mqtt Advent Calendar 2025

[MQTT] MQTT5 Reason Code "Disconnect with Will Message"

Last updated at 2025-12-18Posted at 2025-12-18

はじめに

今回は狭いテーマですが、DisconnectのReasonのひとつである、"Disconnect with Will Message"について確認します。

Reason Code "Disconnect with Will Message"

MQTT3.1.1では異常切断のみWillが発火しましたが、正常切断でも発火してほしい場合があります。MQTT5では、DisconnectのReasonにDisconnectwith Will Message設定すると、異常切断ではなくてもWillを発火させることができます。

規格における、Disconnectwith Will Messageについての記述

The Client wishes to disconnect but requires that the Server also publishes its Will Message.

実践

今回はサーバ側にmosquittoを利用します。
クライアント側からDisconnectの際にReasonを設定してWillが発火することを確認します。

サーバ側 mosquito

下記を参考にdocker上でmqttブローカーを立ち上げる
https://qiita.com/kino15/items/0ef252b10100b2ee2f05

クライアント側 paho

import paho.mqtt.client as mqtt
import threading
import time
from paho.mqtt.packettypes import PacketTypes
from paho.mqtt.reasoncodes import ReasonCode

BROKER = "localhost"
TOPIC = "test/topic"
WILL_TOPIC = "test/will"


# ======================
# Subscriber
# ======================
def on_message(client, userdata, msg):
    print(f"[受信] {msg.topic}: {msg.payload.decode()}")


def on_subscribe(client, userdata, mid, granted_qos, properties=None):
    print(f"[SUBACK] mid={mid}, qos={granted_qos}")


def subscriber():
    sub = mqtt.Client(
        client_id="sub-client",
        protocol=mqtt.MQTTv5,
        callback_api_version=mqtt.CallbackAPIVersion.VERSION2
    )
    sub.on_message = on_message
    sub.on_subscribe = on_subscribe

    sub.connect(BROKER, 1883, 60)

    sub.subscribe(TOPIC)
    sub.subscribe(WILL_TOPIC)

    print("[Subscriber] Start")

    # ★ スレッドでループ開始
    sub.loop_start()

    # このまま待機
    while True:
        time.sleep(1)


# ======================
# Publisher
# ======================
def publisher_with_will_disconnect():
    pub = mqtt.Client(
        client_id="pub-client",
        protocol=mqtt.MQTTv5,
        callback_api_version=mqtt.CallbackAPIVersion.VERSION2
    )

    # Will 設定
    pub.will_set(
        WILL_TOPIC,
        "pub offline via disconnect-with-will",
        qos=1,
        retain=False
    )

    pub.connect(BROKER, 1883, 10)

    for i in range(3):
        msg = f"Hello {i}"
        print(f"[送信] {msg}")
        pub.publish(TOPIC, msg)
        time.sleep(1)

    print("\nPublisher が DISCONNECT with Will Message を送信します (0x04)\n")

    # ★ ReasonCode の value を直接書き換える方式
    rc = ReasonCode(PacketTypes.DISCONNECT)
    rc.value = 0x04    # Disconnect with Will Message

    pub.disconnect(reasoncode=rc)

    print("[Publisher] DISCONNECT(0x04) 送信完了")


# ======================
# メイン
# ======================
if __name__ == "__main__":
    t = threading.Thread(target=subscriber, daemon=True)
    t.start()

    time.sleep(1)
    publisher_with_will_disconnect()

    print("Will を受信するのを待っています...\n")
    time.sleep(10)

Terminal

~/qiita/client]$uv run disconnect_will.py 
[Subscriber] Start
[SUBACK] mid=1, qos=[ReasonCode(Suback, 'Granted QoS 0')]
[SUBACK] mid=2, qos=[ReasonCode(Suback, 'Granted QoS 0')]
[送信] Hello 0
[受信] test/topic: Hello 0
[送信] Hello 1
[受信] test/topic: Hello 1
[送信] Hello 2
[受信] test/topic: Hello 2

Publisher が DISCONNECT with Will Message を送信します (0x04)

[Publisher] DISCONNECT(0x04) 送信完了
Will を受信するのを待っています...

[受信] test/will: pub offline via disconnect-with-will
[~/qiita/client]$

まとめ

必ず切断で発火するようになり、便利そうです。

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