Excessive JVM Memory usage at MQTT Server

The most common reason for excessive memory usage at an MQTT Server is that messages are being stored at the MQTT Server for future delivery.

There are two reasons messages are stored at an MQTT Server:

MQTT clients are connecting with the 'Clean Session' flag set to False
- To receive messages from an MQTT Server, a client establishes a connection and creates subscriptions to the desired topics.
- When a MQTT client connects in a non-persistent session, with 'Clean Sesion' set to True, if the connection between the client and broker is interrupted the client loses its subscriptions and needs to re-subscribe upon reconnection.
- When a MQTT client connects in a persistent session, 'Clean Session' set to False, all relevant client information on the broker is stored ensuring that subscriptions and messages are retained even when the client is offline.
MQTT client publishing with the 'Retain' flag set to True
- When a client publishes a message with the “retained” flag set to true, the broker retains the message. Consequently, any client subscribing to the corresponding topic will receive the most recent retained message, even if no recent publications have occurred.

The chart below summarizes how Quality of Service (QOS), Clean Session flag and Retain Message flag affects what messages are received.

Retain Message flag	Clean Session flag	Subscribe QOS	Publish QOS	Published Message Always Received
False	True	0	0	No
False	True	0	1 or 2	No
False	True	1 or 2	0	No
False	True	1 or 2	1 or 2	No
False	False	0	0	No
False	False	0	1 or 2	No
False	False	1 or 2	0	No
False	False	1 or 2	1 or 2	Yes - all messages
True	True	0	0	Yes - last message only for each unique MQTT topic
True	True	0	1 or 2	Yes - last message only for each unique MQTT topic
True	True	1 or 2	0	Yes - last message only for each unique MQTT topic
True	True	1 or 2	1 or 2	Yes - last message only for each unique MQTT topic
True	False	0	0	Yes - last message only for each unique MQTT topic
True	False	0	1 or 2	Yes - last message only for each unique MQTT topic
True	False	1 or 2	0	Yes - last message only for each unique MQTT topic
True	False	1 or 2	1 or 2	Yes - all messages

How do I identify which MQTT clients are connecting using 'Clean Session' set to False

By default MQTT Distributor com.cirruslink.chariot.server.core.PacketHandler will log each MQTT client connection as shown below:

A [C] in this message indicates Clean Session = True whilst a [P] indicates Clean Session = False

The Chariot UI will display MQTT Client detail under MQTT > Client Status