Nodel is an open source digital media control system for museums and galleries, although there are many valid uses for Nodel outside of this scope where device control, monitoring, and scheduling is required.
In response to the pending renewal at the Australia Centre for the Moving Image, the technical services team decided to trial Nodel to assess it’s suitability as the control system for future exhibits and installations. I put my hand up to build a small Nodel implementation example, and write some documentation on the lessons learnt that would help future employees deploy Nodel around the organisation. This documentation will come in the form of four posts; Basic Concepts, Implementing a Simple Nodel System, Nodel System Design, and Coding Tips.
Implementing a Simple Nodel System, is the second of the write ups that will discuss Nodel. This post will describe how to implement the use case describe in the first post, Basic Concepts, introduce how to work with the Nodel Interface to create nodes, how to customise Nodes using python code, and how to use Nodel Recipes. It is highly recommended that you read Basic Concepts before going ahead and digesting this post.
If you are trying to learn how to work with Nodel for the first time, It is completely recommended to follow each step in this post and using it as a guide to build your first Nodel system. However, there are a few things that will not be covered to keep the post as concise as possible. This includes a how-to on the installation and starting up of a Nodel session. There is already great documentation written about how to do this on Nodel’s wiki for every common operating system (and some more obscure ones to boot!). And while this post will briefly discuss the Nodel scripting toolkit reference, It will not teach you how to code in Python. Fortunately Python is widely known as one of the easiest programming languages to pick up, and if you want to take a primer in the language I recommend you take a look at Learn Python’s free interactive online tutorial which will teach you all you need to know to code with Nodel.
One cool thing about Nodel is that you can design, implement, and test a large part of a system on your laptop/personal computer before migrating it over to the hardware that will be actually be running the system afterwards. We will be exploiting this feature during this tutorial, so try and run the instance of Nodel on the computer your are working with during the tutorial.
The Nodel Scripting Toolkit Reference
Animation 1 – Accessing the Nodel scripting toolkit reference. Click to enlarge.
The Nodel scripting toolkit reference describes all the key functions that are available for us to use once we eventually do get around to coding, so obviously it is something we are going to want to take a look at. It can be accessed within Nodel by going to the Nodel homepage at localhost:8085, clicking on the Nodel logo, and then clicking on the scripting toolkit reference hyperlink. Alternatively you can access it in your browser using http://localhost:8085/toolkit.htm. If you are having trouble with this already, it is probably because your do not have a Nodel session running.
The key functions that we will be using in this post are related to the creation and looking up of Local Actions, Local Events, Remote Actions, Remote Events, and the yet to be discussed Parameters.
The Use Case
Figure 1 – Use case for design problem
In Basic Concepts, we defined a use case involving a TV and a DVD player that needed to be turned on, and a acknowledgment from the TV and DVD player that confirmed the devices had indeed turned on when they were told to do so. Based on this use case, we came up with a Nodel design defining Node 1 as a Management Node, and Nodes 2 and 3 as Device nodes. Two separate implementations were designed, one using Remote Actions for control, and one using Remote Events for control. Believe it or not, defining these designs means half the battle of implementation is already won! We already know the exact nodes that need creating, and the Local Actions, Local Events, Remote Actions, and Remote Events that each node requires. These are shown in Table 1 and Table 2 for clarity. We will now go about implementing both these designs using the Nodel Interface. In order to keep this post a suitable length only the Remote Action control design will be laid out step-by-step, but after following these steps it should become straight forward enough to also implement the Remote Event control design by yourself!
The Nodel Interface
Accessing the Nodel Interface
Once a Nodel session is running, the Nodel interface will be accessible via any modern browser by typing in the IP address of your computer into the browser, and specify the port 8085. A more simple way of doing this would be by using localhost:8085 as shown in Animation 1.
Adding The Nodes
First thing’s first, we need to create our nodes. To do this we click the add node button, enter our node name, and select “add blank”. In the future, we will uses recipes to speed up the development process. But for now we will be creating our nodes from a blank canvas. After creating the nodes, we should be able to see that they are available from the Nodel Interface.
Animation 2 – Creating new nodes. Click to enlarge.
Nodes can be deleted by selecting the node from the Nodel interface, clicking on the Node title, and clicking the delete button. An example of this process is shown in Animation 3. It should be noted that sometimes the Nodel must be restarted for the deletion to be reflected in the interface.
Animation 3 – Deleting a Node. Click to enlarge.
Creating Actions and Events for Device Nodes
Each node once created contains a file entitled “script.py”. This file is where all the magic happens in regards to what a specific nodes Local Actions, Local Events, Remote Actions, Remote Events, and Parameters. It will also contain the functions that actually do something for device nodes. The Nodel interface provides an easy way to edit and update this file as changes are made. Once selecting the Nodel2Tv node from the Nodel home page, click the “enable” check box under the “advanced mode” label, then click the “display editor” checkbox. This will open an editing window where changes to the script.py file for this particular node can be made. For now, select all the sample code within this file, delete it and click the save button. You will be able to observe that once you have saved the changes, the node will automatically refresh the page, and the available Actions and Events (labeled Signals) now appear empty.
