A small library which maps events between MQTT to BPM elements based on Camunda BPM Engine.

MQTT is a well adopted protocol in the area of IoT and sensor applications. Camunda BPM provides a light weight, developer-friendly process engine, speaking BPMN 2.0 which allows to create easy-to understand application. The purpose of this library is to bridge the gap between MQTT and BPMN.

The library provides two abstractions for sending and receiving messages respectively. A message delivered to MQTT topic can be mapped to a Catching Signal Event of BPMN. So the library subscribes to the topic and transmits the payload of the received message to the payload of the BPMN event. This is used to communicate from device to process. A Throwing Message event inside the BPMN process model can be mapped to a message published to a MQTT topic. This is used to communicate from process to device.

In order to use the library, a configuration of MQTT protocol has to be supplied.

mqtt.broker=tcp://localhost:1883
mqtt.qos=2
mqtt.topic.prefix=camunda-bridge

• Weld 1.1
• EJB
• Servlet

In order to send a MQTT message, please use a BPMN throwing message event and provide ${mqttThrowingEvent} as delegateExpression property value. In addition, the MQTT client need the information about the target topic and the message payload. There are two ways to specify those:

You must name the BPMN throwing message-definiton event with the topic name (without prefix) and the process execution must contain a variable called <event-id>_payload.

E.g if the prefix is configured to be camunda-bridge

<bpmn:intermediateThrowEvent id="messageEventBar" name="foo/bar/zoo" />

will lead to a message deliver to the topic camunda-bridge/foo/bar/zoo with the value taken from the process variable messageEventBar_payload.

As an alternative you can use the Camunda Fields extensions (see below)

  <bpmn:messageEventDefinition camunda:delegateExpression="${mqttThrowingEvent}">
    <bpmn:extensionElements>
      <camunda:field name="topic" stringValue="foo" />
      <camunda:field name="message">
        <camunda:expression>${barValue}</camunda:expression>
      </camunda:field>
    </bpmn:extensionElements>        
  </bpmn:messageEventDefinition>

In order to convert an incomming MQTT message to a BPMN Signal, the property mqtt.signals.deliver must be set to true. Then, every message is converted to a signal with the name equals to the topic name without the prefix (and sub-topics, if configured, see below). For example, the following BPMN catching signal event will be triggered on receipt of the message to the topic <prefix>/foo/bar/zoo. Both start process signal event and intermediate signal catching event are supported.

  <bpmn:signalEventDefinition signalRef="Signal_0ohapx3" />
  <bpmn:signal id="Signal_0ohapx3" name="foo/bar/zoo" />

• prefix configuration: mqtt.topic.prefix=camunda-bridge
• topic: camunda-bridge/foo/bar/zoo
• fired BPMN signals: zoo, bar/zoo, foo/bar/zoo
• payload variables:
  • zoo_payload and zoo_topic for process instance triggered by the signal zoo
  • bar/zoo_payload and bar/zoo_topic for process instance triggered by the signal bar/zoo
  • foo/bar/zoo_payload and foo/bar/zoo_topic for process instance triggered by the signal foo/bar/zoo

You can suppress delivery of sub-signals (zoo and bar/zoo) by setting the property mqtt.subsignals.supress to true.