What is an Mqtt 'session'?

Question

In creating an Mqtt client connection using the class AwsIotMqttConnectionBuilder, I encounter a configuration setting called "withCleanSession(boolean)" that is described in the documentation this way:

/**
     * Configures whether or not the service should try to resume prior
     * subscriptions, if it has any
     *
     * @param cleanSession true if the session should drop prior subscriptions when
     *                     a connection from this builder is established, false to resume the session
     * @return {@link AwsIotMqttConnectionBuilder}
     */
    public AwsIotMqttConnectionBuilder withCleanSession(boolean cleanSession) {
        this.config.setCleanSession(cleanSession);
        return this;
    }

What is the meaning of "session" in this context? Is a session something that is local to one computer, or is it a state stored on the network somewhere? Is the client ID of the connection related to its state?

Accepted Answer

Please see here: https://docs.aws.amazon.com/iot/latest/developerguide/mqtt.html#mqtt-persistent-sessions

> Persistent sessions store a client’s subscriptions and messages, with a quality of service (QoS) of 1, that have not been acknowledged by the client. When a disconnected device reconnects to a persistent session, the session resumes, its subscriptions are reinstated, and subscribed messages received prior to the reconnection and that have not been acknowledged by the client are sent to the client.

And: https://docs.aws.amazon.com/iot/latest/developerguide/sdk-tutorials.html

> The message broker receives messages from devices and publishes messages to devices that have subscribed to them. With persistent sessions —sessions that remain active even when the initiating device is disconnected—devices can retrieve messages that were published while they were disconnected. On the device side, MQTT supports Quality of Service levels (QoS) that ensure the host receives messages sent by the device.

Also: https://aws.amazon.com/about-aws/whats-new/2019/01/aws-iot-core-now-enables-customers-to-store-messages-for-disconnected-devices/

`Is the client ID of the connection related to its state?`

Yes, the Client ID is used to identify which session to resume.

What is an Mqtt 'session'?

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