Monday, April 15, 2013

MQTT Transport for Axis2/Synapse

MQTT is an application layer broker based pub/sub protocol built on top of TCP/IP. It is very lightweight and hence ideal for usage in network/memory/computing power constrained environments. If I quote the MQTT spec, the protocol features,
  1. Pub/Sub message pattern
  2. Small transport overhead, fixed length header being only 2 bytes
  3. Supports three QOS for message delivery, at-least once, exactly once and at-most once
  4. etc.
The ability to operate under constrained environment make this protocol an ideal candidate for embedded device communication/pervasive computing.  However smaller footprint is always better. Why not MQTT in enterprise integration ? :)

There are number of MQTT broker implementations out there (you can find the original spec here). Right now, MQTT transport is undergoing OASIS standardization process. Mosquitto is one such broker implementation of MQTT. Eclipse Paho project provides MQTT client library in Java. 

Apache Synapse is  a popular open source enterprise service bus (ESB). Synapse mediation engine relies on Axis2 transport framework to receive messages. Axis2/Synapse  supports many transports out  of the box. I set out to implement a MQTT transport for Axis2/Synapse so that MQTT transport is available for EIP scenarios/etc with Synapse ESB.

The sample message flow,

Setting up the environment and running the code,

1. Download the MQTT broker from and install
2. Download Apache Synapse and install. Due to some API compatibility reasons, the code only works with trunk version of Synapse (1.7.0-SNAPSHOT) as of this writing.
3. Deploy the provided proxy service configuration 'SampleProxy.xml'
The provided proxy service read from a Mosquitto broker topic and writes to a publicly hosted MQTT broker topic.

Next configure the MQTT transport definition in axis2.xml transports section. Here we configure the transport listener and transport sender.



simply copy replace the original axis2.xml file found in synapse distribution with provided axis2.xml.
Now add the eclipse Paho MQTT client jar and the axis2-mqtt transport jar to synapse classPath. That is to $synapse_home/lib/

4. Start Synapse and Mosquitto broker
5. Publish message to topic1 using provided client. Edit the endpoint addresses and payload accordingly.

Once you publish the message to the topic, the listening ESB transport will get notified and proxy service will write the message to the target endpoint.

6. Read/Observer the output in topic2 using the provided receiver. The online hosted MQTT broker is configured as the target endpoint. The http bridge allows you to see the currently published topics to the broker.

I have only demonstrated, MQTT -- > MQTT scenario. The Axis2 transport framework allows you to transport switch messages. Hence X --> MQTT / MQTT--> X is possible. (X is a Axis2 transport). The code is very much in its initial state. The next step would be to improve this and contributing to Axis2 project.

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