Subscribe to a Variable's Last Value

Use the last value (/lv) topic to subscribe to a variable's most recent value. On connect you immediately receive the current value, then get a new message whenever the value changes.

Topic structure

/v1.6/devices/{DEVICE_LABEL}/{VARIABLE_LABEL}/lv

Parameter	Description
`{DEVICE_LABEL}`	Your device's unique label
`{VARIABLE_LABEL}`	The variable you want to monitor
`/lv`	Last-value suffix

⚠️
If the device or variable doesn't exist, the subscription will fail.

Connection parameters

Field	Value	Required
Host	`api.surfact.com`	Yes
Port	`1883` (no TLS) / `8883` (TLS)	Yes
Username	Your Surfact token	Yes
Password	Any character or blank	No
Quality of Service	`0` or `1`	No

Examples

Command line (mosquitto_sub)

mosquitto_sub \
  -h "api.surfact.com" \
  -t "/v1.6/devices/weather-station/temperature/lv" \
  -u "$SURFACT_TOKEN" \
  -p 8883 \
  -q 1

Output:

24.0

Python (paho-mqtt)

import paho.mqtt.client as mqtt

token = "your-surfact-token"
device_label = "weather-station"
variable_label = "temperature"

def on_connect(client, userdata, flags, rc):
    topic = f"/v1.6/devices/{device_label}/{variable_label}/lv"
    client.subscribe(topic)
    print(f"Subscribed to {topic}")

def on_message(client, userdata, msg):
    print(f"Last value: {msg.payload.decode()}")

client = mqtt.Client(client_id="my-device-12345678901234567890")
client.username_pw_set(username=token, password="")
client.on_connect = on_connect
client.on_message = on_message
client.connect("api.surfact.com", 8883)
client.loop_forever()

JavaScript (mqtt.js)

const mqtt = require('mqtt');

const token = 'your-surfact-token';
const deviceLabel = 'weather-station';
const variableLabel = 'temperature';

const client = mqtt.connect('mqtts://api.surfact.com:8883', {
  username: token,
  password: '',
  clientId: 'my-device-12345678901234567890'
});

client.on('connect', () => {
  const topic = `/v1.6/devices/${deviceLabel}/${variableLabel}/lv`;
  client.subscribe(topic, (err) => {
    if (!err) console.log(`Subscribed to ${topic}`);
  });
});

client.on('message', (topic, message) => {
  console.log(`Last value: ${message.toString()}`);
});

Behavior

When you subscribe to an /lv topic:

Immediate delivery — you receive the variable's current last value right away.
Updates — you receive new values whenever the variable is updated.
Retained messages — the last value is always available (see the retain flag).

Use cases

Device initialization — get the current state immediately on connect
State syncing — ensure your device matches the cloud state on startup
Dashboard displays — show the latest value without waiting for an update
Control systems — initialize actuators to their last known state

📘
Need the timestamp and context too? Subscribe to the full dot topic instead.