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mqttclient.createmessage(topic, message, options)

Create a message

Availability

Agent

Parameters

Name Type Description
topic String or blob The topic the message will be published to. Must be valid UTF-8
message String or blob The message to publish
options Table Optional table of publishing parameters

Returns

An mqttmessage object

Description

This method allows you to create a message that can subsequently be posted to the specified topic. The method does not publish the message.

The maximum size of a message (as passed into the message parameter) is currently 256KB. A runtime error will be thrown if you attempt to create a message larger than this.

The options parameter can take a table of message-publishing settings. These are listed in the table below.

Key Type Description
qos Integer Set the Quality of Service level (see below). Default: mqtt.AT_MOST_ONCE
retain Boolean Instruct the broker to retain messages for future subscribers (true), or send to current subscribers only (false). Default: false

QoS

The value of qos can be any of the following:

Constant Value Description
mqtt.AT_MOST_ONCE 0 Only provide the message once or not at all
mqtt.AT_LEAST_ONCE 1 The message may be delivered more than once

If no qos is included in the options table, the default value mqtt.AT_MOST_ONCE is applied.

Example

The following code generates a message (using an mqttclient instance, client) and sends it to the broker referenced when client was created.

server.log("Posting a message...");

local message = client.createmessage("imp.mqtt.test.pings",
                                     "this is a test",
                                     {"qos": mqtt.AT_LEAST_ONCE} );
if (m != null) {
    message.sendasync(function(result) {
        server.log("The message was " + (result != 0 ? "not " : "") + "published");
    });
} else {
    server.error("Could not create message");
}