# MQTT Client

> This section explains how to configure a Pub/Sub integration using the built-in MQTT client.

Source: https://www.thethingsindustries.com/docs/integrations/other-integrations/pubsub/mqtt-client/


This section explains how to configure a Pub/Sub integration using the built-in MQTT client.

Creating a Pub/Sub integration with the built in MQTT client requires you to have an MQTT server running with an available endpoint. [Mosquitto](https://mosquitto.org/) is a popular open source MQTT server you may use, but The Things Stack MQTT client works with any other MQTT server too.

No API key is needed for Pub/Sub messaging. All messages received by the Application Server are assumed to be authorized, so be sure to configure appropriate security on your MQTT server.

In your application, select the **Pub/Subs** submenu from the **Integrations** side menu. Clicking on the **+ Add Pub/Sub** button will open the Pub/Sub creation screen.

Give your Pub/Sub an **ID**, choose a **Pub/Sub format**, and enter a **Base topic** to subscribe or publish to.

Select **MQTT** as provider.

![Pub/Sub creation screen](https://www.thethingsindustries.com/docs/integrations/other-integrations/pubsub/mqtt-client/mqtt.png)


If using client certificates, enable **Secure**. You will be asked to upload the **Root CA certificate**, the **Client certificate**, and the **Client private key.**

Enter the endpoint of the MQTT server in the **Server URL** field, including the protocol and port.

Enter the **Client ID** the Application Server should use to authenticate.

Enable **Use credentials** if your MQTT server uses password authentication. Enter the username and password the Application Server should authenticate with.

![MQTT configuration](https://www.thethingsindustries.com/docs/integrations/other-integrations/pubsub/mqtt-client/mqtt-config.png)


## Message Types

The Application Server can be configured to publish and subscribe to topics for individual events. See [Message Types](https://www.thethingsindustries.com/docs/integrations/other-integrations/pubsub/message-types/) for more information about events and topics.

After enabling events, click the **Add Pub/Sub** button to enable the integration.

## Subscribing to Upstream Traffic

The Application Server publishes messages for any enabled events. For example, when a device sends an uplink, an `uplink` message is published. To view these using a `mosquitto_sub` client connected to your MQTT server:

```bash
mosquitto_sub -h <server_hostname> -p <port> -t '#' -v
# base-topic/uplink {"end_device_ids":{"device_id":"dev1","application_ids":{"application_id":"app1"}},"received_at":"2020-05-12T12:23:07.087614Z","uplink_message":{"session_key_id":"AXIDznz4bnQqtW8T3NsIVg==","f_port":1,"f_cnt":327,"frm_payload":"AQ=="}}
```

## Scheduling Downlinks

You can schedule downlink messages by publishing to a topic. For example, using a `mosquitto_pub` client connected to your MQTT server:

```bash
mosquitto_pub -h <server_hostname> -p <port> -t 'base-topic/push-subtopic' -m '{"end_device_ids":{"device_id":"dev1","application_ids":{"application_id":"app1"}},"downlinks":[{"f_port":1,"frm_payload":"AA==","priority":"NORMAL"}]}'
```

will push a downlink to the end device `dev1` of the application `app1` with a base64 encoded payload of `AA==`, if `push-subtopic`is configured as the Sub topic for Downlink queue push.

This will also result in the following message being published by the Application Server:

```bash
# Received on [base-topic/downlink-queued-subtopic] :
'{"end_device_ids":{"device_id":"dev1","application_ids":{"application_id":"app1"}},"correlation_ids":["as:downlink:01E84EAR5B4NM229NDKE0004J6"],"downlink_queued":{"f_port":1,"frm_payload":"AA==","priority":"NORMAL","correlation_ids":["as:downlink:01E84EAR5B4NM229NDKE0004J6"]}}'
```