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Vizier.md

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Installation

  1. Clone or download the repository.
  2. Follow the installation instruction on the repository.
  3. Install Mosquitto by running sudo apt-get install mosquitto

Overview

Vizier is "A networking framework built on top of MQTT to allow the communication and synchronization of distributed, language-independent resources"

Basically, it allows you to send messages between devices using an MQTT Server to mediate between them. The messages are sent back and forth between "nodes" which in our use case are the Topside Computer and the robot. Each of the nodes has a JSON file that describes what types of messages the node can send and recieve. This allows the python script to request a certain type of message from the server which then sends back the correct information.

JSON Description Files

Purpose

The JSON Description files are used to describe what types of messages a node can send and recieve as well as the name of the node.

Format

The JSON File is formatted in this order:

  1. The end_point (the node) is given a name
  2. A List of the "links" which are the possible messages it can send.
    1. The link has its own list of links
    2. The type that the link is. This can one of two types:
      • STREAM "publishes" the data given to it over the link
      • DATA "puts" the data on the link
    3. The requests that the node can make. These ask the server to respond with the data specified.

This is the JSON description file for the robot node:

{
  "end_point" : "robot",
  "links" :
  {
    "/state" :
    {
      "links" : {},
      "type" : "STREAM"
    }
  },
  "requests" : [{"link": "controller/input", "type": "STREAM", "required": false}]
}

controller is the name of the other node that it is connecting to and it can request the data labeled input.

Syntax

All of the sample code shown here is pulled directly from the robot.py file

First, you need to open the JSON file to get the information from it.

node_descriptor = None
f = open("node_desc_robot.json", 'r')
node_descriptor = json.load(f)
f.close()

This code loads the node description information from a JSON file and stores it in node_descriptor.



Next, you need to start the Vizier Node.

node = vizier.node.Node("localhost", 1884, node_descriptor)

The first argument is the address that the MQTT Server is running at. The second argument is the port number. Finally, the third argument is the node description information pulled from the JSON file as shown earlier.



After this you need to set up the connections for sending and recieving.

publishable_link = list(node.publishable_links)[0]
subscribable_link = list(node.subscribable_links)[0]
msg_queue = node.subscribe(subscribable_link)

The first two lines get the first publishable link and subscribable link from the list of each type of link in the JSON file. Then, the last line sets up a message queue for the recieving link to put recieved messages into.



Next, you need to send or recieve using the links that you have set up.

node.publish(publishable_link,"1")
message = msg_queue.get(timeout = 1).payload.decode(encoding='UTF-8')

The first line sends a string, "1", over the /state link since it is the first publishable link in the JSON file.

The second line recieves a message from the recieving link (timing out if it has not recieved a message after 1 second). This message contains a payload (the information we want) which is in binary form. the call to decode() is to decode this binary into a string using UTF-8.



Finally, when we are done we need to close the connection.

node.stop()