[tahoe-dev] [tahoe-lafs] #795: append-only files

tahoe-lafs trac at allmydata.org
Sat Aug 22 23:19:48 UTC 2009

#795: append-only files
 Reporter:  warner        |           Owner:           
     Type:  enhancement   |          Status:  new      
 Priority:  major         |       Milestone:  undecided
Component:  code-mutable  |         Version:  1.5.0    
 Keywords:                |   Launchpad_bug:           
 It's a long-term goal, but it would be nice to have append-only
 mutable files in Tahoe. Specifically, that means some sort of
 mutable slot that has the following caps:

  * writecap (permits arbitrary modification)
  * appendcap (permits appending new data, not removing existing
  * readcap (permits reading arbitrary data)
  * verifycap

 Note that the appendcap and readcap are peers: neither can be
 derived from the other.

        |      |
        v      v
  appendcap   readcap
        |      |
        v      v

 This requires some tricky encryption, and will rely upon the
 servers to enforce certain types of message updates. (this means
 that a sufficiently-large set of colluding servers will be able
 to roll back appended messages until someone with the writecap
 comes along and flattens them all into the base file). I've got
 some notes on this scheme somewhere, which I'll eventually
 transcribe into this ticket. The basic approach is that the
 appendcap gets you a signing key, and the server will accept
 signed append messages, and put them next to all of the other
 messages. The writecap gets you a different signing key, with
 which the servers will accept arbitrary rewrite operations. I
 also have notes on revocation schemes (where you can create
 multiple "subwritecaps" for an object, but revoke them later),
 which involves even more sets of signing keys.

 This could be integrated with LDMF mutable files, in which the
 servers hold a series of append-only revision nodes, forming a
 version-history graph (which I still think should be based on
 Mercurial's "revlog" format). In this case, writecap holders get
 to add revision nodes, but not remove older ones.

Ticket URL: <http://allmydata.org/trac/tahoe/ticket/795>
tahoe-lafs <http://allmydata.org>
secure decentralized file storage grid

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