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mqtt: receiving messages from an MQTT broker

1: Prerequisites to using the mqtt() source

2: Limitations to using the mqtt() source

3: Options of the mqtt() source

From AxoSyslog version 3.35, you can use the mqtt() source to fetch messages from MQTT brokers.

The mqtt() source builds on the MQTT protocol, and uses its client and broker entities.

Note The rest of this chapter and its sections build on your familiarity with the MQTT protocol, the concept of client and broker entities, and how these entities function within an MQTT system.

Declaration:

   source s_mqtt{
        mqtt(
            address("tcp://<hostname>:<port-number>")
            topic("<topic-name>")
        );
    };

Starting with AxoSyslog version 4.7, mqtt() source automatically sets the ${MQTT_TOPIC} name-value pair for the messages it receives. This is useful when the name of the topic contains MQTT wildcards ($, +, #). For example:

log {
    source { mqtt(topic("#")); };
    destination { stdout(template("${MQTT_TOPIC} - ${MESSAGE}\n")); };
};

Example: Using the mqtt() source in your configuration

The following example illustrates an mqtt() source configured to fetch messages from the MQTT broker running on localhost:4444 using the test/test topic, and send them to the localhost:4445 address.

    @version: current
    @include "scl.conf"
    source s_mqtt {
        mqtt(
            address("tcp://localhost:4444")
            topic("test/test")
        );
    };
    destination d_network {
        network(
            "localhost"
            port(4445)
        );
    };
    log {
        source(s_mqtt);
        destination(d_network);;
    };

1 - Prerequisites to using the mqtt() source

Using the current implementation of the mqtt() source has the following prerequisites:

Installing the eclipse-paho-mqtt-c library.

Note The default package manager for some Linux operating systems contains the eclipse-paho-mqtt-c library, but depending on your OS, you may have to install the library manually. For more information about how you can download and install the eclipse-paho-mqtt-c library, see Eclipse Paho on the Eclipse Foundation website.
Having a broker entity in a functional MQTT system.

Note In your configuration, you will specify the broker entity of your MQTT system in the address() option of your mqtt() source.

2 - Limitations to using the mqtt() source

Using the mqtt() source of AxoSyslog has the following limitations:

You can only use the mqtt() source with AxoSyslog version 3.35 or higher.
You cannot use the mqtt() source without installing the the eclipse-paho-mqtt-c library.
For more information about how you can download and install the eclipse-paho-mqtt-c library, see Eclipse Paho on the Eclipse Foundation website.
The current implementation of the mqtt() source supports versions 3.1 and 3.1.1 of the MQTT protocol

3 - Options of the mqtt() source

The mqtt() source has the following options.

Required options: address(), fallback-topic(), and topic().

address()


Type:	string
Default:	`tcp://localhost:1883`
Required:	yes

Description: Specifies the hostname or IP address, and the port number of the MQTT broker to which AxoSyslog will send the log messages.

Syntax: <protocol type>://<host>:<port>

client-id()


Type:	string
Default:	`syslog-ng-source-{topic option}`
Required:	no

Description: The client-id() is used to identify the client to the MQTT server, which stores session data for each client. The session data can contains information regarding which message has been sent, received. It is not possible to set the client-id() to an empty string. To always start a new session see the cleansession() option.

cleansession()


Type:	`yes` or `no`
Default:	`no`

Description: This option instruments the MQTT broker to clean the session data when connecting. The session data contains information about which message was processed.

http-proxy()


Type:	URL
Default:	N/A

Description: Specifies HTTP/HTTPS proxy for WebSocket connections.

keep-alive()


Type:	positive integer number (in seconds)
Default:	`60`

Description: Specifies the number of seconds that AxoSyslog keeps the connection between the broker and clients open in case there is no message traffic. When keep-alive() number of seconds pass, the connection is terminated, and you have to reconnect.

On the MQTT side, the keep alive function provides a workaround method to access connections that are still open.

password()


Type:	string
Default:	N/A

Description: The password used to authenticate on the MQTT broker.

persist-name()


Type:	string
Default:	N/A

Description: If you receive the following error message during AxoSyslog startup, set the persist-name() option of the duplicate drivers:

Error checking the uniqueness of the persist names, please override it with persist-name option. Shutting down.

This error happens if you use identical drivers in multiple sources, for example, if you configure two file sources to read from the same file. In this case, set the persist-name() of the drivers to a custom string, for example, persist-name("example-persist-name1").

qos()

Type:

number

Default:

`0`

Possible values:

`0` - at most once (the fastest option)

`1` - at least once (a much slower option than `0`)

`2` - exactly once (the slowest option)

Description: The Quality of Service (QoS) level in MQTT messaging is an agreement between sender and receiver on the guarantee of delivering a message.

tls()


Type:	tls options
Default:	n/a

Description: This option sets various options related to TLS encryption, for example, key/certificate files and trusted CA locations. TLS can be used only with tcp-based transport protocols. For details, see TLS options.

The following options are relevant for the mqtt() tls() block: ca-dir(), ca-file(), cert-file(), cipher-suite(), key-file(), peer-verify(), ssl-version(), use-system-cert-store().

topic()


Type:	string or template
Default:	N/A

Description: Required option. Specifies the MQTT topic.

username()


Type:	string
Default:	N/A

Description: The username used to authenticate on the MQTT broker.