MQTT Essentials Part 9: Last Will and Testament
Welcome to the ninth part of MQTT Essentials. A ten-part blog series on the core features and concepts of the MQTT protocol. In this post we explain the Last Will and Testament feature of MQTT.
Because MQTT is often used in scenarios that include unreliable networks, it’s reasonable to assume that some of the MQTT clients in these scenarios will occasionally disconnect ungracefully. An ungraceful disconnect can occur due to loss of connection, empty batteries, or many other reasons. Knowing whether a client disconnected gracefully (with an MQTT DISCONNECT message) or ungracefully (without a disconnect message), helps you respond correctly. The Last Will and Testament feature provides a way for clients to respond to ungraceful disconnects in an appropriate way.
Last Will and Testament
In MQTT, you use the Last Will and Testament (LWT) feature to notify other clients about an ungracefully disconnected client. Each client can specify its last will message when it connects to a broker. The last will message is a normal MQTT message with a topic, retained message flag, QoS, and payload. The broker stores the message until it detects that the client has disconnected ungracefully. In response to the ungraceful disconnect, the broker sends the last-will message to all subscribed clients of the last-will message topic. If the client disconnects gracefully with a correct DISCONNECT message, the broker discards the stored LWT message.
LWT helps you implement various strategies when the connection of a client drops (or at least inform other clients about the offline status).
How do you specify a LWT message for a client?
Clients can specify an LWT message in the CONNECT message that initiates the connection between the client and the broker.
To learn more about how the connection between the client and broker is established, see part 3 of MQTT Essentials.
When does a broker send the LWT message?
According to the MQTT 3.1.1 specification, the broker must distribute the LWT of a client in the following situations:
- The broker detects an I/O error or network failure.
- The client fails to communicate within the defined Keep Alive period.
- The client does not send a DISCONNECT packet before it closes the network connection.
- The broker closes the network connection because of a protocol error.
We will hear more about the Keep Alive time in the next post.
Best Practices – When should you use LWT?
LWT is a great way to notify other subscribed clients about the unexpected loss of connection of another client. In real-world scenarios, LWT is often combined with retained messages to store the state of a client on a specific topic. For example, client1 first sends a CONNECT message to the broker with a lastWillMessage that has “Offline” as the payload, the lastWillRetain flag set to true, and the lastWillTopic set to client1/status. Next, the client sends a PUBLISH message with the payload “Online” and the retained flag set to true to the same topic (client1/status). As long as client1 stays connected, newly-subscribed clients to the client1/status topic receive the “Online” retained message. If client1 disconnects unexpectedly, the broker publishes the LWT message with the payload “Offline” as the new retained message. Clients that subscribe to the topic while client1 is offline, receive the LWT retained message (“Offline“) from the broker. This pattern of retained messages keeps other clients up to date on the current status of client1 on a specific topic.
That brings us to the end of part nine in our MQTT Essentials series. Thanks for joining us. In the last post of this series, we’ll cover the MQTT heartbeat mechanism and how the broker knows a client is online or offline.
Have a great week and see you on the next MQTT Monday!
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