Weaknesses of XML-RPC

The problem with XML-RPC as an object-serialization format is that it just plain does not have enough types to handle the objects in most high-level programming languages. Listing 4 illustrates this shortcoming.

>>> c = C()

>>> c.foo = 'bar'
>>> d = {'foo':'bar'}
>>> print xmlrpclib.dumps((c,d),'PyObjects')
<?xml version='1.0'

?>
<methodCall>
<methodName>PyObjects</methodName>
<params>
<param>
<value><struct>
<member>

<name>foo</name>
<value><string>bar</string></value>
</member>
</struct></value>
</param>

<param>
<value><struct>
<member>
<name>foo</name>
<value><string>bar</string></value>

</member>
</struct></value>
</param>
</params>
</methodCall>

In Listing 4, two things are serialized -- an object instance and a dictionary. While it is fair to say that Python objects are particularly dictionary-like, you lose a lot of information by representing a dictionary and an object in exactly the same way. Additionally, the excessively generic meaning for <struct> in XML-RPC affects pretty much any OOP language, or at least any language that has native hash/dictionary constructs; it is not a Python quirk here. On the other hand, failing to distinguish Python tuples and lists within the <array> type of XML-RPC is a fairly Python-specific limitation.

xml_pickle handles all the Python types much better (including data types defined by user classes, as we saw). Actually, there is no direct pickling of dictionaries in xml_pickle, basically because no one has asked for this (it would be easy to add). But dictionaries that are object attributes get pickled, as shown in Listing 5.

>>> c, c2 = C(), C()
>>> c2.foo = 'bar'
>>> d = {'foo':'bar'}

>>> c.c, c.d = c2, d
>>> print XML_Pickler(c).dumps()
<?xml version="1.0"
?>
<!DOCTYPE PyObject SYSTEM "PyObjects.dtd">
<PyObject class="C" id="1917836">

<attr name="c" type="PyObject"
class="C" id="1981484">
  <attr name="foo" type="string" value="bar" />

</attr>
<attr name="d" type="dict" id="1917900">
  <entry>
    <key type="string" value="foo" />

    <val type="string" value="bar" />
  </entry>
</attr>
</PyObject>

Another virtue of the xml_pickle approach that is implied in the example is that dictionary keys need not be strings. In XML-RPC <struct> elements, <name> keys are always strings. However, Perl, PHP, and most languages are closer to the XML-RPC model in this.