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A Beginner's Guide to oBIX (even if you're not a developer!) - Part 2

  Posted by Gareth Johnson |
Senior Development Engineer
Tridium Europe (based in the UK)

Last month we ran Part 1 below is part 2 and A Beginners Guide to oBIX (even if you're not a developer!) - Part 3 is now online as of 22-Aug-07 11:14 AM EDT Your next question might be, "Ok, but I can already access real-time point information by regularly making HTTP GETs to an oBIX URI".

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A Beginner's Guide to oBIX (even if you're not a developer!) - Part 2
 
In the last installment of this basic oBIX guide, I discussed reading and writing objects in oBIX. This week I'm going to discuss objects in more detail and oBIX Operations. Once we've understood this concept, we can start using oBIX as an API and then dig into Watches and some of the other advanced features.

Before we start invoking Operations, let's go back to an oBIX document and discuss Contracts. As stated last week, an oBIX object is an XML document that can be identified by its own unique URL. However, we need someway to identify 'types' of objects. For instance, we need someway to distingish between a NumericWritable from a SineWave object. This is where Contracts come in. Let's look at the first line of a NumericWritable...

<real val="1.0" href="http://localhost/obix/config/Folder/NumericWritable/"
is="/obix/def/control:NumericWritable /obix/def/control:NumericPoint obix:Point"...

The above 'is' attribute identifies the 'type' of object. You can read the content of this attribute from left to right and see all the Contracts NumericWritable implements. Therefore, a NumericWritable is also a NumericPoint and a generic oBIX Point. For all you developers out there, you can think of this as a flattened inheritance hierarchy.

Now let's talk about Operations. In the last part of the guide, I discussed using HTTP GET and PUT for reading and writing oBIX data. To invoke an oBIX Operation, we need to use an HTTP POST. To execute an Action we simply...

  • Identify the URL of the Action we want to invoke.
  • HTTP POST to this URL with the content being the input argument for the Operation.
  • This invokes the Action and returns any result in the response.

That's it - easy eh? Ok let's try this out with ORT again. As described in part 1, run ORT and a test Station. In your Station, add a Program Object to 'Folder'. Edit the Program Object and add the following code...

public void onExecute()
{
  System.out.println("Hello world!!!");
}

...now save the changes and open web browser. Log in and surf to the 'execute' Action on the Program Object. It should look something like...

http://localhost/obix/config/Folder/Program/execute/

Note how the 'execute' Action is listed as an 'op' XML element. Now paste this as the URL into ORT, select POST from the drop down menu and hit the 'Send' button. Everytime you hit the 'Send' button, you should see the 'Hello World!!!' message appear in the Station output. Congratulations, you can now externally execute a Program Object! And yes you've just implemented a custom fully functioning web service for a Station!

Let's go back at and have a look at the definition for an oBIX Operation...

<op href="http://localhost/obix/config/Folder/NumericWritable/set/" in="obix:real" out="obix:Nil" />

You'll notice I'm now referring to another Operation that now takes an argument. In this case, the above Operation would invoke a 'set' Action on a NumericWritable. The 'in' attribute refers to the Contract of the input argument and the 'out' attribute specifies the output (which in this case is nothing). For the 'set' Action, you would POST...

<real val="12.2" />

Reliable Controls Let's take a more complicated example for invoking the 'override' Action on the same NumericWritable component. Now let's view the 'op' for the override Operation. By surfing to...

http://localhost/obix/config/Folder/NumericWritable/override/

...you should get back...

<op href="http://localhost/obix/config/Folder/NumericWritable/override/"  
in="/obix/def/control:NumericOverride /obix/def/control:Override" displayName="Override" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://obix.org/ns/schema/1.0 /obix/xsd" xmlns="http://obix.org/ns/schema/1.0"/>

So we're interested in the 'in' attribute and we can see it takes an 'NumericOverride' object. Well that's very nice. But how do I find out about the Contract? Well just cut and paste the relative URL into your web browser and surf to it. For example, by surfing too...

http://localhost/obix/def/control:NumericOverride

...you should get back...

<obj href="http://localhost/obix/def/control:NumericOverride/" is="/obix/def/control:Override" display="0.0 : permanent" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://obix.org/ns/schema/1.0 /obix/xsd" 
xmlns="http://obix.org/ns/schema/1.0">
  <reltime name="duration" val="PT0S" href="duration/" display="0ms" displayName="Duration" writable="true"/>
  <real name="value" val="0.0" href="value/" displayName="Value" writable="true"/>
</obj>

Mmmm. There's quite a few attributes in there. The good news is for invoking the 'override' Operation, we don't need to include all the detail as specified in the Contract. Basically, we just need to specify the name and value. Therefore, you would really POST...

<obj>
  <reltime name="duration" val="PT0S" />
  <real name="value" val="32.6" />
</obj>

Now believe it or not, you've just learnt a core piece of oBIX and one that will allow you to do virtually anything. Let me explain...

  • By using oBIX Operations you can execute any kind of Niagara Action. Therefore, if you develop your own components with their own Actions, they automatically get exposed as web services! Wow - neat eh? It certainly takes the pain out of creating external interfaces for custom applications! Consequently you can concentrate on your Niagara components and not anything with angle brackets.
  • An Operation can take one argument (note this could be a structure and hence take as many arguments as you want).
  • An Operation can return a result (another oBIX document).
  • Interacting with Watches, Alarms and Histories all rely on using oBIX Operations. Now you know how to invoke an Operation, you can scout ahead to these oBIX objects, look at their Operations and start invoking them. Eh presto - you're now just using another API.

Phew - well if you didn't quite catch the last bit then don't worry. In the next part, we're going to be looking at using the Watch Service (although if you've just understood everything I've just discussed you should be able to get there without me).

One last thing - thanks for all the positive comments I've had about my blog!

 

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