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revision 1.179 by tjl, Sun Nov 9 19:11:59 2003 UTC revision 1.180 by tjl, Mon Nov 10 07:56:45 2003 UTC
# Line 247  Related work Line 247  Related work
247  In this section, we shall briefly discuss the existing approaches  In this section, we shall briefly discuss the existing approaches
248  to document updates in P2P systems.  to document updates in P2P systems.
249    
250    A trivial approach to P2P versioning
251    is to have a centralized server keep track of the current version
252    of a document, such as in the *Content-Addressable Web*
253    proposal [content-addressable-web]_, in which clients download
254    a file's hash through HTTP (using an HTTP URI),
255    then download the file through, e.g., a filesharing network.
256    This provides some of the efficiency advantages of a P2P Web.
257    However, there is still a single point of failure, and
258    links still break when pages move to a different directory
259    or Web server.
260    In the Content-Addressable Web proposal, the situation
261    regarding versioning is the same as on the current Web.
262    
263    *Freenet*, a P2P network for anonymous publication [freenet-ieee]_,
264    was originally planned to have an
265    update mechanism where the network would remember
266    the current version of a document, and update this information
267    when receiving a message signed by the appropriate key,
268    rather like in OceanStore. However, in practice,
269    Freenet has a rather peculiar combination
270    of two workarounds [freenetfaq]_.
271    In date-based updating, the URI for the current version
272    is formed by concatenating a document's id with the current date;
273    this requires publishers to insert versions of their document
274    every day, otherwise they will not be accessible.
275    In edition-based updating, a version's URI is the document's id
276    concatenated with a version number. Version ``n`` then links to
277    version ``n+1`` using an ``<img>`` link. The image will not show until
278    version ``n+1`` is actually inserted into the system; thus,
279    when the image does show up, the user knows that they should click
280    on it to get to a newer version of the document.
281    In Freenet, files only stay online as long as they are
282    requested regularly; keeping a copy on your harddisk
283    does not keep a page alive.
284    
285  *CFS* [dabek01widearea]_  *CFS* [dabek01widearea]_
286  is a file system based on Chord [stoica01chord]_, storing  is a file system based on Chord [stoica01chord]_, storing
287  data in a distributed hashtable (DHT). CFS identifies  data in a distributed hashtable (DHT). CFS identifies
# Line 290  harddisk to keep the version "alive." Line 325  harddisk to keep the version "alive."
325     semantics; for example, the NFS client for OceanStore only requests     semantics; for example, the NFS client for OceanStore only requests
326     new heartbeats less than 30 seconds old.     new heartbeats less than 30 seconds old.
327    
 *Freenet*, a P2P network for anonymous publication [freenet-ieee]_,  
 was originally planned to have an  
 update mechanism where the network would remember  
 the current version of a document, and update this information  
 when receiving a message signed by the appropriate key,  
 rather like in OceanStore. However, in practice,  
 Freenet has a rather peculiar combination  
 of two workarounds [freenetfaq]_.  
 In date-based updating, the URI for the current version  
 is formed by concatenating a document's id with the current date;  
 this requires publishers to insert versions of their document  
 every day, otherwise they will not be accessible.  
 In edition-based updating, a version's URI is the document's id  
 concatenated with a version number. Version ``n`` then links to  
 version ``n+1`` using an ``<img>`` link. The image will not show until  
 version ``n+1`` is actually inserted into the system; thus,  
 when the image does show up, the user knows that they should click  
 on it to get to a newer version of the document.  
 In Freenet, files only stay online as long as they are  
 requested regularly; keeping a copy on your harddisk  
 does not keep a page alive.  
   
 Finally, a simplistic approach to P2P versioning  
 is to have a centralized server keep track of the current version  
 of a document, such as in the *Content-Addressable Web*  
 proposal [content-addressable-web]_, in which clients download  
 a file's hash through HTTP (using an HTTP URI),  
 then download the file through, e.g., a filesharing network.  
 This provides some of the efficiency advantages of a P2P Web.  
 However, there is still a single point of failure, and  
 links still break when pages move to a different directory  
 or Web server.  
 In the Content-Addressable Web proposal, the situation  
 regarding versioning is the same as on the current Web.  
   
328    
329  Pointer records  Pointer records
330  ===============  ===============

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