...and XML-RPC Oh No!

Web services were born when Dave Winer decided, when developing Frontier, that he needed a ``simple-enough-that-it-will-work'' strategy for calling procedures on remote computers across language and platform boundaries. Then and now XML was all the buzz; so, when cooking-up his solution he decided to sprinkle a bit of XML on the dish. The result was XML-RPC.

XML-RPC is a remote procedure protocol which sends XML missives over the wire using the protocol of love, HTTP. Here, for exmample, is an XML-RPC request to call the procedure examples.getStateName remotely

POST /RPC2 HTTP/1.0
User-Agent: Frontier/5.1.2 (WinNT)
Host: betty.userland.com
Content-Type: text/xml
Content-length: 181 


<?xml version="1.0"?>
<methodCall>
  <methodName>examples.getStateName</methodName>
  <params>
    <param>
      <value><i4>41</i4></value>
    </param>
  </params>
</methodCall>

and here the response to our example query

HTTP/1.1 200 OK
Connection: close
Content-Length: 158
Content-Type: text/xml
Date: Fri, 17 Jul 1998 19:55:08 GMT
Server: UserLand Frontier/5.1.2-WinNT
 
<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
      <value><string>South Dakota</string></value>
    </param>
  </params>
</methodResponse>

In fixing the eye long enough on these simple examples you can ascertain the gist of XML-RPC. The protocol is dead-simple and this is its major strength. XML-RPC, unlike CORBA, does not rely on any fancy-pants ORB that may or may not be able to interoperate with other ORB's, nor does XML-RPC, unlike XDR, rely one some arcane serialization protocol derived in the days when dinasours roamed the earth. XML-RPC is packed with goodness from head-to-toe. It is this which motivated people to seriously start looking into XML-RPC.

While XML-RPC was based upon wholesome goodness from the zenith of its head to the nadir of its toes, in applying it to domains distant from the original Frontier sand-box, it began to show growing pains. For example, the data types which XML-RPC can shuffle to-and-fro are so simple as to work, but a bit too simple as to be the end-all RPC solution. For example, here is the entire type specification for XML-RPC data types from the Vulgate on these matters, the XML-RPC standard

Scalar <value>s

<value>s can be scalars, type is indicated by nesting the value inside one of the tags listed in this table :

If no type is indicated, the type is string.

<struct>s

A value can also be of type <struct>.

A <struct> contains <member>s and each <member> contains a <name> and a <value>.

Here's an example of a two-element <struct>:

<struct>
  <member>
    <name>lowerBound</name>
    <value><i4>18</i4></value>
  </member>
  <member>
    <name>upperBound</name>
    <value><i4>139</i4></value>
  </member>
</struct>

<struct>s can be recursive, any <value> may contain a <struct> or any other type, including an <array>, described below.

<array>s

A value can also be of type <array>.

An <array> contains a single <data> element, which can contain any number of <value>s.

Here's an example of a four-element array:

<array>
  <data>
    <value><i4>12</i4></value>
    <value><string>Egypt</string></value>
    <value><boolean>0</boolean></value>
    <value><i4>-31</i4></value>
  </data>
</array>

<array> elements do not have names.

You can mix types as the example above illustrates.

<arrays>s can be recursive, any value may contain an <array> or any other type, including a <struct>, described above.

The XML-RPC primitive type system, though flexible, isn't as filigreed or festooned as the baroque arabesque that is the XML primitive type system; one can easily glean this by comparing the above lean-mean solution to the more rococo arabesque of figure . This, among other things, was a motivation to introduce WSDL, a similar but all together more advanced RPC protocol, with progenitor XML-RPC, that can make use of the rococo arabesque of figure . However, before revisiting WSDL we'll take a look at the engine behind much of WSDL's success SOAP.