol version being used. The MQTT specification itself does not define a fixed maximum length for the payload of a publish message. However, most MQTT brokers have their own limitations and constraints. In general, MQTT brokers may impose restrictions on the maximum size of a publish message to ensure efficient network usage and prevent abuse or denial-of-service attacks. The maximum length can vary depending on factors such as the available memory on the broker, network limitations, or configuration settings. It is recommended to consult the documentation or specifications of the specific MQTT broker you are using to determine the maximum allowed length for publish messages on topics. The broker's documentation should provide details on any limitations or guidelines regarding the maximum payload size for publish messa...
Read more

Overview of MQTT

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for efficient communication between devices in a network. It was developed by IBM in the late 1990s and has since become an open standard maintained by the OASIS consortium. MQTT is designed to be simple and efficient, making it well-suited for resource-constrained devices and networks with limited bandwidth or intermittent connectivity. It follows a publish-subscribe messaging model, where devices communicate by publishing messages to topics and subscribing to topics to receive relevant messages. Here's a brief overview of how MQTT works: Broker: MQTT uses a central message broker, which acts as an intermediary between publishers and subscribers. The broker receives messages published by devices and routes them to subscribers based on their topic subscriptions. Topics: Messages in MQTT are organized into topics, which are hierarchical and represented by strings. Topics can have multiple levels,...
Read more

Implement Historical Messages

In MQTT, the broker typically does not store or provide access to historical messages by default. MQTT follows a publish-subscribe model where messages are delivered to subscribers in real-time. However, you can implement a solution to store and retrieve historical messages on a specific topic by using additional components or modifying your MQTT setup. Here are a few approaches: Message Persistence: Configure your MQTT broker to use a message persistence mechanism. Some MQTT brokers, such as Mosquitto, support message persistence options like persistence plugins or configurations. By enabling message persistence, the broker will store messages on disk, allowing you to retrieve historical messages later. External Message Storage: Implement an external database or message queue that subscribes to the MQTT broker and stores messages on a specific topic. You can develop a custom application that connects to the broker, subscribes to the desired topic, and saves the received messages to a ...
Read more