The Persistence Of Messages

In MQTT, the persistence of messages on the broker depends on the configuration of the broker and the QoS level used when publishing messages. 


By default, MQTT brokers do not persist messages. When a message is published and delivered to the subscribed clients, the broker does not store the message for future retrieval. If a client is not connected at the time a message is published, it will not receive that message.


However, some MQTT brokers provide features for message persistence, allowing messages to be stored and retrieved even if clients are not currently connected. This feature is typically useful when you want to ensure that messages are reliably delivered to clients, even if they are offline at the time of publishing.


If message persistence is enabled in the broker, it can store messages in various ways, such as writing them to disk or storing them in a database. When a disconnected client reconnects to the broker, it can retrieve any messages that were published while it was offline.


It's important to note that the persistence of messages can have implications for the storage requirements and performance of the MQTT broker. Storing a large number of messages for extended periods can consume significant disk space and impact the broker's performance. Therefore, it's recommended to consider the trade-offs and configure message persistence appropriately based on the specific requirements of your IoT application.


When publishing messages, the QoS level chosen also affects message persistence. MQTT supports three QoS levels:


1. QoS 0 (At most once): The broker delivers the message to connected subscribers but does not store or guarantee delivery to disconnected clients. This level provides the lowest reliability.


2. QoS 1 (At least once): The broker ensures that the message is delivered to connected subscribers and stores it for disconnected clients. It may result in duplicate message delivery in case of network issues or client retries.


3. QoS 2 (Exactly once): The broker ensures that the message is delivered exactly once to connected subscribers and stores it for disconnected clients. This level provides the highest reliability but introduces additional overhead.


When using QoS levels 1 or 2, the broker needs to store messages temporarily to handle message delivery and guarantee reliability.

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