Did you know that the MQTT broker can store new messages for a client after it goes offline? It will send the messages as soon as the client reconnects. This feature in MQTT is called persistent session. In this post, I will explain how it works.
See Also:
Fundamentals of MQTT
MQTT Python With Paho-MQTT Client

Persistent Session or Clean Session

Lets say you have 2 clients connected to a broker. One client is publishing to a topic and the other client is subscribed to that topic. If the client that has subscribed suddenly disconnects from the broker, it will not receive any of the messages published to that topic, until it reconnects.

To solve this problem we use a Persistent Session or a Clean Session flag while connecting to the broker. When a client connects to the broker it can request the broker for a persistent session. The broker will store the client information and the topics it has subscribed to. When the client reconnects after a disconnection, the broker will send all undelivered messages to the client. The broker stores these undelivered messages in a message queue.

How to use a Persistent Session?

A persistent session can be started by the client by setting the cleanSession flag to False when the client connects to the broker. By default this value is set to True.

For a persistent session to work you must note:

  • The broker uses the client ID to recognize clients for a persistent session. So make sure you use a unique client ID for each client.
  • You need to subscribe to the topics with a QoS level of 1 or 2. Only then the broker will publish the undelivered messages.
    See: MQTT QoS Levels Explained
  • The messages also must be published to that topic with QoS Level 1 or 2. The broker will not store messages if a client publishes them with QoS Level 0.
  • If the retain flag is set to True when publishing a message, the client which has a persistent session will still get all the messages published to that topic(not just the last message).

When to use a Persistent Session?

It is important to use a Persistent Session if:

  • The client must receive all (or almost all) the messages from a topic that it has subscribed to.
  • Internet is unreliable for the client.

FAQs

How many messages can be queued by the broker?

The broker stores the messages in the primary memory(RAM). So the maximum number of messages that can be stored by a broker depends on the primary memory.

Mosquitto by default stores upto 100 messages. This setting can be changed.

How do I set a client that is already connected to a broker to have a Persistent Session?

You will need to disconnect the client and reconnect it with the cleanSession flag set to True.