Thinking in G » XML

XML Notepad 2007 – new release

Jim Kring — Fri, 29 Aug 2008 17:33:01 +0000

One of the members of the JKI team just pointed me to the new version (2.5.2798.17141) of XML Notepad. The major improvement is performance — it’s MUCH faster! The old version was pretty clunky (although, very feature rich and easy to use).

This free tool from Microsoft is very useful for editing and debugging XML when you’re working with XML data in LabVIEW (which doesn’t have to be hard). Give it a try, and you’ll be happy that you did.

Creating ordered elements in XML

Jim Kring — Tue, 20 May 2008 07:00:51 +0000

XML doesn't inherently support element ordering. So, if you have XML that looks like this (below), you can't deterministically say that the element with "foo" data comes before element with "bar" data.

   foo

   bar

   monkey

   fish

You can get around this XML limitation by explicitly adding an order/index attribute to your ordered elements. For example:

   foo

   bar

   monkey

   fish

In order to work with data like this, you'll need to pre-populate the "n" attribute before you generate the XML (iterating over the elements and setting the "n" attribute) and you'll need to post-process the "n" attribute after you parse the XML (iterating over the elements to get the "n" attribute and reordering the elements appropriately).

Here's a LabVIEW example that demonstrates this technique (note that this example uses JKI's EasyXML library):

For more tips and tricks for using XML in LabVIEW, and EasyXML in particular, visit the EasyXML Tips and Tricks page on the JKI Software website.

Using XML Data in LabVIEW Just Got Easier

Jim Kring — Sun, 02 Mar 2008 01:15:11 +0000

As I've mentioned before, using XML data in LabVIEW is way too hard. And, according to this poll, 7 out of 10 LabVIEW developers think so, too. LabVIEW's built-in XML schema and support functions are not at all useful for generating and parsing XML schemas defined by others. And, the various tools available to LabVIEW developers for generating and parsing XML schemas defined by others are not easy to use.

For example, there's really no simple and maintainable way to write the code necessary to generate and parse the following XML:

John
Doe

The existing tools require understanding way too much about the details of the XML language (e.g., DOM) and get in the way of the average developer doing useful work with XML data. For examples, this is the code required to read only a single attribute of an XML entity:

When I think easy, I imagine a tool that will allow the developer to:

1.) define a simple LabVIEW data structure that is analogous to an XML data structure of interest and
2.) use it to generate and parse the XML data.

For example, generating and parsing XML data in LabVIEW should be as easy as creating a cluster:

The good news is that JKI Software has just released such a tool that lets you work with XML by simply creating a cluster in LabVIEW. For example, you can create an XML playlist with just one VI and a cluster, as show in the screenshot, below.

And, parsing XML is just as easy, as you can see in this example that downloads the LAVA RSS feed and parses the data into a cluster.

This new tool is called the EasyXML Toolkit for LabVIEW and is a low-cost ($39 introductory price) toolkit that can be installed using VI Package Manager. And, you can even download the demo for free, using VIPM. Now, that's easy.

Using XML Data in LabVIEW is Hard

Jim Kring — Tue, 22 Jan 2008 00:00:34 +0000

XML, which stands for eXtensible Markup Language, is text-based data format (or language) that is human readable and can be used to create arbitrary data structures. It is designed to facilitate sharing structured data across many different systems. Here is a simple example of XML data:

John
Doe

Figure 1 – simple XML data representing a person

You’ll notice that the data looks a lot like HTML, or Hypertext Markup Language (and in fact, XHTML is XML formatted data). In XML a “schema” defines the set of the possible elements and the constraints on those elements (what sub-elements and attributes they can have).

LabVIEW’s XML Schema

There are several tools for dealing with XML data in LabVIEW. First off, LabVIEW includes some VIs for Flattening and Unflattening LabVIEW data to and from XML. Figure 2, below, shows how to use the Flatten to XML function to convert a cluster to an XML string.

Figure 2 – LabVIEW’s Flatten To XML function is used to convert any LabVIEW data to an XML string.

What you’ll notice is that LabVIEW uses a special XML schema for flattening data. The schema that LabVIEW uses is defined by the XSD file located beneath the LabVIEW installation, here: \vi.lib\Utility\LVXMLSchema.xsd. Generally speaking the LabVIEW XML schema uses the following format:

<{Data Type}>
{Data Label}
<{Attribute Name}>{Attribute Value}
{Data Value}

One of the disadvantages of this format is that it does not allow you generate or parse arbitrary XML. For example, the XML data shown in Figure 1, which represents a person, does not conform to the LabVIEW XML schema, so we cannot use LabVIEW’s Flatten To XML or Unflatten From XML functions to generate or parse that data.

Creating Arbitrary XML Data

A natural and straight-forward approach is to use the Format into String function to create XML data when some special schema is required.

Figure 3 – Converting LabVIEW data to an arbitrary XML schema

The downside of the technique shown in Figure 3 is that whenever we want to change the XML schema we have to change the items in the cluster and in the format string. And, if we are changing the number of items in the XML, we have to add or remove items on the Unbundle by Name and Format into String functions — that’s a lot of work and leaves a lot of room for mistakes (a.k.a. “bugs”).

Parsing Arbitrary XML Data

Converting arbitrary XML data back into LabVIEW data is a bit more complicated than it is to convert LabVIEW data into arbitrary XML. For this task, we really need to use an XML parser. An XML parser analyzes an XML string and creates a document object that can be accessed using a Document Object Model, or DOM, API. There are a variety of XML parsers available, including one from National Instruments which is bundled with the LabVIEW Internet Toolkit. Also, the Microsoft XML Parser is available from LabVIEW via ActiveX and .NET and usually pre-installed on most Windows systems (and if it’s not, you can easily download it for free).

Figure 4, below, shows how to read an attribute of an XML element using the functions provided with the LabVIEW Internet Toolkit. As you can see, the process is quite tedious. And, we haven’t even extracted the person’s name, yet. That is even more tedious!

Figure 4 – Parsing an XML string to extract an element’s attribute

Conclusion

Dealing with arbitrary XML data (that does not conform to LabVIEW’s XML schema) is generally very difficult, requiring a lot of duplicate and excessive code that is tedious and risky to maintain. XML parsers do the work of validating and parsing the XML, but there is usually a lot of additional coding required to extract element and attribute values from the document object. Certainly, there must be an easier way…

Are you using XML in your LabVIEW applications? Are there specific XML schema that you have to use? Which aspects of working with XML data do you find difficult? What solutions have you found?

[Update: I have posted a follow-up to this article called Using XML Data in LabVIEW Just Got Easier. I hope you enjoy it.]