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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: XML Binary Characterization WG public list available
 Dennis Sosnoski wrote: > I think that makes ASN.1 a reasonable "one-size-fits-schema" > choice. A lot of the world is still not schema, though - and > even for documents with schemas, would the ASN.1 approach > handle arbitrary documents at runtime, or would it need to > use pregenerated codecs? Just as XML works well in both "schema-based" and "schema-free" environments, it is critical that any alternative binary encoding do the same. But, it is also useful to recognize that when you're working with XML, you are never really in a "schema-free" environment... Even if you don't have an application-specific schema, you've still got the schema for XML itself -- the InfoSet [5]. If you have an application-specific schema, you can do all sorts of optimizations in the binary encodings and get really great compression as well as parsing efficiencies based on your knowledge of the schema. When working with the Infoset alone, you can't get quite as much benefit from a binary encoding, but for many (not all) applications the benefit will still be great enough to justify the effort. For instance, you won't get as much compression with an Infoset based encoding, but you'll still usually benefit from having counted length strings and other benefits that make parsing more efficient. (We exploit these efficiencies at PubSub.com, for more info see [1] below.) As most folk now know, ASN.1 and XML Schema are pretty much interchangeable for describing XML. What you can describe in one you can describe in the other. If you've got an XML Schema, you can translate it to ASN.1 using the ISO defined transformations and then generate 'codecs" for either XML or any of the ASN.1 defined binary encodings. Of course, if you don't want to work with the ASN.1 syntax, there are number of vendors (like OSS Nokalva [2]) who let you skip the ASN.1 stage and generate either XML or binary codecs directly from the XML Schema. (i.e. the XML Schema to ASN.1 Schema conversion step isn't necessary). It is not necessary to have "pregenerated codecs" to work with at least the ASN.1 defined binary formats. While it has traditionally been the case that most ASN.1 work *does* use pregenerated codecs, you can get "compile-and-go" systems (like OSS Nokalva's [3]) generate codecs dynamically at runtime. Thus, a minimal system might be defined to have a pregenerated "codec" for XML Infoset, so that it can work with any XML that arrives, and then use "Compile-And-Go" to generate schema-specific codecs as the need for them arises or as schemas are discovered. Just as you can generate codecs on demand, you would also make the switch from XML to binary encodings on demand. i.e. you probably wouldn't generate a binary codec unless you had a connection with an endpoint that supported binary encodings. I assume that unless some endpoint explicitly supports binary, you would normally just generate XML codecs. Of course, your code shouldn't care which codec is in use -- the interface to the codecs should be the same in all cases. (i.e. your interface might be SAX and you just don't care which XMLReader or XMLWriter implementations are being used...) bob wyman [1] Some discussion of the way we benefit from a mixed XML/ASN.1 environment at PubSub.com. I think we've got the first practical demonstration of a mixed XML/ASN.1 or encoding-neutral environment.: http://bobwyman.pubsub.com/main/2004/02/xml_asn1_and_th.html [2] OSS Nokalva's XML/ASN.1 tools: http://www.oss.com/products/xml.html [3] OSS Nokalva Compile and Go for ASN.1 (I assume that they will be supporting XSD directly in the future): http://www.oss.com/products/cng.html [4] Objective Systems. Another XML/ASN.1 vendor: (Note: In addition to their other products, they offer a free ASN.1 to XSD converter.) http://www.obj-sys.com/ [5] Sun's "Fast Web Services" rely on ASN.1 binary encodings for both "Fast Schema" and "Fast Infoset": https://jwsdp.dev.java.net/fast/ 
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