---
title: MQTT Client
slug: unify/mqtt-client
docTags: 
createdAt: 2025-01-24T19:35:17.943Z
---

The *MQTT Client* is a built-in client for the message broker in Litmus Unify. You subscribe to topics, watch messages arrive, and publish test messages without installing a separate MQTT tool.

Use *MQTT Client* to confirm that a broker is receiving messages. When a device is not reporting, subscribing here tells you whether the data reaches the broker at all, which separates a device problem from a downstream one. When you set up a topic, publishing a test message confirms the path first.

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# Access MQTT Client UI

**To access the MQTT Client pane:**

Navigate to **MQTT Client**.

# Interface

| Element               | Description                                                                                                                           |
| --------------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
| Connection Status     | Whether the client is connected to the broker. Nothing arrives in the feed and nothing can be published while it reads `Disconnected` |
| **Add Subscription**  | Subscribes to a topic. Wildcards are supported, so `+` matches one topic level and `#` matches all remaining levels                   |
| **Filter**            | Narrows what the feed displays by topic or content. It filters the view only and does not change what you are subscribed to           |
| **Auto-scroll**       | Keeps the newest message in view. Turn it off to read one message on a busy topic                                                     |
| Message Feed          | Messages arriving on your subscribed topics in ascending order                                                                        |
| Publish Message Panel | Sends a message to a topic. See field description in **Publish message fields**.                                                      |

## Publish message fields

| Field       | Description                                                                     |
| ----------- | ------------------------------------------------------------------------------- |
| **Topic**   | Topic to publish to                                                             |
| **Message** | Payload to send                                                                 |
| **Retain**  | Tells the broker to keep this message and publish it to new subscribing clients |
| **QoS**     | Delivery guarantee: `0` at most once, `1` at least once, `2` exactly once       |
| **Send**    | Publishes the message                                                           |

# Troubleshooting 

| Symptom                    | What to check                                                                                                        |
| -------------------------- | -------------------------------------------------------------------------------------------------------------------- |
| No messages arrive         | Confirm that the subscription topic path matches the publisher's topic and check if wildcard usage is appropriate.   |
| A message will not publish | Validate that the topic and message payload format are correct.                                                      |

# Considerations

- The feed starts empty and fills as messages arrive. It is not a history, so subscribing after an event does not show past messages.
- **Retain** has a lasting effect that the other fields do not. A retained test message stays on the topic and is delivered to every later subscriber until something overwrites it.