Unlimited keepalive in MQTT

Question

Currently developing something like "smart home" and I have few different devices in my home. All of them connected to OpenHab via MQTT. I'm using Paho MQTT library (Python) for my purposes.

Generally, MQTT has "keepalive" property. This property describes how much time my client will be connected (AFAIK it sends the ping to the server) to MQTT server when there are no updates on the subscribed topic.

But here I have a huge problem. Needed topic could be updated once per hour or even once per few days/months. Let's say that this is indoor alarm. How can I avoid that keepalive timeout or ignore that field? Could it be unlimited?

Answer 1

You have miss understood what the keepalive value represents.

MQTT clients can stay connected indefinitely even if they do not publish or receive any messages. But the broker needs to keep track of which clients are still connected so it knows when to send the Last Will and Testament (LWT) message for the client. In order to do this it uses the keepalive time.

Every time a message is sent or received by the client, the broker resets a timer, if this timer exceeds 1.5 times the value of the keepalive time then the broker marks the client as disconnected and processes the LWT. To prevent clients with very low messages rates from being disconnected, such a client can send a PINGREQ packet at any time (most likely on timeout of the keepalive value) to the server/broker. The server receives the PINGREQ, answers with a PINGRESP packet and it will reset the keepalive timer to zero and leave the client in the connected state.

See Keep Alive section of the MQTT standard: (http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349238)

The Client can send PINGREQ at any time, irrespective of the Keep Alive value, and use the PINGRESP to determine that the network and the Server are working. If the Keep Alive value is non-zero and the Server does not receive a Control Packet from the Client within one and a half times the Keep Alive time period, it MUST disconnect the Network Connection to the Client as if the network had failed