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[Xen-API] Typing in XML-RPC

  • To: Xen-API <xen-api@xxxxxxxxxxxxxxxxxxx>
  • From: Anthony Liguori <aliguori@xxxxxxxxxx>
  • Date: Fri, 23 Jun 2006 14:13:11 -0500
  • Delivery-date: Fri, 23 Jun 2006 12:13:27 -0700
  • List-id: Discussion of API issues surrounding Xen <xen-api.lists.xensource.com>

One of the first things I noticed is that the spec defines some extended types. This is a bit challenging in XML-RPC. It's unclear to me how 64 bit integer is represented (since XML-RPC only defines 32 bit integers). Also, defining void to an empty string is understandable but requires some special casing that really shouldn't be necessary.

Here's what I propose:

As a convention, we never use <struct>'s on the wire to directly represent structures. By convention, structs always appear as:


Where typename is the string representation of the type and typevalue is the type-specific value.

Some common types would be:

struct - use this to represent actual structs. typevalue is the normal encoding of a struct long - 64 bit representation of struct. typevalue is the string representation
void - use to represent None.  typevalue is ignored.

What's nice about this sort of consistent approach is that we can write a marshalling/unmarshalling wrapper for Python that automagically does this conversion. Furthermore, if we did decide to support objects (as the current spec does), we could automagically marshal/unmarshal these over the wire.

The general idea here is that this is an extensible typing system for XML-RPC.

Another thing to consider is having a type of exception. I like exception based APIs (i'll say that for another email) however the format of the standard XML-RPC exception leaves a lot to be desired.



Anthony Liguori

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