wiki:XmlVersionNumbers

XML Versions

This technical document explains the version numbers Vespa writes into the XML (VIFF files) it creates. They have not turned out to be as useful as we expected.

Introduction

In all of the XML (VIFF files) written by Vespa, we write a version attribute on major blocks. This is not to be confused with the Vespa version information which is written into a comment at the top of each VIFF file.

For instance, in the sample below (taken from a real file) there are version attributes on the top level vespa_export element, the dataset element, and the user_prior element --

<vespa_export version="1.0.0">
    <!--
This XML file is in Vespa Interchange File Format (VIFF). You can download
applications that read and write VIFF files and learn more about VIFF here:
http://scion.duhs.duke.edu/vespa/

It was created with Vespa version 0.6.4.
-->
    <timestamp>2013-01-18T17:28:39</timestamp>
    <comment />
    <dataset id="2b92bc53-d98d-48ea-a60e-d374658926b2" version="1.1.0">
        <user_prior version="1.0.0">
            <auto_b0_range_start>1.7</auto_b0_range_start>
            <auto_b0_range_end>3.4</auto_b0_range_end>
            <auto_phase0_range_start>1.85</auto_phase0_range_start>

Purpose

The purpose behind these was to allow us developers to change the XML format as necessary without breaking our ability to read in XML files written by older versions of Vespa. For instance, in version 1.0.0 of Analysis' dataset XML, there was a block_basic element that moved to a user_prior element in subsequent versions. Our code can read both the older and newer formats.

In Practice

In practice, the XML versions aren't very useful for two reasons.

First, as far as I can tell, in every instance where we've changed an XML version number, the change to the version number has been superfluous. For instance, in the example mentioned above where block_basic was eliminated in favor of user_prior, the XML version is just a proxy for the presence of a block_basic element.

Our current code does this --

if xml_version == "1.0.0":
    for i, block_element in enumerate(block_elements):
        if block_element.tag == "block_basic":
            break
    # ...change block_basic into a user_prior element...

But it could just as well do this --

block_basic = block_elements.find("block_basic")
if block_basic is not None:
    # ...change block_basic into a user_prior element...

IMO the second version is clearer because it explicitly tests for the condition in which it's really interested.

The fact that the version number is superfluous was emphasized by the fact that Vespa 0.6.1 changed the dataset XML format, but the XML version number wasn't changed due to an oversight. The practical consequences of this were nil (or None, to be Pythonic).

A second, less important reason that the XML version numbers are not as useful as they could be is that the format is unnecessarily complex. I (Philip) made the bad choice of using app-style version numbers in x.y.z format. A simple integer would have been sufficient and easier to work with (especially in comparisons).

The x.y.z format orders as one would hope (e.g. '1.0.0' < '1.0.1', and '2.9.9' < '3.0.0') but such ordering is not as intuitive as simple int comparison.

Conclusion

I'm not ready to advocate abandoning the XML version numbers yet. There might be a case where they're necessary, we just haven't come across it yet. Currently it costs us next to nothing to keep writing them to the XML, and if we find we need them we'll be glad they're there.

Last modified 6 years ago Last modified on Jan 25, 2013, 3:45:23 PM