[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Streams, protocols, documents and fragments
Instead of separation characters, I would just label the fragments (borrowed from XML linking): <xf:fragment location="ROOT() CHILD(1,stocks) (1700,stockPrice)"> <stockPrice timestamp="19992402141500"> <ticker>MSFT</ticker> <price>1000</price> </stockPrice> </xf:fragment> <xf:fragment location="ROOT() CHILD(1,stocks) (1327,stockPrice)"> <stockPrice timestamp="19992402132540"> <ticker>ICI</ticker> <price>1010</price> </stockPrice> </xf:fragment> The xf:fragment element identifies each fragment in terms of its location in the entire document. In this case it assumes a document structured as <stocks> <stockPrice>..</stockPrice>* </stocks> where the prices desired are the 1700th and 1327th price entries. It may be a bit verbose, but allows a document tree to be transmitted piecemeal and reassembled. Access to any node not yet downloaded could be requested (for instance if stockPrice 1553 were desired). The reassembled tree caches the subset of elements that an application is interested in, but has all of the holes to access additional elements. Internet protocols are supposed to ensure an error free transmission. Any ordering problems are resolved by the location description. Marc B McDonald Principal Software Scientist Design Intelligence, Inc www.design-intelligence.com ---------- From: Mark Birbeck [SMTP:Mark.Birbeck@iedigital.net] Sent: Wednesday, February 24, 1999 7:18 AM To: xml-dev list Subject: RE: Streams, protocols, documents and fragments > From: Borden, Jonathan [SMTP:jborden@mediaone.net] > My sole purpose in discussing 'document > fragments' was because the thread had gotten stuck on the notion that > a > continuous XML stream would contain a single long document (perhaps > w/o a > closing tag) and the actual PDU's consist of document fragments ... > the > point is that if we create a protocol on a stream which transmitts > multiple > documents, there is no loss of functionality over a solution employing > 'document fragments' > I agree with this. And the point I was trying to get to was that therefore we don't need to introduce loads of terms on top of XML 1.0 to understand the concepts. I still think all of this is being over-complicated - but then maybe I'm the one who's missing something, so let's see. I don't follow why so many suggestions to resolving this problem involve stepping 'outside of' XML 1.0. We have suggestions for sync characters like ^C and ^L, we have the proposal that XML 1.0 should be fundamentally altered to allow the concept of a 'not well-formed' document (or one that may *become* well-formed at some point in the future), we have proposals for documents that contain subsets of validity. All of these suggestions seem to go against the grain of what XML is about. XML 1.0 already copes with streams and files. A physical XML document is a linear sequence of characters conforming to certain rules. You can't tell whether those rules have been met until you have received the entire sequence of characters. You know when you've reached the end by the closing tag. That's it! There's not much else you can do about it, because that's what XML is all about - well-formed, possibly validated documents conforming to certain rules. Now, the fact that the beginning and end of this sequence of characters may be presented to the parser eight hours apart is to me an application problem. If someone has a document that takes eight hours to arrive then maybe they should re-think how they're setting the system up. If it's a massive document that can only be processed in its entirety, and if any part fails to arrive the whole document fails, then sure, you have to go ahead and send it over eight hours. But the stock ticker example is not like this. If I miss the stock price for Microsoft at 11am, then I can still make use of the stock price for Microsoft at 11.20am. It will affect my historical archives, but at least I have something to display. It is not an 'all or nothing' situation. So, accepting for a moment that we should transmit many documents throughout the day, rather than one big one, it leaves the question of demarcation. And here I'm surprised that people want to step outside of XML to find a solution. Say we send the following: ^L <stockPrice timestamp="19992402141500"> <ticker>MSFT</ticker> <price>1000</price> </stockPrice> ^L <stockPrice timestamp="19992402132540"> <ticker>ICI</ticker> <price>1010</price> </stockPrice> ^L If the data link is 100% reliable then we have encoded redundant information because the document name - the element for stockPrice - already tells us where one starts and ends. So, we don't need the ^L. But if the data link *isn't* reliable then adding a few ^L characters doesn't help a lot, because if we lose the following sequence we have no way of knowing: <price>1000</ticker> </stockPrice> ^L <stockPrice timestamp="19992402132540"> <ticker>ICI</ticker> If this sequence is taken out of the above two documents then you now have the wrong price for Microsoft and nothing for ICI, and your application is none the wiser. I think if 100% data reliability is required then we need a few streaming-related attributes that we can add to our documents, such as: <stockPrice timestamp="19992402141500" streamns:packetID="55"> <ticker streamns:packetID="55">MSFT</ticker> <price streamns:packetID="55">1000</price> </stockPrice> <stockPrice timestamp="19992402132540" streamns:packetID="56"> <ticker streamns:packetID="56">ICI</ticker> <price streamns:packetID="56">1010</price> </stockPrice> These would be added by a 'sending' application as a separate layer to the original document generation, and would allow the receiving application to process all the 'streamns' packets before actually processing the nodes - say, storing or displaying the stock prices. You could remove 'invalid' nodes from the tree (well-formed at the XML level, but with the wrong packet ID), and then while your main application is getting on and acting on the stock data, the receiving process could be re-requesting the lost data. In the illustration above, after losing the packet, we would now have: <stockPrice timestamp="19992402141500" streamns:packetID="55"> <ticker streamns:packetID="55">MSFT</ticker> <price streamns:packetID="56">1010</price> <--- error here </stockPrice> and the 'streamns' processing would spot and re-request the missing data easily (both packet 55 and packet 56). To be honest, I'm not suggesting what I've said here as some new standard. There are lots of ways what I've described could be achieved, for example: <stockPrice timestamp="19992402141500" streamns:packetID="55" streamns:checksum="556543"> <ticker>MSFT</ticker> <price>1000</price> </stockPrice> <stockPrice timestamp="19992402132540" streamns:packetID="56" streamns:checksum="771239"> <ticker>ICI</ticker> <price>1010</price> </stockPrice> takes up less space, and would still spot the same errors. I'm just trying to illustrate how solutions can be found that don't involve smashing XML 1.0 to bits. At the end of the day this is an application problem, not an XML one. Regards, Mark xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@ic.ac.uk Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ and on CD-ROM/ISBN 981-02-3594-1 To (un)subscribe, mailto:majordomo@ic.ac.uk the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@ic.ac.uk the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@ic.ac.uk) xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@ic.ac.uk Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ and on CD-ROM/ISBN 981-02-3594-1 To (un)subscribe, mailto:majordomo@ic.ac.uk the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@ic.ac.uk the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@ic.ac.uk)
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