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
    • 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.

    • 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.

  • 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 persisted.

Retain Message flagClean Session flagSubscribe QOSPublish QOSPublished Message Persisted
FalseTrue

0

0No
FalseTrue01 or 2No
FalseTrue1 or 20No
FalseTrue1 or 21 or 2No
FalseFalse00No
FalseFalse01 or 2No
FalseFalse1 or 20No
FalseFalse1 or 21 or 2Yes - all messages
TrueTrue

0

0Yes - last message only for each unique MQTT topic
TrueTrue01 or 2Yes - last message only for each unique MQTT topic
TrueTrue1 or 20Yes - last message only for each unique MQTT topic
TrueTrue1 or 21 or 2Yes - last message only for each unique MQTT topic
TrueFalse00Yes - last message only for each unique MQTT topic
TrueFalse01 or 2Yes - last message only for each unique MQTT topic
TrueFalse1 or 20Yes - last message only for each unique MQTT topic
TrueFalse1 or 21 or 2Yes - all messages


Chariot MQTT Server

The Chariot UI will display MQTT Client details under MQTT > Client Status including the Clean Session flag, the subscribe QOS and all Retained Messages.

MQTT Client Clean Session

MQTT Client Subscription QOS


MQTT Client Retained Messages

MQTT Distributor

MQTT Client Clean Session

By default MQTT Distributor com.cirruslink.chariot.server.core.PacketHandler will log each MQTT client connection indicating the 'Clean Session' flag as shown below:


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


MQTT Client Subscription QOS

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

The [] after the topic subscription indicates the quality of service. For example:

  • spBv1.0/STATE/IamHost is subscribing with QOS 1
  • spBv1.0/# is subscribing with QOS 0

MQTT Client Retained Messages

Retained messages are currently not available to view in MQTT Distributor




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