Room control
MQTT: messages, state and availability in an AV interface
MQTT can carry device information to an AV platform through a broker that distributes messages. Before choosing an integration, define the data you need, who publishes it and how you will recognize that updates have stopped.
Who publishes and who interprets
A device or service publishes a message to a topic, a name used to organize information. The broker distributes it to clients with matching subscriptions. The application defines the message content, or payload; two MQTT products do not necessarily understand the same names or values.
Record protocol, firmware, broker and platform versions. This guide refers to MQTT 5.0, the OASIS Standard dated March 7, 2019, and Home Assistant MQTT documentation showing version 2026.10.4 when consulted. Do not assume an existing installation uses that combination.
A reading example with fictional names and values
Suppose a platform receives environmental data to display alongside AV state. These topics only illustrate how to separate concepts; they are not addresses from an installation or a configuration to copy.
- Device/service → documented topic and payload.
- Broker → distribution to the subscribing platform.
- AV interface → interpreted value and missing-update indication.
| Example topic | Example payload | What it represents |
|---|---|---|
| demo/espacio/temperatura | 23.4 | A reading interpreted as °C only if that unit is agreed with the publisher. |
| demo/espacio/disponibilidad | online | Publisher availability according to its implementation. |
| demo/espacio/medicion_hora | 10:30:00 | Fictional time: a real design needs a date, time zone and reference clock. |
Retained does not mean newly measured
A retained message can provide the last stored value when a client subscribes. This can restore an indication, but it does not establish that a new measurement was taken. MQTT 5.0 Message Expiry Interval can limit message lifetime; the interface also needs a policy for values it has already received.
As a platform example, Home Assistant MQTT Sensor distinguishes state_topic, availability_topic and expire_after. Its documentation warns that replaying retained state after a restart or reload can make an expired reading appear available again. Review freshness rules against the actual publishing behavior.
Message delivery and device confirmation
QoS levels describe MQTT message delivery, not the AV result. QoS 1 permits duplicates, and QoS 2 avoids duplicate delivery within its protocol exchange; neither establishes that a projector is ready or an image is visible.
Availability needs a mechanism defined by the device or service. Birth and Last Will messages can help communicate connection or disconnection, but their configuration and timing need checking. A broker or platform reporting online does not establish the state of every device.
What to share for an integration review
Start with the information you want to display and an equipment diagram. As the review progresses, agree on who maintains the broker, defines messages and retains manual AV control. This scope covers noncritical information and coordination; it excludes safety functions, access control and machinery.
- Known device models, firmware and MQTT versions.
- Topic, payload, unit and availability documentation without credentials.
- Stale-data criteria and behavior after reconnection.
- Local, account or internet dependencies and required AV feedback.
Questions that help with selection
Do I need an internet-hosted broker?
MQTT does not require an internet-hosted broker. Choose its location and dependencies for the platform and project; the solution’s continuity still needs its own verification.
Can I display the last value while the connection returns?
You can define that display behavior, but it should distinguish the last known value from a current reading. Agree on how to show its age and when it becomes unavailable.
Does MQTT make any device compatible with my platform?
No. Review the message schema and the functions at both ends. A gateway or documented translation may be needed; sharing a transport is not enough.