Animation 3 – Editing python code with the nodel interface. Click to enlarge.
Using the functions that we had a look at previously it is relatively easy to write the code necessary to create the required Actions, Events, and Functions. The Actions and Events are created in the main function. The handler function called “TurnOn” handles the turning on of the TV, but for simulation purposes it simply prints that the function has been called. A handler function called “UpdateStatus” handles the emitting of the local status event. Again this function is a placeholder for simulation purposes and will randomly emit “ok” or “not ok”. It is called every time the TurnOn placeholder function is called.
By copying and pasting the above code into the Nodel2Tv node’s editor and saving, it should be possible to observe the creation of all of the required Actions and Events. This step is demonstrated in Animation 4. This completes all that is required to configure the Node2Tv node.
Animation 4 – Coding for Node2Tv to create Actions and Events using the above code. Click to enlarge.
By making some minor changes to the names in the previous code, we can also finish up with the Node3Dvd Device node. The following code demonstrates this. Go to the Nodel home page, click on the Node3Dvd node, paste the code in, and observe the creation of the required Actions and Events. Also note how Device Nodes are implemented in a similar way, which will make the use of Nodel Recipes much clearer when described later on.
Animation 5 – Coding for Node3Dvd to create Actions and Events using the above code. Click to enlarge.
Creating Actions and Events for Management Node
Following the same procedure as before, the Management node can be completed. The Actions and Events are created in the main function. A handler function called “TurnOn” handles the calling of the TV and DVD node remote actions and this function is called every time a use clicks on the “TurnOn” Action button. A handler function called “UpdateStatus” handles the aggregation of the TV and DVD node status messages and emits the Management Nodes local status Event. It is called every time a status is reported (Local Actions from the Device Nodes are Emitted) from the TV or DVD nodes.
By copying and pasting the above code into the Node1Management node’s editor and saving, it should be possible to observe the creation of all of the required Actions, Events, and Bindings required. After clicking the save button under the bindings section, the Remote Actions and Remove Events will become “Wired”. This can be observed by looking for the green arrows in the binding section. This step is demonstrated in Animation 5. Believe it or not, this completes all that is required to configure the Management Node. We can now move on to testing.
Animation 6 – Coding for Node1Management to create Actions and Events using the above code. Click to enlarge.
Testing the System
A good way of testing the the system is by placing each node in a different browser tab and observing all the interactions. Animation 7 shows the result of calling the Node1Management nodes TurnOn local action. It calls both Device Nodes “TurnOn” Local Actions and emits the status Local Events. We can conclude after observing this that the system is working as intended!
Animation 7 – Testing the system by clicking the TurnOn local action on the Node1Management node and observing the result. Click to enlarge.
While this was a good exercise, It should be apparent now that it is undesirable to have to write code for each individual node in a system. This would take a large amount of time and for the most part Management Nodes have very similar coding requirements. To solve this issue, we can use Recipes.
A Recipe is a template for a node. It is simply a script.py file. Museums Victoria has a Github Repository that contains official Nodel Recipes that can used for a wide range of Device and Management Nodes. Recipes should be stored in the Recipes directory (/nodel/recipes/), and will become available for use as node templates if stored in this directory. Animation 8 shows the availability of using Recipes as a node templates when they have been put in to this directory. In this case, a BrightSign player Recipe is used as a template for the creation of a new node.
Animation 8 – Using a Recipe as a template for a node. Click to enlarge.
Creating a Management Node Recipe Using Parameters
Figure 2 – An example of a Parameter from an node created using the official BrightSign recipe.
To unleash the full potential of Recipes we must use Parameters. Parameters allow user input. This user input can be used for many purposes. It could define a target node and target Local Action for a Remote Action, or it could define an IP address for when creating a device node. The format of the Parameter might be a string, an integer, a dropdown box, or an array including a combination of any of the three. The following code provides an example of different kinds of parameters used in a node, how they are created, and how the entered values can be used. In this case the values are printed inside the console window. Animation 9 shows the result.
Animation 9 -Example of the different kind of Parameter input methods. Click to enlarge.
Now we know what Parameters are let’s recreate the Node1Management node using Parameter inputs. To do this we will create a generic Management Node Recipe, and use it to configure the Node1Management node with a user input instead of hard coding the target nodes, Actions, and Events. The following code shows the resulting recipe. Animation 10 shows how the recipe is used to create a node, and configure the necessary bindings to Node2Tv and Node3Dvd.
Animation 10 – Using a Management Node Recipe. Click to enlarge.
Hopefully following this tutorial has demystified some of the abstract concepts covered in the first post, Basic Concepts. We have covered the Nodel interface and how to actualise the abstract system design using code. We have also covered what Parameters and Recipes are, and how they can be used to speed up the implementation process. it should be apparent by now that limiting the amount of time designers are coding is key to Nodel being practical in the real world. Just how to approach Nodel design with minimal coding is explained in the next post, Nodel System Design, where it will be shown how to design a large Nodel system and how to design Recipes that allow non-software engineering types to design and implement Nodel systems.