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literal caps and immutable directories 

[Imported from Trac: page Capabilities, version 5]
davidsarah 2010-07-01 02:01:35 +00:00
parent fef3a33b09
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52
Capabilities.md

@ -1,18 +1,24 @@
# Capabilities
An overview from the mailing list archives - originally [Feb 2009 - Brian Warner](http://allmydata.org/pipermail/tahoe-dev/2009-February/001110.html):
An overview from the mailing list archives - originally by [Feb 2009 - Brian Warner](http://allmydata.org/pipermail/tahoe-dev/2009-February/001110.html),
but updated to take into account literal caps and immutable directories:
```
1: immutable file read-only capability string URI:CHK:
2: immutable file verify capability string URI:CHK-Verifier:
1: immutable file read-only capability string URI:CHK:
2: immutable file verify capability string URI:CHK-Verifier:
3: immutable LIT file read-only capability string URI:LIT:
3: mutable file read-write capability string URI:SSK:
4: mutable file read-only capability string URI:SSK-RO:
5: mutable file verify capability string URI:SSK-Verifier:
4: mutable file read-write capability string URI:SSK:
5: mutable file read-only capability string URI:SSK-RO:
6: mutable file verify capability string URI:SSK-Verifier:
6: directory read-write capability string URI:DIR2:
7: directory read-only capability string URI:DIR2-RO:
8: directory verify capability string URI:DIR2-Verifier:
7: immutable directory read-only capability string URI:DIR2-CHK:
8: immutable directory verify capability string URI:DIR2-CHK-Verifier:
9: immutable LIT directory read-only capability string URI:DIR2-LIT:
10: mutable directory read-write capability string URI:DIR2:
11: mutable directory read-only capability string URI:DIR2-RO:
12: mutable directory verify capability string URI:DIR2-Verifier:
In Tahoe, directories are built out of mutable files (a directory is really
just a particular way to interpret the contents of a given mutable file), and
@ -22,28 +28,32 @@ uploaded into immutable files by default.
Some capabilities can be used to derive others. If you have #1, you can
derive #2 (but not the other way around). The full table is:
#1->#2
#3->#4->#5
#6->#7->#8
#1 -> #2
#4 -> #5 -> #6
#7 -> #8
#10-> #11-> #12
Deriving a weaker capability from a strong one is called "diminishing" the stronger one.
So we use "filecap" to talk about #1+#3+#4, but (since most files are
immutable) we're usually talking about #1. We use "dircap" to talk about #6
and #7. We use "readcap" to talk about #1,#4, and #7, but usually we refer to
#7 as a "directory readcap". We use "writecap" to talk about #3 and #6.
So we use "filecap" to talk about #1..6, but (since most files are immutable)
we're usually talking about #1. We use "dircap" to talk about #7..12.
We use "readcap" to talk about #{1,3,5,7,9,11}, but usually we refer to
#{7,9,11} as a "directory readcap". We use "writecap" to talk about #4 and #10.
A "literal cap" or "LIT cap" stores the contents of a small file (#3) or
directory (#9) in the capability itself.
A "verifycap" is the weakest capability that still allows every bit of every
share to be validated (hashes checked, signatures verified, etc). That means
#2, #5, and #8.
#{2,6,8,12}.
When we talk about a "repaircap", we mean "the weakest capability that can
still be used to repair the file". Given the current limitations of the
repairer and our webapi, that means we're talking about #1, #3, or #6.
repairer and our webapi, that means we're talking about #{1,4,7,10}.
Eventually we'll fix this limitation, and any verifycap should be useable as
a repaircap too. (there's much less work involved to let #2 repair a file..
it's just an incomplete API, rather than a fundamental redesign of the server
protocol).
a repaircap too. (There's much less work involved to let #2 repair a file or
#8 repair a directory... it's just an incomplete API, rather than a fundamental
redesign of the server protocol.)
We then use the somewhat-vague term "rootcap" to refer to a cap (usually a
directory write cap) that is not present inside any directory, so the only