Archive of UserLand's first discussion group, started October 5, 1998.

Re: Lightweight Distributed Objects (LDO); was Re: Encodings

Author:Ken MacLeod
Posted:1/30/1999; 9:15:58 AM
Topic:XML-RPC Spec Comments
Msg #:2587 (In response to 2585)
Prev/Next:2586 / 2588

>>How are we going to resolve the differences in the names of the tags?

XML-RPC, WDDX, and LDO XML and binary serializations all are at the same layer, so in effect they should be plug-compatible. Once the issue of protocol versioning is resolved we could simply say a new serialization plug-in is available and still stay backward and forward compatible.

Regarding versioning, I don't recall anyone mentioning one of the possible solutions that's intended for that: the DOCTYPE declaration! Specifically the PUBLIC identifier. The doctype declaration MAY be used for validation, but it is not required to exist if you're only interested in well-formedness or are using a different schema for validation. Possible doctype declarations for XML-RPC and LDO XML serialization are:

The server can then select plug-ins based on the doctype. They don't have to be really parsed, it could simply be a matter of looking for For backward and cross compatibility, I've been working on a plug-in selector that goes something like this:

1) Grab the first available bytes from the stream or HTTP content-body

2) Search for ``magic'' (in the Unix `file' command sense), if magic exists then we're done

3) Is it an HTTP request (GET, PUT, POST, etc.)?, then we're done, hand it off to the HTTP receiver

4) Is it XML (starts with `<')?

4a) does it have a doctype declaration? then we're done

4b) Parse the document until the first open-tag, use that to select the receiver

>>I don't know when LDO appeared, but I wasn't aware of its existence when we designed XML-RPC.

LDO binary started probably about the same time-frame as XML-RPC (February-March 1998). Shortly after I wrote the Frontier-RPC module and posted my post-mortem to the frontier-xml I created the LDO XML format in response to a request for a possible update to XML-RPC. Somewhere around then the frontier-xml list went away and we just kept on working on it within Casbah. Since then we've had a somewhat different target audience (XML-RPC is closer to traditional procedural RPCs and LDO is closer to distributed object systems) so it wasn't clear that there'd be interest in changing.

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